Liberty Series II. Liberty. Liberty Series II. Analytical Methods book. Publication No October (i) Publication date: 10/99

Transcription

1 Liberty Series II Liberty Liberty Series II Analytical Methods book Publication No October 1999 Publication date: 10/99 (i)

2 Liberty Series II Varian offices Varian has offices in most countries. The major offices for optical spectroscopy products are listed below: Varian Australia Pty Ltd (Manufacturing site) 679 Springvale Road Mulgrave, Victoria 3170 Australia International telephone: International fax: Varian Instruments 2700 Mitchell Dr. Walnut Creek, CA USA Phone: International telephone: International fax: Varian Chrompack Benelux, Analytical Instruments Boerhaaveplein 7, 4624 VT, Bergen op Zoom International telephone: International Fax: Internet The Varian Internet home page can be found at: Varian Australia Pty Ltd is the owner of copyright on this document and any associated software. Under law, the written permission of Varian Australia Pty Ltd must be obtained before either the documentation or the software is copied, reproduced, translated or converted to electronic or other machine-readable form, in whole, or in part. First published 1996 in Australia. Reprinted Comments about this manual should be directed to the Marketing Communications Manager, Varian Australia at the address above. Varian Australia is ISO9001 certified Varian Australia Pty Ltd (A.C.N ) All rights reserved (ii) Publication date: 10/99

3 Liberty Series II Contents 1 Introduction to ICP-AES General introduction to optical emission spectrometry Historical background of the inductively coupled plasma Instrumentation for ICP-AES Overview Sample introduction Plasma torch Radio-frequency supply Optical spectrometer Detectors and readout systems Computers The ICP as an atomization and excitation source for emission spectrometry Nature of the plasma Analyte emission in the ICP Zones in the ICP Comparison of the analytical performance of ICP-AES and some other techniques Comparison of ICP-AES and flame-aas Comparison of ICP-AES with graphite furnace-aas Comparison of ICP-AES with other plasma sources for atomic emission Comparison of ICP-AES with ICP-mass spectrometry Direct analysis of solid samples by ICP-AES Overview of techniques for direct analysis of solids Direct introduction of solid samples into the ICP Analysis of gaseous samples by ICP-AES Sources of further information on ICP spectrochemical analysis Books Periodicals Operating Principles Introduction Checking the torch and nebulizer 2-1 Publication date: 10/99 (iii)

4 Liberty Series II 2.3 Selection of analyte lines Correction of spectral interferences Background correction Non-spectral interferences Transport interferences Solute volatilization interferences Excitation interferences Adaptation effects Organic solvents Liberty description Introduction Hardware Sample introduction Sample excitation Spectrometer Axially-viewed ICP Limits of Detection Implementation of the axially-viewed ICP Limitations of the axially-viewed ICP Nebulizers Introduction Ultrasonic nebulizers Pneumatic nebulizers Glass concentric nebulizers Cross-flow nebulizers V-groove nebulizers Summary Optimization Optimization with Liberty Emission line selection Effect of resolution Predicting trends during optimization The origin of emission lines Plasma viewing height Effect of RF power Nebulizer pressure Gas flows Peristaltic pump speed 5-9 (iv) Publication date: 10/99

5 Liberty Series II Filters Effect of the analytical search window Effect of integration time Nebulization efficiency Nebulizer options Internal standardization Torch injector tube Standard and sample preparation Apparatus Water Reagents Blank Standard preparation Sample preparation Storage Samples Standards Calibration Multi-element standards Calibration graph Internal standards Analytical methodology Agriculture Soil Plants Fertilizer, animal feed Biology Blood, serum, urine Soft and hard tissues Animal samples Sampling Geology Minerals and rocks Rare earth elements Coal and coal ash Solid sample introduction Environment Air analysis Industrial air 7-28 Publication date: 10/99 (v)

6 Liberty Series II Determination of mercury Sediment Sewage sludge Particulates in rain water Foods and beverages Agricultural crops Milk Total diet samples Tea Citrus juice Beer Wine Metallurgy Ferrous Non-ferrous Organics Wear metals in used oils Crudes, refined fuels Organic solvents Water Water quality monitoring Sampling Sample storage Problems encountered with water analysis Water analysis Miscellaneous Radioactive materials Cements, ceramics and glasses Semiconductor industry Brines 7-59 References (vi) Publication date: 10/99

7 1 Introduction Introduction to ICP-AES to ICP-AES 1.1 General introduction to optical emission spectrometry It has long been known that distinctive colors are produced when compounds of certain metals are vaporized in flames. In a common high school experiment, salts of sodium produce a yellow light in the flame of a Bunsen burner, calcium salts color the flame brick-red, barium salts green, and so on. It was shown in the last century that the optical spectra of these colored flames contain lines or bands at wavelengths characteristic of the particular elements. With the development of quantum mechanics in the early part of the twentieth century, the lines and bands were shown to arise from electronic transitions in specific atoms or molecules. The yellow sodium flame, for example, results from a pair of lines at nm and nm, emitted by sodium atoms, while the characteristic colors of the calcium and barium flames arise principally from molecular emission bands of the monohydroxides. In the 1860s Bunsen and Kirchhoff established flame spectroscopy as a highly sensitive and specific means of identifying minute quantities of certain elements. Bunsen discovered rubidium and caesium in German mineral waters after observing lines in the flame spectra that could not be attributed to any known element. Several other new elements were discovered following observations of their characteristic spectral lines. Perhaps the most noteworthy of these was helium, whose existence was postulated in 1868 to account for an unidentified line in the spectrum of the sun, some twenty-seven years before the element was finally detected on the earth in gases from a uranium mineral. From these promising beginnings, optical emission spectrometry developed into a powerful method of chemical analysis, in which the concentration of a specific element in a sample is related to the intensity of lines in its optical spectrum. Flame emission spectrometry, for example, was developed by Lundegardh and others in the 1930s as a series of refinements of the flame spectroscopic methods used by Kirchhoff and Bunsen and their successors in the late nineteenth century. The sample solution is introduced into the flame as a fine spray, generated at a uniform and reproducible rate. The intensities of the characteristic spectral lines of the elements of interest are then Publication date: 10/99 1-1

8 Introduction to ICP-AES measured with a filter spectrometer or simple monochromator with electronic detection and readout. Flame spectrometry has been very widely used for the determination of sodium, potassium and several other elements by measuring the intensity of light emitted at a wavelength characteristic of an analyte element when samples are sprayed into the flame. The relationship between light intensity and analyte concentration is established by measuring the light emission when calibrating solutions of known concentration (often, though inaccurately, called standards ) are sprayed into the flame. Modern inductively coupled plasma atomic emission spectrometry (ICP-AES) relies on the same simple principles to determine minute amounts of a very wide range of elements, even in the presence of much greater quantities of other elements. An energetic plasma replaces the flame, and the simple monochromator and detector are replaced by a highly precise optical spectrometer. Figure 1 shows schematically the main components of an ICP-AES system. The main steps in analyzing samples by ICP-AES may be stated quite simply: 1. The elements to be determined are selected. 2. Solutions of the samples are prepared, using the conventional techniques of quantitative chemical analysis. 3. A set of calibrating solutions is prepared. Each solution contains accurately known concentrations of the analyte elements, and the range of concentrations for each element in the set is chosen to include the expected concentration of that element in the sample solutions. 4. The calibrating solutions and sample solutions are sprayed into the plasma and the intensities of appropriate emission lines are recorded. 5. Calibration graphs ( analytical working curves ) are prepared for each element from the emission intensities of the calibrating solutions. 6. The concentrations of the elements in each sample solution are determined from the calibration graphs. The concentrations in the original sample are then calculated from the measured concentrations of the elements in the sample solution and the known dilution factor. Automated spectrometers, with computers and programmable sample changers, are generally used. This can make steps 4 to 6 automatic, greatly simplifying the task of the operator and increasing the speed of analysis. 1-2 Publication date: 10/99

9 Introduction to ICP-AES 1.2 Historical background of the inductively coupled plasma A plasma is simply a gas whose properties are influenced by the presence of a significant, if often rather small, concentration of ions and electrons. These exist in approximately equal numbers over the volume of the plasma, so overall electrical neutrality is maintained. The generation of plasmas by inductive heating of gases at reduced pressure was first explored by Hittorf in 1884, and subsequently by J. J. Thomson. It was not until 1941 that G. I. Babat experimented with atmospheric pressure radio frequency (RF) inductively coupled plasmas (ICP) with a view to industrial applications (1). The first major application of an ICP was reported in 1961 by Reed (2,3), who used it to grow crystals of refractory (high melting point) compounds such as alumina. He found that refractory powders introduced into the central axis of the ICP were completely vaporized. Reed s ICP was similar to those used in modern spectrochemical analysis. Following the publication of Reed s work, Greenfield and his associates (4) in Britain and Wendt and Fassel (5) in the USA independently developed ICP systems for spectrochemical analysis. Greenfield, like Reed, used a torch consisting of three concentric quartz tubes with the innermost tube being used for injecting sample material directly into the hot core of the plasma. The spectrometer was set up to view light emitted from the somewhat cooler tail flame, downstream from the fireball of the plasma. This allowed measurements of analytical lines to be made against a relatively low-intensity background. The emitting analyte atoms in the tail flame were located in an optically-thin central channel, which favored the collection of emitted light by the spectrometer and promoted a linear relationship between light intensity and analyte concentration over a wide range of concentrations. The crucial importance of injecting sample material directly into the plasma along its central axis was not at first realized by other workers. In Wendt and Fassel s system, sample was swept around the plasma rather than through a central channel. Veillon and Margoshes (6) found severe inter-element effects and poor detection limits with this arrangement. Later, Fassel s group developed an ICP torch that permitted the injection of sample aerosol into a central channel through the plasma, and in 1969 Dickinson and Fassel (7) reported detection limits in the range mg/l for many elements - at least one hundred times better than had been achieved previously. Commercial ICP-AES instruments using the relatively low-power ICP torches developed by Fassel s group became available in 1974 and since then ICP-AES has become increasingly well established in laboratories the world over. The technique is now recognized by authorities such as the United States Environmental Protection Agency (USEPA) as an approved method for many trace element determinations. Publication date: 10/99 1-3

10 Introduction to ICP-AES 1.3 Instrumentation for ICP-AES transfer optics monochromator RF supply photo-multiplier plasma torch spray chamber nebulizer signal processing electronics flow gauge pressure gauge peristaltic pump computer Ar supply pressure regulator drain sample Figure 1.1 The components of an ICP atomic emission spectrometer Overview A typical arrangement for ICP-AES is shown above and consists of: 1. A sample introduction system 2. The ICP torch and its associated gas supplies 3. A radio-frequency generator 4. An optical spectrometer 5. Detectors and associated electronics 6. Computerized instrument control, data collection and analysis Sample introduction For most analyses the liquid sample is pumped into a pneumatic nebulizer, where it is converted to a fine aerosol by a stream of argon. After passing through a spray chamber to remove unfavorably large 1-4 Publication date: 10/99

11 Introduction to ICP-AES droplets, the aerosol enters the plasma through the inner, or injector, tube of the plasma torch. Sample introduction will be covered in more detail in a subsequent chapter Plasma torch The plasma is formed in a fused-silica torch consisting of three concentric tubes. The main argon stream (the plasma gas) is introduced tangentially between the intermediate tube and the outer tube. This confines the plasma and helps prevent the torch from overheating. A plasma is initiated by a brief spark discharge into this gas stream. In the induction coil region electrons from the spark provide a path for energy transfer between the coil and the argon, and a self-sustaining plasma is quickly established. Energy is transferred into the plasma most effectively in the outer regions of the plasma, nearest the coil. As a result, the lower part of the plasma assumes a doughnut shape. The argon stream carrying the sample aerosol, emerging from the innermost tube, passes into the central hole in the base of the plasma and forms a distinctive axial channel through the plasma. An intermediate gas flow, provided in the space between the inner tube and the intermediate tube, is useful in stabilizing the plasma under certain circumstances, such as when the analytical solutions contain organic solvents. A small flow of oxygen added to the argon intermediate gas is particularly useful in eliminating the problems of plasma instability, carbon build-up and spectral interference that can otherwise occur in the analysis of organic solutions Radio-frequency supply The basic circuit for a RF generator is simple, consisting of a capacitor and inductor in either a series or parallel configuration. This is called a tank circuit and is tuned to resonate at the desired operating frequency. Radio-frequency generators are grouped into two categories, depending on whether the resonant circuit acts as an amplifier or as an oscillator. In free running oscillators, the basic frequency is determined by the tank circuit but the actual frequency of oscillation is allowed to vary to accommodate changes in plasma impedance without loss of power. In crystal controlled oscillators a piezoelectric crystal is used to maintain a fixed operating frequency. This type of oscillator usually operates in the low milliwatt output range, as the crystal cannot carry large currents. The low-level oscillation is then amplified by a chain of amplifier stages to the kilowatt level required for the ICP. Publication date: 10/99 1-5

12 Introduction to ICP-AES In early crystal-controlled RF generators a change in plasma impedance resulted in a loss of power delivered to the plasma. Automatic power stabilization is now readily provided by automatic impedance-matching networks and automatic power-level control electronics. In modern practice, the power stability of both types of generator can be controlled to better than 0.1%. Operating frequencies are commonly either MHz or MHz, which are frequencies set aside in most countries for industrial use. The higher frequency is increasingly preferred for ICP-AES, because the plasma is less disturbed by the introduction of different materials into the central channel Optical spectrometer Some ICP-AES instruments detect and measure many analytical lines simultaneously, while others operate sequentially. Simultaneous instruments use some type of polychromator, with a detector for each analytical wavelength. These instruments are capable of impressive speed, but they are limited to measurements at pre-selected lines that are usually rather difficult to change. Simultaneous instruments are best suited for laboratories needing to determine the same set of elements routinely in the same sorts of samples. An optimized set of analytical wavelengths can then be chosen to suit both the elements and the samples. Sequential instruments offer complete freedom of choice in the selection of wavelengths, at the expense of speed of measurement. Sequential ICP-AES instruments are also generally smaller and less costly. Because of the benefits in cost, size and versatility, fast sequential spectrometers have become the more widely used optical spectrometers in ICP-AES. The optical elements common to both types of ICP-AES instrument are focusing optics, a grating spectrometer and a detector (many detectors in the simultaneous instruments). Focusing optics collect light from the plasma and focus it onto the entrance slit of the spectrometer. Lenses or mirrors can be used, but the latter are preferable because their focal lengths do not change with wavelength. The grating spectrometer produces a spectrum of the light collected from the plasma, so that light intensity can be measured at very precisely defined wavelengths. The relationship between the wavelength of the diffracted light and the spectrometer parameters is nλ=d(sinι ± sin α) where λ is the wavelength of the diffracted light, d is the spacing of the grooves on the diffraction grating, ι is the angle between the incident beam and the normal to the grating and α is the angle between the 1-6 Publication date: 10/99

13 Introduction to ICP-AES incident beam and the diffracted beam at the grating. The term n can have the values 0,1,2,3... and is called the order of diffraction. When the incident and refracted beams are on the same side of the normal, the sine terms in the equation are summed. If the beams are on opposite sides of the normal, they are subtracted. When the beams are on opposite sides of the normal, and the angles are equal, the right hand side of the equation becomes zero, and consequently n must also be zero. In this zero order position, incident light of all wavelengths is reflected without dispersion and the grating acts as a plane mirror. When n=1 the spectrum is called a first order spectrum, when n=2 the spectrum is second order and so on. For a given grating position, the wavelengths reaching the detector will be λ, λ/2, λ/3 and so on. If the spectrometer is set to measure a line at 420 nm in first order, it will also transmit a line at 210 nm in second order. Usually, optical filters called order sorting filters are placed in the light path to ensure that only radiation of the selected wavelength reaches the detector. A Czerny-Turner monochromator is commonly used in sequential ICP-AES spectrometers. Light comes into the monochromator through the entrance slit, and is reflected onto the diffraction grating by a collimating mirror. Light is reflected from the grating onto a second collimating mirror, which focuses the spectrum in the plane of the exit slit. The spectrum is scanned by moving it across the exit slit by rotating the diffraction grating. Scans across a very small range of wavelengths can be made by keeping the grating fixed and rotating a refractor plate positioned between either the exit slit or the entrance slit and the grating, close to the slit. For measurements of emission lines having wavelengths below 190 nm, the monochromator is evacuated and the optical path between the monochromator and the plasma is purged with argon, to minimize attenuation of the light by oxygen in the air. More details on monochromators can be found in Chapter Detectors and readout systems At present, the photometric detectors in ICP-AES are almost universally photomultiplier tubes. They show extremely high photometric sensitivity while at the same time the dark current (the current generated while the device is in total darkness) is very low. The maximum usable current is usually about 100 million times the dark current. There is much interest in applying the new generation of solid state optical detectors in ICP-AES (8), but costs are at present prohibitive. Publication date: 10/99 1-7

14 Introduction to ICP-AES Photomultiplier tubes insensitive to wavelengths much above 300 nm, known as solar blind photomultipliers, are useful in ICP-AES because they can be used to detect wavelengths below 300 nm in second and higher order spectra without interference from longer wavelength light in the first order spectrum Computers Modern ICP-AES instruments make extensive use of personal computers for the collection and analysis of data. Typical functions include spectral background correction, preparation of calibration graphs, and the calculation and statistical analysis of results. Computers also allow convenient access to databases of spectral emission lines, and are also used for instrument control. 1.4 The ICP as an atomization and excitation source for emission spectrometry Nature of the plasma Energy is transferred into the ICP by the interaction of ionized argon with the electromagnetic field of the induction coil. The positive argon ions and the electrons are both accelerated by the high-frequency field of the coil, but because of their far smaller mass the electrons are accelerated to much higher velocities than the ions and energy transfer into the plasma is dominated by processes involving electrons. When a spark is passed through argon in the presence of the RF field of the induction coil to initiate the plasma, some electrons in the spark gain sufficient energy to undergo inelastic collisions with argon atoms. An energetic electron colliding with an argon atom may transfer enough energy to ionize the argon, releasing another electron which is then available to participate in the transfer of energy from the coil to the gas. A steady-state plasma is produced when the rate at which electrons are released by ionizing collisions equals the rate at which they are lost by recombination. Ion-electron recombinations emit light, producing a continuous spectrum corresponding to the distribution of ion kinetic energies in the plasma. Unlike the energy levels of emitting atoms, electron kinetic energies are not quantized. The spectrum of the bluish-white light emitted from the ICP consists of the line spectrum of atomic argon superimposed on a continuous spectrum from ion-electron recombinations. 1-8 Publication date: 10/99

15 Introduction to ICP-AES Analyte emission in the ICP A rather complex chain of events must occur to convert the material dissolved in the sample solution into emitting atoms or ions in the plasma. First, the solution has to be nebulized, then the aerosol droplets must evaporate to form very small particles of dry solute. These must next be vaporized, and the resulting molecules dissociated into free atoms. Further energy transfer to the atoms is required to raise their electrons to the excited energy states which can then emit the characteristic line spectra. Atoms may also gain enough energy to ionize, and the resulting ions may in turn be raised to excited electronic states and produce characteristic line spectra. Spectral lines from singly-charged ions are prominent in the ICP spectra of many elements. Both ionic and atomic spectra are useful analytically. To obtain the highest analytical sensitivity and reliability, all factors affecting atom and ion production and excitation must be optimized and maintained as constant as possible. Clearly, any changes in these factors during the course of an analysis will contribute to errors in analytical measurements. Some of the variables affecting the production and excitation of atoms and ions in the ICP are discussed below. Nebulization Only a relatively small fraction of the sample solution is converted to the very fine aerosol that ultimately reaches the plasma. Most of the sample aerosol, containing unfavorably large droplets, is separated in a spray chamber and drained away to waste. The efficiency of the nebulizer in converting solution into usable aerosol is affected by such parameters as the viscosity and surface tension of the solution and the flow rates of the solution and the nebulizing gas, as well as by the design of the nebulizer. High levels of dissolved solids may increase the viscosity of the sample solution and reduce the efficiency of nebulization, thus ultimately reducing the number of atoms and ions formed in the plasma. On the other hand, solutions in organic solvents frequently have lower viscosities than aqueous solutions and show enhanced efficiency of nebulization. Desolvation Desolvation takes an appreciable fraction of the few milliseconds available for the sequence of events leading up to the emission of light by analyte atoms and ions. Large droplets take longer to desolvate, and may pass through the hottest region of the plasma before the steps of vaporization, atomization, and ionization are completed. The droplet size distribution in the aerosol reaching the plasma depends primarily on the design of the nebulizer and spray chamber, and ideally should be around µm. The flow-rate of aerosol into the Publication date: 10/99 1-9

16 Introduction to ICP-AES plasma affects the residence time of droplets in the plasma and thus the degree of desolvation. The plasma power and the flow rate of argon in the outer tube affect the size and temperature of the plasma and so may also influence desolvation. Vaporization Vaporization of the solid particles left after desolvation is affected by many of the same parameters influencing desolvation. Anything that increases the time particles spend in the plasma, or increases the temperature and size of the plasma, will improve the efficiency of vaporization. In addition, vaporization is influenced by the nature of the vaporizing particle. For instance, a particle of aluminium oxide vaporizes more slowly than a particle of sodium chloride of the same size. If the analyte is contained in a particle of aluminium oxide, it will vaporize to a smaller extent than it would in a particle of sodium chloride. This effect is important in the analysis of complex samples, where the matrix may alter the processes of vaporization and atom and ion production and is then said to interfere with the determination of the analyte. Fortunately, the high-temperature ICP provides an environment favorable for both desolvation and vaporization, and such interferences are minimized. Dissociation and atomization In some cases vaporization may lead directly to atomization. More commonly, evaporating solid particles form molecules which then require further dissociation to free atoms. Many oxides and hydroxides do not dissociate readily even in an intensely hot plasma, and often re-associate in the cooler regions. Under some circumstances interferences may be negligible at low observation heights in the plasma but reappear at greater heights, where the plasma temperature is lower. Excitation and ionization The processes which excite free atoms to higher energy states, to ions and then to higher energy states of the ion, are not at present fully understood. The most plausible mechanisms are similar to those described in section for the electron-argon collisions that sustain the plasma: e - + M M * + e - e - + M + M + * + e - where M denotes an atom, M + an ion, and M * and M + * an atom and ion respectively in an excited energy state. These reactions can proceed in either direction. In the forward reaction, the electron must have at least the transition energy involved. Ionization occurs by the following reaction: e - + M M + + 2e Publication date: 10/99

17 Introduction to ICP-AES which normally leads to ions in their lowest-energy (ground) state. In this reaction the electron energy must be at least equal to, or greater than, the ionization energy of the atom. The reverse reaction involves the three-body recombination: M + + 2e - e - + M * and provides both a mechanism for ion removal and another route by which excited-state atoms can be produced. In plasmas (like the ICP) that are not highly ionized this three-body reaction is relatively rare, and removal of ions by radiative recombination is more important: M + + e - M * + hν cont which also produces an excited atom and a photon of continuous radiation (hν cont ). Analyte line emission occurs when excited-state atoms and ions are de-excited with release of a photon of radiation: M * M + hν line M + * M + + hν line These reactions are thought to be the most important of those responsible for the characteristic line spectra observed in ICP-AES Zones in the ICP The various processes occurring in the ICP lead to several rather distinct zones, with very different spectrochemical characteristics. A system of nomenclature for these zones was proposed by Koirtyohann, Jones and Yates (9). Familiarity with these zones is very helpful in optimizing an ICP and in diagnosing faults. The most obvious zone in the ICP is the intensely luminous region, called the induction zone, where energy from the induction coil is coupled into the plasma. This zone, the temperature of which may reach K, is a source of background in the analytical spectra. The aerosol injector gas passes axially through the induction zone, forming a darker central channel in which the processes leading to emission of light by analyte atoms take place. The temperature in the hottest part of the central channel can exceed K in a typical analytical ICP - much cooler than the surrounding induction zone, but very much hotter than the flames used in flame spectrochemical analysis, which do not exceed K. When solutions of certain elements are introduced, the emission of visible light allows the various zones in the axial region of the ICP to be viewed directly. For example, when a solution of 1 g/l sodium is sprayed, a bright yellow bullet-shaped region is seen. This is called the initial radiation zone (IRZ). The intense yellow emission of sodium atoms in the IRZ shows that sodium salts have been dissociated into free atoms. Publication date: 10/

18 Introduction to ICP-AES Directly below the IRZ is the pre-heating zone (PHZ) where desolvation, evaporation and dissociation take place. Just above the IRZ, the yellow sodium emission is no longer visible. The temperatures here are so high that sodium is almost completely ionized. The majority of elements emit most intensely in this region, and it is therefore called the normal analytical zone (NAZ). Above the NAZ, the yellow sodium light becomes visible again as the gas stream cools. This region, extending several centimetres above the top of the induction zone, is called the tail flame. As the flow rate of nebulizer gas is increased (by increasing the nebulizer pressure) the PHZ, the IRZ and the NAZ move higher with respect to the induction zone. The same effect is seen as the power is decreased. A better view of the axial zones is obtained when a solution of 1 g/l yttrium is sprayed. The IRZ is then red, from molecular emission bands of yttrium monoxide, while the NAZ shows up in blue from the ionic lines of yttrium. The viewing position should be set to view the NAZ. Red emission from YO appears again in the tail flame. It is very instructive to observe the appearance of the plasma when the yttrium solution is sprayed, and then to vary the power levels and the nebulizer gas pressures. Remembering that the blue region corresponds to the region giving useful analytical signals for most elements, while the red corresponds to incomplete atomization, the operator can quickly develop an appreciation of the effects of power and nebulizer gas flow-rate on the spectrochemical characteristics of the plasma. The appearance of the plasma when a yttrium solution is sprayed is an extremely useful test to confirm that all is well with the plasma, or to give an indication of what may be wrong. If yttrium solutions are not available, a scandium solution can be used instead. The NAZ is then a deep indigo color, while the IRZ is red. 1.5 Comparison of the analytical performance of ICP-AES and some other techniques Comparison of ICP-AES and flame-aas Detection limits The detection limit is the concentration of analyte corresponding to the smallest signal that can be distinguished from random fluctuations in the background according to a statistical measure of significance (generally at the 3σ level, where σ is the standard deviation of the background emission signal). The detection limits with ICP-AES generally lie in the range mg/l, and for the vast majority of elements are from 1 to 50 times better than with flame-aas. They are 1-12 Publication date: 10/99

19 Introduction to ICP-AES better still for elements such as B, Ge, Hf, Nb, Re, Ta, Th, U, W, Zr and most of the lanthanides. Flame-AAS determination of these elements is limited by the formation of refractory oxides or carbides that resist dissociation to free atoms. This problem is largely overcome in the ICP by the much higher temperature and the inert argon environment. On the other hand, flame-aas shows better limits of detection than does ICP-AES for the alkali metals. This is because the alkali metals are almost completely ionized in the ICP, the resulting ions having extremely stable electronic configurations and consequently not producing intense emission spectra at ICP temperatures. Detection limits for many of the first-row transition metals (Co, Cr, Cu, Fe, Mn, Ni) and also for Ag, Au, Bi, Ga, In, Mg, Pb, Sb, Te and Tl are similar for both techniques. Apart from the alkali metals, those elements which can be determined by flame-aas in an air-acetylene flame show similar detection limits in flame-aas and ICP-AES. Those elements whose determination by flame-aas requires the nitrous oxide/acetylene flame have much better detection limits by ICP-AES than by flame-aas. In all, ICP-AES is suitable for the determination of at least 72 elements. The non-metals I, P and S are best determined at wavelengths requiring a vacuum-ultraviolet spectrometer. The alkali metals, particularly Rb and Cs, are better determined by either flame- AAS or flame-aes than by ICP-AES. Analytical working range The linear dynamic range of an analytical technique for a given element corresponds to the range of concentrations for which the calibration graph, i.e. the plot of signal against concentration, is linear. Bending of the graph at high concentrations increases the error in calculation of concentration. For optimum precision, perform determinations on concentrations ranging from not less than about 10 times the detection limit to not more than that where the calibration graph begins to depart significantly from linearity. Ideally an analytical technique should have a large linear dynamic range so that major, minor and trace constituents can be determined in the same sample without the need for dilution. This is particularly desirable in a multi-element technique. ICP-AES has excellent linearity up to concentrations of times the detection limit, but flame-aas shows nearly linear absorbance response up to concentrations of only times this limit. Thus the determination of major and minor constituents by flame-aas often requires either a time-consuming dilution step, with the attendant risk of contamination or dilution error, or the selection of a less sensitive line, if one is available. Publication date: 10/

20 Introduction to ICP-AES Interferences As mentioned earlier, the ICP is better able to dissociate refractory compounds than is the flame. It is not surprising, therefore, that the ICP is far less susceptible than the flame to volatilization interferences (often called chemical interferences ). Frequently in flame-aas the presence of one or more chemical additives is necessary to suppress volatilization and ionization interferences, while complex samples may require solvent extraction or ion exchange to remove interference caused by major constituents. This not only increases the complexity of the analysis but decreases its speed. While such interferences are not entirely absent in the ICP, their magnitude is many times less than in flame-aas; furthermore the operating parameters of the ICP can usually be optimized to minimize the effects of volatilization interferences. Spectral interferences, however, which are very uncommon in flame-aas, can be a significant problem in ICP-AES. The spectral selectivity of AAS is achieved with a low resolution monochromator isolating the appropriate resonance line from a hollow-cathode lamp. In ICP-AES, a very high resolution monochromator is needed to keep spectral interferences at manageable levels Comparison of ICP-AES with graphite furnace-aas Graphite furnace-aas (GF-AAS) provides excellent detection limits, up to times better than either ICP-AES or flame-aas (10). Unfortunately, volatilization interferences tend to be troublesome in GF-AAS, and the methods for their alleviation are somewhat complex. In addition, GF-AAS, normally a single-element technique, is relatively slow. Recent developments have made possible multi-element determination with GF-AAS, either with line sources or with continuum sources (11). However, there are some difficulties in arriving at a set of multi-element operating parameters. Continuum source multi-element AAS is further limited by deficiencies in continuum light sources, which make the limits of detection for elements with absorption lines near 200 nm, such as Se, As and Zn, poor in comparison with the corresponding detection limits with line sources. The chief advantages of GF-AAS are excellent detection limits and relatively low cost. However, the technique is applicable only to about 50 elements, fewer than can be determined by ICP-AES. Many elements are difficult to determine by GF-AAS because of the formation of refractory carbides. GF-AAS and ICP-AES are best viewed as complementary, rather than competitive, techniques Publication date: 10/99

21 Introduction to ICP-AES Comparison of ICP-AES with other plasma sources for atomic emission Apart from the ICP, some other types of plasma are used for optical emission spectrochemical analysis. The more important of these are the microwave-induced plasma (MIP) and the direct-current plasma (DCP). The MIP is sustained by microwave energy (generally at 2.45 GHz) within a resonant cavity or similar device. The principal advantages of the MIP are the relatively low cost of microwave generators and the low power and gas-flow requirements (12,13). Another advantage of the MIP is its efficient excitation of the optical emission spectra of nonmetals. Inter-element effects are common in the MIP and consequently the most effective application of this plasma has been as an element-selective detector for chromatography (14), where the analyte is separated from potential interferents. The DCP is typically maintained in argon by an arrangement of three electrodes, and the plasma itself has a shape similar to an inverted Y (15). It has similar physical properties and detection limits to the ICP, but is more subject to solute-volatilization and ionization interferences. The principal advantage of the DCP is that it less easily becomes clogged by sample solutions containing high concentrations of dissolved solids. The DCP is very rugged and is well suited for qualitative analysis (16). For quantitative work, it is often necessary to use matrix matched calibrating solutions or to add appropriate chemicals to samples and calibrating solutions to suppress interferences. The ICP has been more widely accepted and applied than either the MIP or DCP, principally because of its greater freedom from interference effects Comparison of ICP-AES with ICP-mass spectrometry Inductively coupled plasma mass spectrometry (ICP-MS) offers remarkably good detection limits and the capability of determining isotopic ratios with great speed and convenience (17,18). Detection limits in the range mg/l are obtainable with the same sample introduction systems used in ICP-AES. Significantly, most elements show similar detection limits, whereas detection limits for ICP-AES vary much more from one element to another. Because they both use the ICP source, ICP-AES and ICP-MS show similar solute volatilization and ionization interferences, which are usually quite small. However, ICP-MS suffers from some additional Publication date: 10/

22 Introduction to ICP-AES problems because of the possibility of interactions in the vacuum interface and because a cool surface susceptible to condensation and deposition is introduced into the plasma. For this reason the level of dissolved solids in the analytical solutions should be kept relatively low. Precision is usually rather worse in ICP-MS than in ICP-AES, but this can be compensated for by the use of an internal standard, or by isotope dilution methods, which enable the achievement of precision and accuracy similar to that of ICP-AES. The principal disadvantage of ICP-MS is the high cost of instrumentation. 1.6 Direct analysis of solid samples by ICP-AES Overview of techniques for the direct analysis of solids So far we have assumed that the sample can be presented to the instrument as a solution, but in many cases it is desirable to analyze solid samples directly. Solid sampling techniques are applicable to reasonably homogeneous samples; for non-homogeneous samples, dissolution followed by solution sample introduction is preferable to ensure more representative analysis. Classical methods of DC arc and high-voltage spark optical emission spectrometry are still used in many laboratories for the analysis of solids, but not often for solutions. However, precision and accuracy are poor by modern standards. A recent innovation is the use of glowdischarge spectrometry for conductive solids. The glow-discharge source uses an electrical discharge in argon at reduced pressure to sputter material from the sample surface by bombardment with argon ions. The glow-discharge shows better linear dynamic range and greater freedom from inter-element effects than does the high-voltage spark, thus allowing in many cases the use of a single analytical working curve for a range of materials of differing composition. X-ray fluorescence (XRF) spectrometry is commonly used for the analysis of various types of solid sample. This technique measures radiation associated with the transitions of inner-shell electrons, and these transitions are unaffected by the nature of the chemical bonding. In ICP-AES the characteristic optical spectra are produced by transitions of outer-shell electrons, and any chemical bonds must be broken before these transitions can occur. In this respect XRF spectrometry has an important advantage over ICP-AES for the direct analysis of solids. XRF spectrometry gives better precision than ICP-AES for major constituents but suffers from 1-16 Publication date: 10/99

23 Introduction to ICP-AES matrix interferences, so that it is generally necessary to match samples and calibrating materials closely. The speed and convenience of solid-sample handling accounts for the wide acceptance of XRF spectrometry in such applications. XRF spectrometry can also be applied to the analysis of solutions, but detection limits are around 1000 times worse than for ICP-AES (19) Direct introduction of solid samples into the ICP ICP-AES is usually regarded as a solution analysis technique, but methods have been developed for the direct analysis of solids (20,21). These include: 1. Direct aspiration of powdered samples into the plasma (5) 2. Fluidized-bed introduction of powders (22) 3. High-voltage spark nebulization (23) 4. Laser ablation (24) 5. Vaporization of microsamples from a graphite furnace (25) 6. Slurry introduction of powders (26,27) 7. Direct insertion on a graphite rod (28) The direct aspiration of powders is limited by the segregation of particles of different sizes and densities. Fluidized-bed introduction is a more promising approach. A fluidized bed is a gas-solid mixture with many of the properties of a liquid. It can be maintained by the continuous input of mechanical energy by flowing gas or by vibration. Introduction of samples as slurries (suspensions of a finely-divided powder in a liquid) offers considerable promise for the direct analysis of solids by the ICP. Nebulizers that are used for solutions can be used for slurries, provided that they are resistant to clogging. Other advantages of slurry introduction are that calibration graphs can conveniently be prepared using solutions and that analyte addition methods are readily applied. Possible problems with slurry introduction, as with other procedures for the introduction of powders, include the variable transport properties of different-sized particles and the possibility of incomplete evaporation and excitation of particles in the plasma. Finally, a procedure that has received much attention for the direct analysis of solid samples is the insertion of samples directly into the plasma in a small cup on the end of a graphite rod, either through the ICP torch or, less frequently, from the side. A complication with direct insertion is that different substances vaporize at different rates. Other Publication date: 10/

24 Introduction to ICP-AES problems are that the precision of the technique is critically dependent on the exact positioning of the sample cup during successive insertions, and that the presence of the graphite rod tends to reduce the temperature of the plasma, which may lead to volatilization interferences. 1.7 Analysis of gaseous samples by ICP-AES This subject is reviewed very comprehensively by Caruso, Wolnik and Fricke (29). The analysis of gases such as air, combustion gases and industrial exhausts by direct introduction of samples into the injector gas stream is possible in principle, but does not appear to have found wide application. There has been more interest in ICP-AES as an element-selective detector for gas chromatography, but only a few practical applications have been published (29). By far the most important application of gaseous sample introduction in practical analysis is hydride generation. Samples are reacted with NaBH 4 to generate gaseous hydrides of As, Bi, Ge, Pb, Sb, Se, Sn, and Te. Under favorable conditions, efficiency of transfer of analyte from the sample to the plasma can approach 100% for most of these elements, in contrast to the 1 or 2% typical of solution nebulization. Sensitivity is greatly enhanced, and spectral interferences are reduced because the analyte is separated from major concomitants. Conditions for hydride generation differ from element to element, so compromise conditions for multi-element determinations involve some loss of sensitivity. Despite this, hydride generation-icp-aes has found wide application in the determination of these elements in real analytical samples (30). Several devices for hydride generation are available commercially. Varian s VGA-77P vapor generation accessory is designed for use with the ICP. A gas-liquid separator, having a pumped drain, is connected directly to the sample inlet of a concentric glass nebulizer. The nebulizer provides an easily controlled flow of injection gas, that sweeps the small flow of gas from the VGA- 77P into the plasma. 1.8 Sources of further information on ICP spectrochemical analysis Books A handbook of inductively coupled plasma spectrometry. M. Thompson and J.N. Walsh, Blackie and Son Ltd, Glasgow (1983). Inductively coupled plasmas in analytical atomic spectrometry. A. Montaser and D. W. Golightly, Eds., 2nd edition, VCH Inc., New York (1992) Publication date: 10/99

25 Introduction to ICP-AES Inductively coupled plasma emission spectroscopy. Part 1: methodology, instrumentation and performance. P. W. J. M. Boumans, Ed., John Wiley and Sons Inc., New York (1987). Inductively coupled plasma emission spectroscopy. Part 2: applications and fundamentals. P. W. J. M. Boumans, Ed., John Wiley and Sons Inc., New York (1987). Introduction to inductively coupled plasma atomic emission spectrometry. G. L. Moore, Elsevier, Amsterdam (1989) Periodicals The following periodical is devoted exclusively to ICP spectrochemical analysis: ICP Information Newsletter, Department of Chemistry, GRC Towers, University of Massachusetts, Amherst, Massachusetts, , U.S.A. Original papers on aspects of ICP spectrochemistry are published in a number of journals, including: Journal of Analytical Atomic Spectrometry, Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge, CB4 4WF, U.K. Spectrochimica Acta, Part B, Pergamon Press Inc., Maxwell House, Fairview Park, Elmsford, NY 10523, U.S.A., Pergamon Press plc, Headington Hill Hall, Oxford OX3 0BW, U.K. Applied Spectroscopy, Society for Applied Spectroscopy, P. O. Box 1438, Frederick, MD 21701, U.S.A. Analytical Chemistry, American Chemical Society, th St., N. W., Washington, D. C., 20036, U.S.A. Publication date: 10/

26 Introduction to ICP-AES This page is intentionally left blank Publication date: 10/99

27 2 Operating Principles Operating principles 2.1 Introduction As indicated in chapter 1, analysis of a sample by ICP-AES is, in principle, extremely simple. Typically, the sample is taken into solution and diluted by a known factor. Next, the relationship between analyte concentration and instrument response at the wavelength of a spectral line characteristic of the analyte is established with a set of calibrating solutions, on the assumption that this relationship will be the same for the sample. The instrument response for the sample solution is then measured, and finally the concentration of analyte in the sample is calculated. Any factor that causes the relationship between instrument response and analyte concentration for the sample solution to differ from that for the calibrating solutions will lead to an incorrect result, and is called an interference. The identification and control of interferences is one of the most important aspects of the practical application of any analytical technique. Other important considerations are the selection of operating parameters to give the sensitivity and precision necessary for the analysis, and the choice of the sample preparation procedure. It is not intended here to give details of specific analytical methods, but rather to outline the more important factors that need to be taken into account when developing a method for analyzing a new type of sample by ICP-AES. The analytical performance of ICP-AES has been treated in a number of publications. The review by Thompson (31) is a first-rate introduction to the subject. 2.2 Checking the torch and nebulizer Before beginning any analytical work, it is a good idea to check that the torch and the nebulizer are performing correctly. The plasma should be located centrally in the torch and should look symmetrical. Any yellow or orange coloration of the plasma may indicate that the torch is overheating. If such coloration is seen, the plasma should be switched off and the position of the torch in the induction coil checked. Publication date: 10/99 2-1

28 Operating principles If it is not located centrally in the coil, or if it is too high, the torch is very likely to overheat and may be permanently damaged. If the position of the torch seems to be satisfactory, and there is still evidence of overheating, the plasma gas flow rate is probably too low for the power level being used. A good test of the nebulizer and plasma torch is to nebulize a solution of 1 g/l yttrium to reveal the various plasma zones, as described in chapter 1. The normal analytical zone (NAZ), which is blue, should extend above the outer quartz tube of the torch. This zone should be well defined and stable, without any perceptible flickering. It does not matter if there is some movement of the tail flame above the NAZ. Normally, the red initial radiation zone (IRZ) below the NAZ should be visible between the turns of the induction coil. If it extends above the top of the torch, the nebulizer gas flow is too high for the plasma power level being used. Failure to achieve a correctly positioned IRZ at powers around 1000 W and the recommended nebulizer pressure may indicate that the gas orifice in the nebulizer has been damaged and is passing too much gas. If this is so, the nebulizer must be replaced. Sometimes, when the yttrium solution is nebulized, the characteristic red color of the IRZ can be seen on the outside of the lower part of the plasma. This indicates that some of the aerosol, instead of passing up through the plasma, is going around it because the torch is positioned too low in the induction coil. Raising the torch slightly will cure this problem. Care must be taken not to raise the torch too high, because the plasma may then be too low, causing rapid overheating of the torch and permanent damage. 2.3 Selection of analyte lines It might be thought that the selection of the spectral line for the determination of an element by ICP-AES is simply a matter of choosing the most intense emission line of that element from tables of spectral lines. In practice, however, there are several other factors to be considered. Firstly, is the line detectable with the spectrometer to be used for the analysis? The wavelengths of the most intense emission lines of some elements are below 190 nm, and a vacuum monochromator is needed for their measurement. Secondly, will lines emitted by other elements in the samples, or by species present in the ICP, overlap with the chosen line? Thirdly, is the analyte element likely to be present in such high concentrations that a less intense line is more appropriate? The possibility of overlap of the analytical line with some other spectral feature arising from the other elements in the sample, or from the ICP, is probably the most difficult problem in the development of ICP-AES methods. Fortunately, a great deal of information has been 2-2 Publication date: 10/99

29 Operating principles accumulated over the years to guide the analyst in the selection of analyte lines. A comprehensive review by Zander of line selection and spectral interferences gives a detailed discussion of the subject, with plenty of examples, together with a list of useful references (32). While reference to the literature and to tables of ICP-AES emission lines is a useful guide, the analyst will need to carry out some tests to confirm that the chosen analyte line is appropriate for the task at hand. The line should be scanned while a solution containing a suitable concentration of the analyte element is nebulized. This test solution should be made from a spectroscopically pure compound and should contain as few other elements as possible; its concentration should be in the range expected for the analyte element in the sample solutions. This test will show whether the spectral line is sufficiently sensitive for the analysis. The viewing height must be optimized; it is usually better to optimize for best signal-to-background ratio rather than for the most intense signal. The nebulizer pressure, plasma power, and gas flows can also be optimized, but optimization then becomes very timeconsuming. It is usually satisfactory to leave these parameters fixed at compromise conditions appropriate for the types of samples being analyzed and to restrict the optimization to the viewing height. Scanning the spectrum around the analytical line while nebulizing pure water (or other appropriate solvent) can be very informative. It will reveal, for example, any peaks from molecular emission bands which might compromise the identification and measurement of the analyte line. If a potentially interfering spectral feature is found, it may be possible to remove it by altering the plasma power or the viewing height. Should this not be effective, it may be necessary to choose another analyte line. The next step is to examine the spectrum around the chosen line when a reagent blank is nebulized, to detect any interfering spectral features introduced by materials present in the blank. If a peak is found at the wavelength of the analyte line, it may indicate contamination of the blank with the analyte element. It may also originate from some other element present in the blank. If it is a result of contamination of the blank with analyte, lines will also be seen at the wavelengths of all other analyte spectral lines of similar sensitivity. If the line is due to some other element, it may be possible to avoid the problem by finding another analyte line. It is always best if the blank does not show lines close to the analyte line as, while it is possible to correct for such lines in the blank subtraction, the precision of the analysis is inevitably reduced. Peaks may appear in the background scan due to molecular band emission such as from OH, NO, as well as from argon emission lines. Refer to the examples shown in chapter 5 Optimization. Publication date: 10/99 2-3

30 Operating principles In most analyses, spectral interferences arise neither from the plasma nor from the reagent blank but from other elements present in the samples. To obtain accurate analytical results, it is essential to avoid these interferences or to correct for them. Obviously, the most likely sources of spectral interferences will be those elements present in the highest concentrations and those having a large number of intense lines in their spectra. If the major elements in the sample have line-rich spectra, the potential for spectral interferences is very great. The analyst should be aware of the major elements likely to be present in the samples. In addition, it is very useful to scan the spectrum of a sample solution for the major emission lines of elements that may introduce spectral interferences. Once these elements have been identified, the more severe spectral interferences can often be pinpointed with the aid of tables of spectral lines. This, however, is only the first step. It is always a good idea to examine the spectrum of each potentially interfering element in the ICP, under the same conditions to be used in the analysis. Spectroscopically pure solutions of the relevant interfering elements are required for this work. Unless pure solutions are used, the analyte element may be present as a trace impurity and its emission line may easily be mistaken for a line from the interfering element. Such contamination can sometimes be confirmed by checking for the presence of the other characteristic spectral lines of the analyte at their appropriate wavelengths. If there is no spectral line at or near the wavelength of the proposed analyte line, it can safely be assumed that the potentially interfering element will not cause spectral interference. If an interfering line is detected, it is always better to look for an alternative, interference-free analyte line than to attempt to correct for the interference Correction of spectral interferences As a last resort, the effects of a spectral interference can be minimized by applying a correction factor. The relationship between the concentration of the interfering element and the instrument response at the analyte wavelength has to be found by making measurements at that wavelength on a series of spectroscopically pure solutions of the interfering element, over the range of concentrations likely to occur in the sample solutions. During the analysis of the samples the concentration of the interfering element, as well as that of the analyte, must be measured. The correction factor is then used to calculate the apparent concentration of analyte corresponding to the spectral interference from the measured concentration of the interfering element, and this is subtracted from the measured concentration of analyte. When a scanning monochromator is used, correcting spectral interferences in this way substantially increases the analysis time. 2-4 Publication date: 10/99

31 Operating principles 2.4 Background correction A good check for spectral interference is to analyze a typical sample for each element at several analyte wavelengths, using single-element calibration solutions. If the results differ significantly from line to line, the line which indicates the lowest concentration of analyte should be chosen. Spectral interferences can only increase the instrument response, so any line suffering spectral interference will indicate an erroneously high concentration of the analyte element. When a spectral line is selected, the instrument software will choose the recommended spectral order and filter. It is possible for the analyst to select different spectral orders and different filters, and this can sometimes be useful. For example, in determining trace levels of metals in fresh water, it may be possible to achieve better light transmission at some wavelengths, and consequently somewhat improved limits of detection, by not using an optical filter. This is so because interferences from radiation in other spectral orders may not be a problem with samples containing very low concentrations of elements emitting spectral lines in the ICP. Spectral emission lines in ICP-AES are always superimposed on a continuous background consisting of detector dark current, and any spectral continuum that may be emitted by the ICP. The level of continuum background radiation varies considerably with wavelength, with plasma power, and with viewing height. Ideally, the background should be the same for all analytical solutions, as it will then be accounted for by the calibration routine and will require no further consideration. However, sample constituents often cause the background to change from one sample to another. It is then necessary, with each solution, to estimate the intensity of the background at the wavelength of the analytical line, and correct the measured intensity of this line for the contribution of the background. Changes in background from one solution to another can be caused by a number of factors. Intense lines of concomitant elements can be quite broad, with extended wings, causing changes in background at wavelengths some distance away, and of course the effect is worse when the analyte line is close to the concomitant line. Very intense lines from concomitants can cause background changes at distant wavelengths by stray light effects. These are minimized by careful design of the optical spectrometer and by the use of order-separating filters. Other factors contributing to background changes are continuous spectra from ion-electron recombinations, and molecular band spectra. The latter are particularly troublesome because they can be highly structured, and if so it may not be possible to estimate their intensity at the wavelength of the analyte line from measurements at adjacent wavelengths. Publication date: 10/99 2-5

32 Operating principles 2.5 Non-spectral interferences In developing an analytical method, it is important to examine a range of typical samples for background changes and to choose an appropriate background correction technique. The ability to scan the region of the spectrum around the analytical wavelength with the aid of a refractor plate is extremely useful in identifying the need for background correction, and makes possible several different ways of estimating the background intensity at the analyte line. If the intensity of the background is essentially constant over a short range of wavelengths centered on the analyte line, background correction may require only a single additional measurement on one side of the line. In other cases, a better estimate can be obtained by making measurements on each side of the line and calculating the background at the analyte line on the assumption that the intensity of the background changes linearly with wavelength. When the background changes non-linearly with wavelength, it may still be possible to estimate the background under the analytical line with appropriate computer software. Early studies on the ICP showed that it was remarkably free from interferences (33). While the ICP is certainly superior to most, if not all, other spectroscopic sources in this respect, subsequent studies have shown that interferences do exist in the ICP, and can at times be quite severe. Spectral interference is an obvious example. There are, in addition, other types of interference that can cause problems when real samples are analyzed by ICP-AES. One of these, shared by ICP and the various flame techniques, is transport interference Transport interferences If the physical properties of the sample solutions differ significantly from those of the calibrating solutions, the rate of generation and transport of appropriately sized aerosol droplets to the plasma may differ. This will obviously lead to inaccurate analytical results. Transport interferences are likely to occur when the samples contain constituents that change the viscosity, density or surface tension of the solutions (34,35). These interferences can be identified by carrying out recovery tests on typical samples. The concentration of analyte is measured both in a sample solution and in a sample solution to which a known amount of analyte has been added. The difference of these two concentrations is expressed as a percentage of that expected from the amount of analyte added; a recovery of close to 100% indicates that interferences are insignificant. 2-6 Publication date: 10/99

33 Operating principles There are several approaches to overcoming transport interferences. It may be possible to match the physical properties of the calibration solutions with those of the sample solution by adding pure chemicals to the calibration solutions to equal the concentration of these substances in the sample. This technique, called matrix matching, is feasible only if the major components affecting the properties of the sample solution are known and are available free from contamination with analyte elements. Another approach is the analyte addition method (frequently, though inaccurately, called the standard addition method ). The sample solution is divided into at least three aliquots, to all but one of which known increasing amounts of analyte are added, and the solutions are made up to the same volume. They are then analyzed, and the relationship between instrument response and concentration of added analyte is established, either as a graph or as a mathematical function in software. Extrapolation of the graph to the instrument response corresponding to that for a blank solution allows the concentration of analyte in the sample solution to be calculated. Once such a calibration has been done for one sample, it can be used in the analysis of similar samples. The analyte addition method, like the ordinary calibration procedure, demands careful attention to background correction and selection of interference-free analyte lines. The aim of the method is to calibrate the instrument with solutions having chemical and physical properties closely matched to those of the sample. Contributions to the instrument response from background or from other emission lines are indistinguishable from analyte emission, and will lead to incorrect results. Sample transport interference can also be corrected by internal standardization. The same concentration of a carefully selected reference element is added to sample solutions and calibration solutions, and all measurements of the instrument response at analyte lines are calculated relative to the instrument response at a wavelength characteristic of the reference element. The reference element must be chosen to have similar spectrochemical characteristics to the analyte element (36, 37). Spectral interferences and background changes have to be considered for the reference element, as well as for all the analyte elements. The use of internal standardization, not only to correct for transport interferences but also to achieve improvement in analytical precision, has been carefully studied in recent years (38,39). However, with sequential instruments internal standardization is generally unattractive because it is impossible to make truly simultaneous measurements of analyte and reference elements, and because the extra time needed to make the necessary measurements on the reference element in each analytical solution inevitably reduces the speed of analysis. Publication date: 10/99 2-7

34 Operating principles Solute volatilization interferences Interferences caused by suppression of the volatilization of analyte elements by the formation of refractory compounds are well known in flame spectrochemical analysis. Such interferences are much less severe in the ICP because of its higher temperature; if care is taken in the choice of operating parameters, the interferences can be reduced to negligible levels (33,40). However, with inappropriately chosen operating conditions, solute volatilization interferences can be quite severe. It is instructive to examine plots of signal intensity versus viewing height for a pair of solutions, one of which contains only the analyte element and the other the analyte element at the same concentration together with an excess of some element likely to cause solute volatilization interference. Typically, such plots show significant interference low in the plasma, reducing to negligible levels in the normal analytical zone. As the plasma power is decreased, the region of significant interference moves higher in the plasma. A similar effect is seen as the injector gas flow is increased. These observations are easily understood on the assumption that the time taken to volatilize a particle varies according to its chemical composition and physical size, and unless sufficient time is allowed for this process volatilization interferences will occur. In practice, a suitable combination of plasma power, injector gas flow rate and viewing height will usually reduce these interferences to acceptable levels. Finding the right combination of operating parameters requires careful measurement of test solutions containing known concentrations of analyte and concentrations of potential interferents covering the range likely to occur in the sample solutions. Any residual interferences are best overcome by matrix matching the calibration solutions with the samples. Experience with solute volatilization interferences in flame spectrometry (41) suggests that the analyte addition method may be inappropriate. Unless the amount of interferent is sufficient to combine with both the analyte from the sample and with any added analyte, the two solutions will form different chemical compounds as the aerosol evaporates, and the assumptions underlying the analyte addition method will not be valid. For the same reason, solute volatilization interferences may not be revealed by recovery tests Excitation interferences Even when there is no spectral interference, and no reason to suspect transport or solute volatilization interference, the intensity of analyte lines for a given concentration of analyte can sometimes be altered by the presence of relatively high concentrations of other substances in the sample. It is assumed that these interferences are caused by 2-8 Publication date: 10/99

35 Operating principles changes in the excitation conditions in the plasma; the effect is variable and is not fully understood. Interferences of this type have been reported to be caused by a range of substances, including salts of the alkali metals (42,43), calcium (35,43), and magnesium (43), by dilute mineral acids (43,44), organic acids (45,46), and organic solvents (45). They are best overcome by matrix matching of the calibration solutions to the sample solutions, though in some cases optimization of plasma power and gas flows may greatly reduce the interference (47). The analyte addition method should also be applicable to interferences of this type, and recovery tests should give a valid indication of the extent of the problem. In extreme cases, chemical pre-treatment of the samples to separate the analyte from the interferent may be necessary Adaptation effects A peculiar effect was observed by Maessen et al (43) in their studies on interferences in the ICP. When a solution of a certain concentration of HCl, HNO 3 or H 2 SO 4 had been nebulized, the analytical response from a solution with a different concentration of acid took up to 5 minutes to reach equilibrium, the time depending on the difference in acid concentration between the two solutions. This observation shows that sufficient time for equilibration must be allowed, and that it would be good practice to have the same acid concentration not just in the sample solutions and calibration solutions, but in the rinse liquid as well. 2.6 Organic solvents The analysis of organic materials by ICP-AES has been reviewed in detail by Boorn and Browner (48). All the types of interference found with aqueous samples can occur with organics also, and in many cases they are worse. For example, the introduction of organic solvents into the ICP can result in spectral interferences from molecular emission by species such as C 2 and CN (49-51). Transport interferences may be a problem because of the relatively large changes of viscosity, density and surface tension that can occur when samples are dissolved in organic solvents (34,51). The most conspicuous effect of introducing organic solvents into the ICP, however, is a change in excitation conditions. This is evidently caused by changes in the thermal and electrical properties of the plasma brought about by molecular fragments produced during the pyrolysis of the organic compounds. In some cases, the changes in plasma properties on the introduction of an organic solvent can extinguish the plasma. The tolerance of the ICP to organic solvents varies, depending on the design of the torch, the RF generator frequency, the plasma power and the gas flows. The nature of the organic liquid is of prime importance, the more volatile liquids being generally more troublesome because of the large amount Publication date: 10/99 2-9

36 Operating principles of solvent vapor introduced into the plasma (51-54). This problem has been addressed by placing a cooled condenser between the spray chamber and the plasma torch (55) or by cooling the spray chamber (56,57). In many instances, organic solvents are present in sample solutions merely as diluents, and the analyst can avoid a great deal of trouble simply by selecting an appropriate solvent. For example, Botto (58) achieved a worthwhile reduction in matrix interferences in the analysis of oils by using tetralin (1,2,3,4-tetrahydronaphthalene) as a diluent instead of xylene. Ordinary PVC pump tubes are generally unsuitable for use with organics, but special tubes can be supplied for pumping hydrocarbons and other organic solvents, including ketones such as 4-methyl-2- pentanone (MIBK). Before starting to analyze organic solutions, the analyst should ensure that the nebulizer and spray chamber are free from any aqueous residues. Any water in the spray chamber may lead to poor drainage and very bad analytical precision. A good way to prepare the spray chamber for organic solvent work is to rinse it thoroughly with deionized water, then twice with acetone, and finally twice with the solvent that is to be used in the analysis. Optimum operating conditions will vary, depending on the solvent, but the plasma power, the outer (plasma) gas flow and the intermediate (auxiliary) gas flow must generally be higher than is usual for aqueous solutions. A useful indicator of the suitability or otherwise of the operating conditions is given by the intense green tongue of C 2 emission in the initial radiation zone. With correctly chosen conditions, this will be visible between the turns of the induction coil but will not extend above the top of the torch. A green tongue extending beyond the torch indicates that there is insufficient power to break up the pyrolysis products of the organic solvent. Conditions should be adjusted to reduce the amount of solvent reaching the plasma, or to increase the plasma power. The amount of solvent reaching the plasma can be reduced by reducing the pressure of argon supplied to the nebulizer, reducing the internal diameter of the pump tube or by reducing the pumping rate. If the plasma power is increased, it may also be necessary to increase the outer (plasma) and intermediate (auxiliary) gas flows to prevent overheating of the torch. The addition of oxygen to the intermediate (auxiliary) gas is very beneficial in the analysis of solutions in organic solvents. The oxygen assists the decomposition of the solvent, and the resulting molecules are evidently far less detrimental to the plasma than those produced when the solvent is pyrolyzed in argon. The deposition of carbon at the tip of the injector tube and around the intermediate tube of the ICP torch is entirely prevented when oxygen is used, whereas in the absence of oxygen deposition of carbon can occur quite rapidly. Accumulation of carbon can cause drift in the analytical signal and 2-10 Publication date: 10/99

37 Operating principles eventual overheating of and damage to the torch. An additional benefit of adding oxygen to the intermediate (auxiliary) gas is that the molecular emission bands of C 2 and CN are greatly reduced in intensity (49). An accessory for the introduction of a controlled flow of oxygen into the intermediate (auxiliary) gas stream is available from Varian. Solutions in organic solvents which are impossible to analyze without the addition of oxygen can be analyzed with very little trouble using this device. While the combination of oxygen and organic solvents is potentially hazardous, the addition of oxygen to the intermediate (auxiliary) gas ensures that the oxygen and solvent aerosol are kept apart until just below the plasma, where they react harmlessly. Addition of oxygen to the nebulizer gas is potentially EXTREMELY DANGEROUS and should not be attempted. Publication date: 10/

38 Operating principles This page is intentionally left blank Publication date: 10/99

39 3 Liberty description Liberty description 3.1 Introduction Simultaneous ICP-AES instrumentation was introduced to the elemental analysis market in The multi-element capabilities and increase in sample throughput offered by the new technique made it an attractive alternative to the wide range of atomic absorption spectrophotometers available at the time. However, for ICP buyers, the savings from these advantages were offset by the high initial outlay and the high ongoing operating costs. The main reasons for the high cost involved with early simultaneous ICP instrumentation were the requirements for high power (typically 2 5 kw) radio frequency generators and an expensive array of photomultipliers for multi-element analyses. Added to this initial outlay were substantial operating costs due to the high gas consumption (typically 20 L/min) required to sustain the plasma and to cool the ICP torch at high operating powers. The early 1980s saw the introduction of sequential scanning ICP spectrometers. These offered the flexibility needed to cope with any sample type and its associated spectral problems, at a lower cost. The need for rapid scanning, high resolution and low stray light spectrometers kept the cost of sequential ICPs high. ICP technology has progressed to the stage where low cost, benchtop sequential ICPs are now common. Along with the cost benefits, however, there has sometimes been considerable compromise in the analytical performance. This chapter describes the features and functions of the major components of the Liberty sequential ICP spectrometer, and their role in achieving a benchtop instrument that offers both outstanding analytical performance and low operating costs. Publication date: 10/99 3-1

40 Liberty description 3.2 Hardware RF generator spectrometer torch interface and PC sample introduction system Figure 3.1 ICP components The major components of an ICP-AES system are identified below: i) the sample introduction system ii) the ICP torch iii) the RF generator iv) the spectrometer, and v) the interface and computer Sample introduction Sample introduction systems The first stage in the analysis of any sample by ICP is its introduction into the ICP torch. The sample introduction system consists of a nebulizer and spraychamber. There are a number of different nebulizer/spraychamber options (refer to the following chapter for a more detailed account) but the key criterion in the selection of the appropriate sample introduction system is its suitability to a particular sample type. Not only do solid, liquid and gas samples require different sample introduction systems, but so may the different types of liquid samples, that is, aqueous solutions, high salt solutions, hydrofluoric acid solutions, slurries and organic solvents. 3-2 Publication date: 10/99

41 Liberty description Each type of liquid sample has different physical properties, such as viscosity, volatility and percentage of dissolved solids, and therefore may require different sampling systems to obtain the high transport efficiency required for optimum analysis, as well as to achieve high analytical precision Nebulizers As discussed in the following chapter, there are a number of different nebulizers which can be used for liquid sampling. The main function of each nebulizer is to produce a fine aerosol from the bulk solution. Liberty employs one of the most commonly used nebulizers, the V-groove nebulizer (pictured below). The inert, V-groove nebulizer can be used for analysis of solutions containing hydrofluoric acid or very high levels of dissolved solids, and slurries. Figure 3.2 The V-groove nebulizer The efficiency of the nebulizer plays a critical role both in the sensitivity and the precision of ICP analysis. Sensitivity is related to the amount of sample that reaches the plasma and good precision (better than 1% RSD) is related to the ability of the nebulizer to generate a steady, fine aerosol. Even very minor changes in gas flow affect the aerosol production rates, resulting in changes in the amount of sample reaching the plasma, and in the plasma excitation conditions. These fluctuations are then reflected in the accuracy and precision of the analysis. Other nebulizers available for use on Liberty include the glass concentric and ultrasonic nebulizers. The ultrasonic nebulizer is useful for analyses at very low concentrations where the sensitivity of the pneumatic type nebulizer is not sufficient. Detection limits obtained with the ultrasonic nebulizer are typically 10-fold better than those obtained with the glass concentric nebulizer. This high performance stems from the very high rate of generation of fine droplets when the sample is passed over the transducer plate vibrating at a very high frequency (typically 1.4 MHz). The efficiency is so high that a desolvation chamber is typically built into such equipment. Publication date: 10/99 3-3

42 Liberty description Spraychambers The spraychamber is also critical in maintaining the stable environment required to provide high analytical precision. The main function of the spraychamber is to remove the larger droplets generated in the nebulization process. For most nebulizer/ spraychamber systems a large proportion of solution goes to waste. Less than 2% of the bulk sample reaches the plasma. Figure 3.3 Sturman-Masters spraychamber Figure 3.4 Rapid signal equilibration and cleanout of the Sturman-Masters spraychamber The Sturman-Masters spraychamber, made from inert material, can be used for all sample types. The double-pass inner cylindrical tube efficiently sorts the very fine droplets that are carried through to the plasma resulting in a strong and reproducible signal. The Sturman-Masters spraychamber ensures high sample throughput by fast cleanout and stabilization times. Both of these parameters are a function of spraychamber volume and shape. With stabilization times in the order of 20 seconds, little time is lost while waiting for the maximum signal to be reached before measurements are taken. Similarly, cleanout times in the order of 20 seconds ensure that little time is lost while waiting for the signal to return to the background levels before a new sample is introduced. A glass version of this spraychamber, fitted with a water jacket, is available for the analysis of volatile organic solvents. The chamber is cooled to -10 C by an external chiller that circulates a 1:1 mixture of water and ethylene glycol through the water jacket. This reduces the vapor pressure of the organic solvent, and consequently less solvent vapor is transported into the plasma. It is recommended that this spray chamber be used in conjunction with the AGM-1 oxygen addition accessory, which allows a controlled amount of oxygen to be added to the intermediate gas of the ICP torch. The oxygen converts the carbon from the organic solvent vapor into CO or CO 2, and prevents the accumulation of carbon on the torch. 3-4 Publication date: 10/99

43 Liberty description The glass cyclonic spraychamber is particularly useful for the axiallyviewed ICP. Its aerosol throughput for a given nebulizer is greater than that of the Sturman-Masters chamber. The difference presumably arises from the different droplet size distributions in the aerosols from the two spraychambers. This is less significant in the axially-viewed ICP, because light is collected from the entire central channel and the fluctuations arising from the spraychamber aerosol are, to some extent, averaged out Peristaltic pump Liberty employs a dual channel peristaltic pump to drain the excess liquid from the spraychamber. This removes one source of imprecision by controlling both the liquid flow rate to the nebulizer and the rate at which it is drained. Liberty s 10 roller peristaltic pump provides better precision by reducing the pulsating effect which is more evident with pumps with fewer rollers. Liberty s computer controlled variable-speed pump can be set to different flow rates depending on the sample type, thus reducing nebulization problems associated with variations in solution viscosity. The capability to control the rate at which the solution is pumped through the nebulizer is particularly important with organic samples where nebulization efficiencies can be much higher than for aqueous solutions Sample excitation Figure 3.5 Excitation stages in the ICP, revealed by introducing a solution of yttrium The very high temperatures in the plasma (typically over K) and the chemical inertness of the argon results in complete vaporization of the sample followed by a dissociation into free atoms resulting in a very high proportion of excited atoms. Publication date: 10/99 3-5

44 Liberty description Torch The torch is made up of three concentric fused silica glass tubes. The auxiliary gas passes through the intermediate tube. This gas may be used to raise the plasma relative to the torch and therefore avoid melting of the torch. The auxiliary gas is also helpful in preventing build-up of salt or carbon (in the case of organic solvents) on the tip of the injector tube. The aerosol carrier gas flows through the innermost tube, called the injector tube. The carrier gas punches a tunnel through the base of the plasma. The atoms are restricted to passing through this tunnel rather than spreading through the whole of the plasma. This results in a high emission per unit volume and therefore the excellent detection and sensitivity of the technique. The diameter of the injector tube affects the residence time and dispersion of the sample in the plasma. Typically, larger diameter injector tubes are used for salt solutions because of the tendency of these samples to form deposits on the inside of the injector tube near the base of the plasma. Smaller diameter injector tubes are desirable for analyzing organics. The design of the torch must ensure low detection limits, high excitation temperatures and linear calibration curves covering 5-6 orders of magnitude and a large analytical zone. Liberty employs two torch designs to cope with different sample types. Liberty s standard torch can be used for general applications and organics. It is specially designed to achieve the analytical performance of conventional torches, but at reduced powers and gas flows. This torch has been operated at powers as low as 0.45 kw and at gas flows as low as 7.5 L/min. For organic solvents, typical operating power is 1.1 kw. Typical operating conditions outer tube Nebulizer gas Auxiliary gas Plasma gas Power 0.7 L/min 0.75 L/min 10 L/min 1.0 kw injector tube intermediate tube Figure 3.6 Liberty s low flow, low power torch 3-6 Publication date: 10/99

45 Liberty description The second option, the demountable torch, has a wider internal diameter injector tube (1.8 mm) for analysis of samples having very high dissolved salt contents. The injector tube, made of quartz, can be replaced with an alumina injector tube for solutions containing hydrofluoric acid and fusions RF generator The function of the radio frequency (RF) generator is to supply an alternating current at a desired frequency to the induction coil which is used to form and sustain the inductively coupled plasma. There are currently two types of generators, referred to as free running and crystal controlled, used for ICP. A free running generator allows the frequency of the oscillating current to vary as the impedance of the plasma changes. The crystal-controlled generator maintains constant frequency regardless of plasma impedance. The Series II Liberty instruments feature a free-running 1.7 kw RF generator with a nominal operating frequency of 40 MHz. The generator has been designed for operation with Varian s Direct Serial Coupling (DISC) arrangement, whereby the plasma induction coil is part of the tuned circuit. This ensures that the circuit responds to changes in the plasma load in the shortest possible time, resulting in a highly stable and robust plasma. The generator is designed and built for reliability. The RF system has no moving parts, and is based on a rugged triode vacuum tube specially developed for power oscillator applications. The tube is aircooled for simplicity and reliability. The plasma is initiated by passing a spark through the argon gas, making it electrically conductive. When a high frequency current passes through the induction coils, a rapidly varying magnetic field is generated within the coil. The interaction between the oscillating magnetic field and the flowing, electrically conductive gas generates the plasma. The resulting plasma continues to run as long as high frequency current is supplied to the load coil and gas is supplied to the torch. The whole of the plasma ignition process is under computer control and is initiated at the touch of a button. The signal intensity may be strongly dependent on the plasma power settings (refer to the chapter 5 Optimization ). Short-term fluctuations (due to variations in the line voltage) result in the deterioration of achievable detection limits and precision of analysis. Therefore, generators used in ICP are commonly quoted as having a power stability of better than 0.05%. Publication date: 10/99 3-7

46 Liberty description Pb220 Pb405 Figure 3.7 Effect of power on Pb ionic line (220 nm) & Pb atomic line (405 nm) Early ICP spectrometers employed 27 MHz generators while more modern instruments have standardized on 40 MHz generators. A parameter frequently used to determine the analytical performance of an ICP system is the signal to background ratio. A study of the effect of frequency found that the signal to background ratio improved with higher operating frequencies mainly because of a reduction in background. Further, the improved sensitivity at the higher operating frequencies was also complemented by the requirement for lower forward powers to sustain the plasma. This means that by using a 40 MHz RF generator, the ICP can be operated at lower powers, reducing the ongoing operating costs without compromising the analytical performance. The efficiency of power transfer to the load coil is also an important criterion, particularly for the analysis of organics. Liberty s RF generator is directly coupled to the load coil and provides higher efficiency than conventional dual matching network power supplies. The higher efficiency provides the capability to analyze even the most volatile of organic solvents (e.g. methanol) without the need to perform any dilutions. High efficiency RF generators therefore provide enhanced trace element detection limits for organic solvent samples. Table 1 Detection Limits in Methanol Wavelength D.L. Wavelength D.L. (nm) (ppb) (nm) (ppb) Ni Cd Mn Cu Zn Pb zzzzz Spectrometer Each element possesses a unique, characteristic set of energy levels. When an atom is in an energized or unstable state, the atom releases the excess energy in the form of radiation at wavelengths 3-8 Publication date: 10/99

47 Liberty description corresponding to the transitions between the different energy levels as it returns to its stable state. The different elements can be identified by the unique spectrum of wavelengths in the emitted radiation. The function of the spectrometer is to resolve the radiation from the plasma into its component wavelengths with a diffraction grating. The light intensity is then measured with a photomultiplier tube at a specific wavelength for each element. The photomultiplier tube converts the light intensity into an electrical signal that can be quantified and related to the concentration of the particular element in solution. There are two types of spectrometers available, simultaneous and sequential scanning spectrometers. The simultaneous spectrometer uses an array of detectors to simultaneously measure emissions at fixed wavelength settings. This makes the simultaneous spectrometer both faster and more expensive than the sequential spectrometer, which uses a mechanically driven grating to scan a set of wavelengths sequentially. A spectrometer must provide a high degree of resolution to achieve good separation of nearby spectral lines, thus minimizing interelement interferences. The spectrometer must also exhibit excellent light gathering capabilities and minimal stray light to ensure good sensitivity. Further, the spectrometer must be mechanically and thermally stable to ensure both the precision of the analysis and the ability to locate the correct analytical line Monochromator There are a number of different types of scanning spectrometers but the Czerny-Turner mounting pictured below is the most common. PMT A PMT B Refractor plate Grating Exit slit Entry slit Filter wheel Hg lamp Scan mirror Window Plasma torch Figure 3.8 Czerny-Turner Monochromator Liberty s 0.75 m Czerny-Turner monochromator incorporates two large concave mirrors to achieve maximum light throughput. One mirror is focussed to provide maximum illumination of the grating, and the Publication date: 10/99 3-9

48 Liberty description second mirror is focussed onto the exit slit. A computer controlled wavelength drive mechanism, which incorporates a stepper motor driven grating, allows the monochromator to slew between wavelengths. When the grating reaches the required wavelength, a refractor plate, located near the exit slit, performs a scan over a narrow wavelength range to accurately define the peak position and characterize the region surrounding the analyte peak Gratings Liberty s holographic grating, with closely spaced grooves etched on the grating surface, is the dispersive element used to separate incident radiation into the component wavelengths of the emission spectra of the individual elements. When light strikes the grating surface, only a narrow band of wavelengths passes through the monochromator for a given angle of incidence. Different wavelengths can be selected simply by changing the angle that the light strikes the grating. There are two types of gratings that can be used in ICP spectrometers, ruled and holographic gratings. Ruled gratings, produced by ruling V-shaped grooves onto the grating surface, contain imperfections of various kinds and are of limited size. The imperfections can give rise to spectral ghosts, which are lines appearing in the wrong part of the spectrum causing interference problems. Holographic gratings are characteristically larger in size and are also free of the imperfections which cause ghosts to appear in the spectrum. Holographic gratings also typically provide orders of magnitude better stray light rejection characteristics. Liberty s 1800 grooves/mm holographic grating provides excellent light gathering capabilities and resolution. The resolution, which is measured by the width of the peak at half the peak height, indicates the ability of the grating to separate nearby analytical lines in order to minimize the occurrence of spectral overlap. Accuracy and precision of analysis is therefore ensured because analysis is performed on an interference free peak without the need for any mathematical corrections. Spectral overlap may not always be avoidable, particularly when two peaks directly overlap, although most inter-element interferences can be avoided by use of a high resolution spectrometer. Figure 3.9 Liberty resolution 3-10 Publication date: 10/99

49 Liberty description The grating must also be able to cover the wavelength range of interest, typically nm. If too high a groove density grating is used, the grating will not be able to cover the whole wavelength region. This problem can be overcome in one of two ways. One way is to use a high resolution grating that covers the wavelength region where most of the emission lines can be found, for example, nm, and then use a second lower resolution grating for the wavelength range nm where there are less emission lines and therefore less likelihood of inter-element interferences. This approach, however, adds instrumentation time delays due to the requirement for a grating calibration for each grating changeover and also introduces another source of mechanical instability. The Liberty 1800 grooves/mm grating covers the full nm wavelength range. Liberty s holographic grating provides high intensity spectra in first, second, third and fourth orders, thus allowing coverage of a very wide wavelength range with excellent resolution. The flexibility to select the grating order for individual wavelengths means that analytical methods can be set up to suit both the resolution and sensitivity requirements for individual applications. The one grating system also provides better mechanical stability than the two grating system. Figure 3.10 Liberty achieves high intensity spectra in all four orders, as shown by the Al 167 nm line in the above example Filters The overlapping of spectral orders can be potentially troublesome. Emission lines at 200 and 300 nm in the first order will coincide with lines at 400 and 600 nm respectively in the second order and 600 and 900 nm respectively in the third order. Liberty uses spectral filters to eliminate spectral overlap from higher orders. The solar blind PMT also acts as an order sorter, because it responds only to ultraviolet light. Publication date: 10/

50 Liberty description Figure 3.11 Order Interferences These order sorters, or filters, not only remove unwanted orders but also reduce stray light. Figure 3.12 Stray light reduction. 0.1 ppm Zn in ppm Ca Wavelength calibration A calibration must be performed prior to analysis to provide the necessary reference when slewing to a particular wavelength. The known wavelengths from the emission spectrum of a mercury lamp are used to calibrate the spectrometer. This calibration also characterizes the minor deviations from the linear wavelength calculation which arise from the small imperfections found in grating drive mechanisms. Liberty s 11 point mercury wavelength calibration of the full wavelength range ensures accurate and reproducible peak location Publication date: 10/99

51 Liberty description Wavelength drive mechanism The speed and accuracy of the wavelength drive mechanism impacts on sample throughput and therefore is an important feature to consider for a rapidly scanning sequential spectrometer. The leadscrew and sine bar drive mechanism provides fast coverage of the wavelength range as well as an accurate linear representation of wavelength. The precision-ground lead screw is driven by a high speed DC motor and also fitted with a precision shaft encoder. The wavelength drive mechanism is called a sine-bar drive because any change in the angle of the grating, which is related to the number of steps moved, is linearly related to a change in wavelength. This linearized approach offers high accuracy and precision of wavelength location. The grating is moved to the point where the peak is expected, as defined by the grating calibration. A refractor plate located near the exit slit then scans the spectrum using smaller, precise steps. The intensity data taken across this scan is then fitted to a mathematical expression which determines the true peak position. The refractor plate is then positioned directly on the peak and replicate measurements performed. This is known as measurement at the peak (MAP) and results in high analytical precision. When the measurement is complete, the monochromator slews to the wavelength for the next element and repeats the process Peak location levels The peak location level is a function of integration time. Peaks in the scan performed by the refractor plate are identified provided the measured signal is greater than 4 times the standard deviation of the noise. The amount of time the refractor plate measures each individual data point in the scan window is dependent on the integration time. Low integration times result in high noise levels which therefore make peaks at the detection limit level difficult to identify. By setting a higher integration time, noise is significantly reduced making it possible to distinguish weak signals from the background. Therefore setting a high integration time provides the best conditions for accurate peak location, and high analytical precision, both of which are essential for trace level analysis. Publication date: 10/

52 Liberty description Figure 3.13 Peak location levels Using a high integration time improves signal to noise Viewing position The intensity of the emission of individual lines varies according to the observed region in the plasma. The optimum viewing position is not necessarily the same for each element or analytical line because each line has different characteristics. Further, the optimum viewing position varies for different sample types and also for different operating conditions. Figure 3.14 Intensity vs Viewing Height Diagram A mirror under stepper motor control allows measurement at different heights in the plasma by focusing the viewing optics to different positions of the plasma. Therefore, it is possible to view the region of the plasma where the optimum signal for individual lines can be observed Detection The detection device used in the Liberty ICP spectrometer is the photomultiplier tube (PMT). The function of the photomultiplier is to convert the radiation at a particular wavelength into an electrical signal in order to allow quantification. When photons reach the cathode inside the photomultiplier, the cathode releases electrons. The emitted electrons are amplified into a current pulse by a series of nine electrodes (dynodes). The first dynode is operated at a high negative potential, the last is near ground potential. An electron striking the first 3-14 Publication date: 10/99

53 Liberty description dynode releases many secondary electrons. These strike the second dynode, releasing more electrons, and so on. The resulting anode current produced is dependent on the intensity of the incident radiation. This electrical current therefore becomes a measure of concentration of the particular element in the sample. The capability of the ICP technique to determine up to 5 orders of magnitude of concentration requires that the photomultiplier must be able to cope with a wide intensity range. This is dealt with by using a photomultiplier tube with gain control which changes the PMT voltage as the instrument scans individual wavelengths. Liberty employs two PMTs for optimum detection across the full wavelength range (figure 3.15). A solar blind PMT is used for detection at wavelengths below 300 nm. The fact that the solar blind PMT has a cut off point at 300 nm makes it an excellent order sorter. It also offers excellent stray light rejection characteristics since most elemental wavelengths that produce stray light are above the cut off (300 nm). A computer controlled mirror automatically switches between the solar blind PMT and the second, wide range PMT. Figure 3.15 PMT response curves Vacuum spectrometers Vacuum spectrometers are employed for measurements at wavelengths lower than about 190 nm and as far down as 160 nm. Elements such as S and Br have analytical emission lines that can only be found at these lower wavelengths while the most sensitive Al, P and I lines can also be found in this region. The vacuum spectrometer components are mounted in an air tight box. The monochromator is evacuated by pumping down to below 1 Torr and then purging with an argon flow rate of about 0.5 L/min until a steady vacuum of approximately 10 Torr is achieved. Publication date: 10/

54 Liberty description Nitrogen purged spectrometers The Varian AGM-2 auxiliary gas module is an accessory designed to extend the wavelength range of an air-path monochromator. It does this by purging the monochromator and the pre-optics with nitrogen. This removes oxygen from the optical path and extends the wavelength range down to at least 175 nm, allowing the instrument to be used for the determination of some important additional elements, such as sulfur. 3.3 Axially-viewed ICP For many years the conventional way to collect light from the ICP has been by side-on or radial viewing. The pre-monochromator optics view the plasma from the side, at ninety degrees to the longitudinal axis of the torch. An alternative way of collecting light from the ICP is by end-on or axial viewing. The pre-monochromator optics face into the end of the torch to view the plasma. From this perspective, the region where the analyte atoms emit light appears as a small circle, surrounded by a bright annulus of highly luminous plasma. The axially-viewed ICP has become popular because it can provide limits of detection significantly better than those obtained with the radially-viewed ICP. A brief review of the factors that affect the limit of detection in ICP-AES will help to explain how axial viewing can give better limits of detection than radial viewing Limits of Detection The limit of detection (LOD) is defined as the concentration that gives a signal equal to three times the standard deviation of a blank solution: LOD = 3 x SD blank(c) where SD blank(c) is the standard deviation of the blank in concentration units Optimizing for lowest limits of detection To see the effect of instrument parameters on the limit of detection, it is necessary to express the detection limit in terms of the instrument signal (i.e. in counts per second). The relationship between the instrument signal and the concentration is the sensitivity, S (units: counts per second per unit concentration). Therefore: LOD = 3 x SD B /S where SD B is the standard deviation of the blank in counts per second. From this equation it is clear that to get the best (i.e. lowest) limit of 3-16 Publication date: 10/99

55 Liberty description detection we need: (a) The lowest possible standard deviation of the blank, AND (b) The greatest possible sensitivity. It is useful to normalize both the sensitivity, and the standard deviation of the blank, to the blank signal B. This can be done by using the relative standard deviation of the blank, RSD B = SD B /B, and the blank (or background) equivalent concentration, BEC = B/S The limit of detection is then: LOD = 3 x RSD B x BEC and we want both RSD B and BEC to be as small as possible. Another way of normalizing the sensitivity to the blank (or background) signal is to use the signal-to-background ratio for unit concentration of analyte, SBR = S/B. In this case the limit of detection is: LOD = 3 x RSD B /SBR and we want RSD B to be as small as possible and SBR to be as large as possible. Note that SBR in this equation means the signal-to-background ratio for a solution of unit concentration. If the test solution used to measure the signal-to-background ratio is not of unit concentration, then the limit of detection equation becomes: LOD = 3 x RSD B x C /SBR where C is the concentration of the test solution. The BEC approach automatically takes care of the possible confusion over the concentration of the test solution. However the SBR approach is very widely used, so it is important to be aware of it. In the discussion that follows, the term SBR is to be understood to mean the signal-tobackground ratio for unit concentration. The relationship between RSD B and the blank (or background) signal B has been explained in great detail by Dr PWJM Boumans There are several sources of instability (or noise) in the background signal; two are particularly important in practice. If the blank signal is large, the main source of instability will be SOURCE FLICKER NOISE, arising from instabilities in the nebulizer, spray chamber and plasma. When this noise is dominant, the RSD B is essentially constant and the detection limit depends entirely on the BEC (or SBR). If the blank signal is very small, the main source of instability will be PHOTON SHOT NOISE, arising from the random arrival of photons at the detector. When this noise is dominant, the RSD B is inversely Publication date: 10/

56 Liberty description proportional to the square root of the background signal: LOD = 3 x k x B -0.5 /SBR where k is a constant of proportionality. Since SBR = S/B, LOD = 3 x k/(s/b 0.5 ) that is, the detection limit is inversely proportional to (S/B 0.5 ) - the signal for unit concentration divided by the square root of the background (blank) signal 441. In practice, both source flicker noise and photon shot noise come into play. However, we recall that when source flicker noise dominates we need to maximize S/B, while when photon shot noise dominates we need to maximize S/B 0.5. In either case, the detection limit is improved if we can simultaneously: (a) Maximize sensitivity - that is, get as much signal as possible for unit concentration of analyte (b) Minimize the background signal The effect of torch viewing on detection limits With a radially-viewed plasma the pre-monochromator optics form an image of the plasma on the monochromator entrance slit, with the image of the central channel aligned with the slit. Obviously, light that does not fall on the entrance slit cannot pass into the monochromator. When the central channel is imaged on the slit, light is collected not only from the emitting atoms in the central channel but from those parts of the surrounding luminous plasma that lie directly in front of, and directly behind, the central channel. This light from the plasma around the central channel contributes to the optical background. The intensity of this light varies along the axial direction, because of the shape of the ICP discharge. So, too, does the intensity of the analyte radiation, but its variation with position is not the same. The adjustable viewing height allows the part of the plasma image that is viewed by the monochromator to be selected for the maximum signal-tobackground ratio. The detection limit advantages of the axially-viewed ICP can be obtained only if the pre-monochromator optics produce an image of the plasma in such a way that the circular image of the central channel falls on the slit, while the annular image of the surrounding luminous plasma does not. Light is thus efficiently collected from the emitting analyte atoms, while analytically useless light from the surrounding plasma is excluded. Consequently, the signal-to-background ratio is better than it is with radial viewing. As well as excluding more of the analytically useless light emitted from 3-18 Publication date: 10/99

57 Liberty description the plasma, axial viewing collects light from the entire length of the axial channel, where the analyte atoms are emitting. Radial viewing, in contrast, collects light only from that part of the axial channel that is imaged on the entrance slit. As a result, the sensitivity (i.e. signal per unit concentration) is better with axial viewing than with radial viewing. The axially-viewed ICP, by providing better sensitivity and better signal-to-background ratio, provides better limits of detection than the radially-viewed plasma. The improvement can be a factor of five to ten or more Implementation of the axially-viewed ICP Optical Design As has been explained earlier, the pre-monochromator optics have to be specifically designed for axial viewing, so that light from the central channel enters the monochromator while light from the surrounding luminous plasma does not Removal of the plasma tail plume from the optical path To achieve best results from an axially-viewed ICP, it is necessary to get rid of the tail plume that forms as the plasma from the central channel cools down in the ambient air. The tail plume contains neutral analyte atoms that can absorb radiation emitted from analyte atoms in the axial channel. It may also contain molecules that can absorb or scatter analyte radiation, or emit molecular spectra that could contribute to the optical background or cause spectral overlap interferences. Apart from these analytical factors, it is necessary to prevent the tail plume from impinging upon the entrance optics, to protect the optics from damage by heat and to prevent contamination of the entrance window by materials in the tail plume. One way of dealing with the problems associated with the tail plume is to use a stream of gas to shear off the tail plume , 451. The disadvantage of this arrangement is the high flow-rates of gas required. If analytical emission lines in the far ultraviolet need to be measured, the gas has to be high-purity nitrogen or argon, and this can be expensive. Another approach is to view the plasma through a hole in a water-cooled metal cone placed in front of the entrance optics, and to pass a low flow of gas through this hole towards the plasma 450. This method has been used in the Varian axially-viewed plasma instrument. The cooled cone interface is made part of a tuned RF circuit to prevent electrical discharges between the plasma and the metal cone Torch design Publication date: 10/

58 Liberty description To minimize cooling of the horizontally-mounted plasma by entrained air it is advantageous to use a torch that extends further out from the induction coil than is usual for radially-viewed plasmas. This extends the region in the central channel in which conditions are favorable for the emission of light by analyte atoms, and thus improves the sensitivity Choice of spraychamber With the axially-viewed ICP, light is collected from the entire axial channel. This means that fluctuations in light intensity that occur in radial viewing when only part of the channel is viewed are to some extent averaged out by axial viewing. Consequently, the signal from the axially-viewed plasma is less perturbed by fluctuations arising from the droplet size distribution in the aerosol, and the requirement for the spray chamber to remove droplets from the aerosol is less critical. It is possible to use a spray chamber that transmits more aerosol to the plasma, and this gives an increase in sensitivity that more than compensates for any increase in background noise. The glass cyclone spray chamber has been found to give very satisfactory performance with the axially-viewed ICP Limitations of the axially-viewed ICP Analysis of samples having matrices that emit high levels of light in the ICP In the preceding discussion, it has been assumed that the sample matrix does not contain species that contribute significantly to the background (or blank) signal. If this is not the case, then the advantages of axial viewing may be reduced, or even disappear. Sample matrix components can emit continuum radiation and/or molecular band spectra in the central channel of the plasma, along with the analyte atoms. The optical system collects light from the matrix just as well as it collects that from the analyte, so the signal-tobackground advantage could be lost. Analysis of samples having matrices that emit molecular band spectra in the ICP Some samples emit distinctive molecular bands in the ICP. These bands can contribute structure to the optical background that can cause spectral overlap interferences and also degrade the detection limit. In the radially-viewed ICP the viewing height can be adjusted to minimize the effect of molecular bands; this is not possible with the axially-viewed plasma. It may be possible to reduce the effect of molecular bands by careful choice of nebulizer gas flow and plasma power Publication date: 10/99

59 Liberty description Analysis of organic solvents Organic solvents introduced into the ICP emit light in the axial channel, principally from molecules such as C 2 that are formed as the solvent molecules decompose. Light emitted by these molecules can produce high levels of background in the plasma. This background consists of continuum emission and molecular emission bands. With the radially-viewed plasma it is often possible to minimize the background by appropriate choice of viewing height. This option is not available with the axially-viewed plasma. Consequently, when organic solvents are analyzed the detection limits with the axially viewed ICP may be worse that those with radial viewing. There is, however, a way to reduce the background emission during the analysis of organic solvents. The addition of oxygen to the intermediate gas flow of the ICP torch results in the destruction of the molecules that produce the background emission. When this is done the detection limit advantages of axial viewing can be achieved even with organic solvents. The Varian AGM-1 gas control module (P/N ) is specifically designed for the addition of oxygen to the intermediate (auxiliary) gas. Analysis of samples for elements subject to volatilization interferences or to interferences from easily-ionizable elements With the radially-viewed ICP, it is possible to change the viewing height (i.e. to select the portion of the axial channel that is viewed by the monochromator) to minimize (or even eliminate) various interferences. With axial viewing, light is collected from the entire length of the axial channel. The analyst no longer has the option of choosing to observe only that part of the channel proving the conditions of minimum interference. For this reason, interferences can be more of a problem with the axially-viewed ICP than they are with the radially-viewed plasma. However other means of minimizing interferences can often be employed, such as careful choice of nebulizer gas flow rate and plasma power. Analysis of samples containing high levels of dissolved solids Another limitation of the axially-viewed plasma is that the longer, horizontally-mounted torch is more susceptible to contamination by materials introduced into the plasma. For example, if solutions containing high levels of dissolved salts are nebulized into the axiallyviewed plasma, salts may be deposited on the walls of the torch. These deposited salts can react with the quartz, ultimately causing it to break. Publication date: 10/

60 Liberty description This page is intentionally left blank Publication date: 10/99

61 4 Nebulizers Nebulizers 4.1 Introduction While the inductively coupled plasma (ICP) is the best available general-purpose atomization and excitation source for atomic spectrometry, it does have its own special requirements for the introduction of analytical samples. If these are not met, the precision and accuracy of the results will be compromised. The situation is complicated by the fact that a sample introduction system that may be highly satisfactory for one type of sample can be completely unsuitable for another. Even when the most appropriate sample introduction system is used, it is still very likely to be the major factor limiting analytical precision and accuracy (59). A number of different techniques have been developed for introducing samples into the ICP (60). Some of these have found only limited use, while others are used in one form or another in almost all ICP spectrometers. Techniques have been described for the introduction of solids, liquids and gases. All have had to allow for the injector gas flow into the ICP being typically no more than 1 L/min. The introduction of solids and liquid has also been limited by the inability of most ICPs to evaporate and atomize particles efficiently if the particle diameter is much greater than a few µm (61). Many different sample introduction devices are available commercially, and others can be built in the laboratory using information supplied in the original papers. Because of this, an ICP spectrometer should have provision for the easy installation of devices other than those supplied as standard items by the manufacturer. The Varian ICP spectrometer has a spacious working area beneath the torch, with plenty of room for alternative sample introduction devices. While many different sample introduction techniques are possible, and may be useful in certain applications, in practice the vast majority of ICP-AES analyses are done on liquid samples. These are usually introduced into the plasma as aerosols produced by pneumatic nebulizers. Another approach to nebulizing liquid samples, ultrasonic nebulization, is also quite widely used. Publication date: 10/99 4-1

62 Nebulizers 4.2 Ultrasonic nebulizers Ultrasonic nebulizers were used in some of the very early work on ICP spectrochemical analysis (5,7,62). Instead of relying on the energy of a high-velocity gas to nebulize a liquid, ultrasonic nebulizers apply energy to the liquid with a piezoelectric transducer driven at ultrasonic frequencies. This has the advantage of making the generation of aerosol independent of the flow rate of the gas transporting the aerosol into the plasma, so the two parameters can be optimized independently. More importantly, the efficiency of aerosol generation is typically up to ten times better than that of conventional pneumatic nebulizers (63). Consequently, the sensitivity is greatly improved. Early studies (7) showed that it was advantageous to remove most of the liquid from the aerosol stream by passing the aerosol through a heated duct and then into a condenser, where much of the liquid, but very little of the desolvated aerosol, drained away to waste. This is particularly important when the nebulizer is used with the lowerpowered ICPs (around 1000 W) typical of modern ICP-AES instruments (63). These plasmas can become unstable when large concentrations of molecular species, such as solvent vapors, are introduced. Early ultrasonic nebulizers had a reputation for being rather unstable, washout times were prolonged, and interferences were worse than with pneumatic nebulizers, often because the desolvation process introduced one more stage in which the samples could behave differently from the calibration solutions (64). Despite these shortcomings, interest in ultrasonic nebulizers persisted, and improved versions have been developed (65,66,67,68). A modern, highly effective ultrasonic nebulizer and desolvator developed by CETAC Technologies Inc., is recommended for use with the Varian ICP-AES spectrometer when particularly low detection limits are required. Despite the improvements in sensitivity and limits of detection made possible by ultrasonic nebulizers, pneumatic nebulizers became firmly established as the most popular means of generating aerosols for ICP spectrochemical analysis. The present chapter discusses some of the more widely used pneumatic nebulizers as a guide to the analyst. Readers seeking more information should refer to the books (69-74) and review articles (59,60,75,76), and the original papers cited therein. 4.3 Pneumatic nebulizers Pneumatic nebulizers and spray chambers for ICP spectrometry are discussed in a review by Sharp (75,76). As well as reviewing the extensive literature, this article discusses the mechanism of operation of pneumatic nebulizers and outlines the characteristics of the 4-2 Publication date: 10/99

63 Nebulizers pneumatic nebulizers used in ICP spectrochemical analysis. Sharp s review is essential reading for anyone interested in pneumatic nebulizers and spray chambers. Pneumatic nebulizers use a jet of gas, which in ICP spectrometry is nearly always argon, to break the liquid sample into small droplets. The resulting aerosol is not suitable for passing directly into the ICP because it contains some unfavorably large droplets. These are removed by passing the aerosol through a spray chamber, where they settle out and are drained away to waste. The spray chamber also damps out some of the fluctuations in the rate of aerosol generation and transport that would otherwise contribute noise to the analytical signal. Unless care is taken, however, the spray chamber can become a source of noise (77-79). For best precision, the internal surfaces of the spray chamber must be uniformly wet and the drain must operate smoothly. The inside surfaces of the spray chamber supplied with the Varian ICP spectrometer are sandblasted to promote even distribution of liquid over the walls, and the drain is pumped to allow smooth removal of waste liquid (76,80). This spray chamber evolved from a design by L. S. Dale and S. J. Buchanan (Journal of Analytical Atomic Spectrometry, 1, 59-62, 1986). The published design was modified for manufacture in an inert engineering plastic, rather than glass, so that samples containing hydrofluoric acid could be analyzed. The aerosol outlet was then completely redesigned to provide a double pass arrangement that promotes the removal of large droplets. The rapid equilibration and washout times of the original Dale and Buchanan spray chamber are retained in the new spray chamber. The assistance of Mr. L. S. Dale in providing one of his spray chambers for evaluation is gratefully acknowledged. It is good practice to clean the spray chamber regularly by soaking it in hot water and laboratory detergent, preferably in an ultrasonic bath. This will assist in maintaining good drainage. The addition of a small amount of a surface-active agent, such as Brij 35 (Merck) or Triton X- 100 (Rohm & Haas), to the analytical solutions has been found to be beneficial in promoting even drainage from an ICP spraychamber (81). In some pneumatic nebulizers, for example those used in flame atomic absorption spectrometry, the pressure drop created by the gas jet is sufficient to allow the nebulizer to act as its own sample pump. While some nebulizers suitable for ICP spectrometry can also act as pumps, the pumping rate is affected by the hydrostatic head of the sample solution. The uptake rate (and hence the analytical signal) can change, depending on the vertical distance between the top of the liquid in the sample container and the nebulizer. To overcome this problem, it is common practice to use an external pump to supply the nebulizer with solution at a constant rate. An external pump is essential for other pneumatic nebulizers, such as V-groove nebulizers, which do not act as pumps at all. Publication date: 10/99 4-3

64 Nebulizers Peristaltic pumps are commonly used to supply analytical solutions to ICP nebulizers (82), and eliminate the effect of hydrostatic head. Care needs to be taken that pulsations in the rate of delivery of solution caused by the pump do not lead to a fluctuating signal and consequent loss of analytical precision. Pulsations can be minimized by careful adjustment of the pressure bar which clamps the pump tube against the rollers. The pressure should be only slightly greater than the minimum required to start pumping. Because low-frequency pulsations are not easily damped by the spray chamber, the pump should be run at a relatively high speed with a pump tube selected to give a sample uptake rate suitable for the nebulizer. A well-adjusted pump, run at an appropriate speed, should give an analytical signal at least as stable as that obtained with the nebulizer acting as its own pump (74). Pumping rates of 1-3 ml/min are typical. Peristaltic pump tubes are consumable items, and need to be replaced from time to time; loss of sensitivity, or a deterioration in precision, often indicates that a tube needs to be replaced. The useful lifetime of a pump tube will be extended if the tube is not left stretched over stationary pump rollers: when the pump is not in use, the pressure bar should be released and the pump tube unhooked. A tube that has become flattened from being left stretched over the pump rollers will almost certainly give poor analytical performance, and should be replaced. As indicated in Sharp s review (75), the first pneumatic nebulizers and spray chambers used in ICP spectrometry were essentially miniaturized versions of those originally developed for flame spectrometry. Refinement of these, and development of new devices, was done empirically, on the basis of measurements of the analytical performance of ICP spectrometers. Pneumatic nebulizers for ICP spectrometry may be divided into several broad categories, of which the most important are concentric, cross-flow and V-groove. Each of these categories contains many different versions of the basic design and it would not be appropriate to discuss them all here. In the following sections some of the advantages and limitations of the more widely-used pneumatic nebulizers will be outlined Glass concentric nebulizers liquid aerosol gas Figure 4.1 The glass concentric nebulizer 4-4 Publication date: 10/99

65 Nebulizers A glass concentric nebulizer was used by Gouy over a century ago in his studies of colored flames (83). A similar device was developed for use with ICPs by J. E. Meinhard in 1974 (84,85). This glass concentric nebulizer, which has become accepted in laboratories around the world as one of the most useful general-purpose nebulizers for ICP spectrochemical analysis, is manufactured by J. E. Meinhard Associates Inc. of California. Similar nebulizers are also manufactured by several specialist suppliers of scientific glassware. A description of the fabrication of a glass concentric nebulizer has been given by Scott (86). In assessing literature reports on the performance of glass concentric nebulizers, it is important to be aware that the design of nebulizers has undergone important refinements over the years. The most significant of these is the development of the Type C recessed-tip nebulizer. Compared to the original Type A nebulizer, the Type C produces a greater pressure drop for a given argon flow rate and the analytical signal is more stable (87). More recently, J. E. Meinhard Associates have introduced the Type K nebulizer, designed to operate at the lower gas flows required by some modern low-flow, low-power ICP torches. The Type K nebulizer can also be used on all instruments previously fitted with earlier models of the glass concentric nebulizer (88). Early versions of the glass concentric nebulizer were prone to clogging with salts deposited from analytical solutions, and efforts were made to reduce this problem by saturating the nebulizer argon with water and by injecting water into the gas inlet at regular intervals (71,85,89-91). Subsequent studies with more recent nebulizers have shown that glass concentric nebulizers can handle quite concentrated salt solutions (for example 410 g/l hydrated aluminium sulfate) provided the nebulizer is not allowed to aspirate air, which initiates crystallization of salts from sample liquid around the tip of the nebulizer (92,93). Use of an argon-saturation device enhances the ability of the nebulizer to handle concentrated salt solutions in routine analysis, and is recommended. A suitable device is available from Varian. Meinhard glass concentric nebulizers are available to various specifications of gas pressure and liquid uptake rate (94). The gas throughput of a concentric nebulizer is determined by the operating pressure and the area of the annular gas orifice at the tip of the nebulizer. If the gas flow rate is excessive, aerosol will pass through the plasma too rapidly for efficient atomization and excitation of the analyte. If the flow rate is too small, the emission of light by analyte atoms or ions will occur low in the plasma and may be obscured by the wall of the torch or by the induction coil. Excessively low gas flowrates can lead to the overheating of the injector tube in some plasma torches, and can cause permanent damage. Publication date: 10/99 4-5

66 Nebulizers While the gas flow-rate through a given nebulizer can be adjusted by changing the gas pressure, large departures from the nominal operating pressure may result in a decrease in the efficiency of the nebulizer or a loss of analytical precision. It is obviously important to select a nebulizer matched to the requirements of the ICP torch with which it is to be used. Glass concentric nebulizers are specified in terms of the operating pressure required to produce an argon flow of 1 L/min (0.7 L/min for the type K), and the water uptake at this argon flow-rate. The range of both these parameters for a given model is rather large (85), but Meinhard nebulizers are supplied with a certificate stating the characteristics of the individual nebulizer. If a nebulizer is replaced with one of significantly different characteristics, the sample pumping rate, gas pressure and viewing height will need to be re-optimized. Nebulizers can be optimized to give maximum signal, maximum signal-to-background ratio (SBR) or best precision. Traditionally, SBR is the preferred choice. However, for routine analysis of samples with analyte concentrations well above the limit of detection, precision of measurement may be more important and the conditions which give the best SBR may not always give the most stable analytical signal. For example, with a 10 mg/l manganese solution measured at the nm manganese line a particular nebulizer gave the best SBR at a gas pressure of 140 kpa and a viewing height of 14 mm above the induction coil, while best precision was obtained when the gas pressure was increased to 200 kpa. It is worthwhile to spend some time investigating the effect of gas pressure and viewing height on the analytical performance, remembering that optimum conditions can vary significantly from one spectral line to another. Maintenance of glass concentric nebulizers A very useful article on the maintenance of glass concentric nebulizers has been written by Meinhard (95), and should be studied carefully by all users of glass concentric nebulizers. The central sample capillary in a glass concentric nebulizer is subject to blockage by particulate material or fibres that may be present in the sample. Care needs to be taken to prevent such material from entering the nebulizer. Filtering the samples is an obvious, but timeconsuming, way of protecting the nebulizer from blockage. Another way, suggested many years ago for cross-flow nebulizers, is to place a guard capillary at the inlet end of the sample uptake tube (96). The guard is simply a short length of plastic capillary, with an internal diameter less than that of the glass capillary in the nebulizer. Any foreign matter will collect in the guard capillary, which can be discarded and replaced. Naturally any obstruction or blockage of the sample inlet line, whether in the nebulizer, in the plastic uptake tubing, or at the guard capillary, will affect the sample flow rate and cause drift in the analytical signal. Arranging matters so that the blockage occurs in the guard tube simply makes the problem easier to correct. 4-6 Publication date: 10/99

67 Nebulizers To avoid carryover, it is good analytical practice to nebulize a rinse solution between samples. As Meinhard explains in his article (95), rinsing can also help prevent sample material from accumulating on the glass nebulizer capillary. The rinse liquid should preferably not contain dissolved solids, and its ph should be similar to that of the sample. If the ph of the rinse differs greatly from that of the samples, there is a risk that material from the samples may be precipitated in the nebulizer capillary. If the samples are made up in an organic solvent, the rinse should be the same, or at least a very similar organic solvent. If care is not taken in the choice of rinse liquid, rinsing can actually promote blockage of the nebulizer by causing precipitation of material from the analytical solutions (95). If the sample capillary does become blocked, great care is needed to remove the blockage without damaging the delicate glass capillary. It is sometimes possible to clear a blocked capillary without removing the nebulizer from the spray chamber by injecting water into the sample inlet with a hypodermic syringe (71). If that is not successful, it may be possible to dislodge a blockage by applying a vacuum at the sample inlet and drawing clean water through the capillary in the reverse direction to the normal sample flow. Manabe (97) suggests that this can be done without taking the nebulizer out of the instrument, or even extinguishing the plasma, with the aid of a device that allows water to be injected into the gas inlet. Alternatively, the nebulizer may be removed from the spray chamber and the tip dipped into clean water while vacuum is applied to the sample inlet. Where the nature of the material clogging the capillary is known, the blockage can sometimes be dissolved away by soaking the entire nebulizer in an appropriate liquid. Ultrasonic cleaning is not recommended, because of the risk of damage to the fragile glass capillary in the nebulizer (95). Attempting to remove a blockage by probing carries the risk of breaking the capillary and ruining the nebulizer, and should be tried only when all else fails. If probing is attempted, nylon filament (fishing line) should be used rather than metal wire (71). Organic material such as textile fibres can be removed by soaking the nebulizer in a hot solution of potassium dichromate or sodium dichromate in concentrated sulfuric acid. Warning Hot sulfuric acid solutions are very corrosive and highly toxic, and dichromates are carcinogenic. EXTREME CARE is required when using this cleaning mixture. Publication date: 10/99 4-7

68 Nebulizers The gas annulus at the tip of the nebulizer is also susceptible to blockage. Any particles present in the gas stream are very likely to become lodged in the annulus. To help prevent this, an in-line filter is built into the argon inlet line of the Varian ICP spectrometer. When the nebulizer is removed for cleaning, care must be taken that no foreign matter enters the gas inlet. If particles become lodged in the gas annulus, they can sometimes be removed by gently tapping the sample inlet end of the nebulizer on a wooden surface so that any loosened particles fall back into the nebulizer body, where they can be removed through the gas inlet. Failing that, it may be possible to blow trapped particles back into the nebulizer body by applying gas pressure to a plastic or rubber tube attached to the tip of the nebulizer. Water (or another appropriate solvent) drawn through the nebulizer body by applying a vacuum to the gas inlet while the tip is immersed in the liquid may also be effective. The gas annulus may also become blocked by solids deposited by the evaporation of analytical solutions. This problem is particularly likely to occur if the nebulizer is left wet with analytical solution when the gas is turned off. The liquid runs into the gas channel, dries, and any dissolved solids are deposited. Rinsing after every sample will help to prevent this problem. If blockage does occur, the deposited solids can usually be removed by soaking the nebulizer in a suitable solvent. Sometimes, chemical reactions occur on drying, forming material that is more difficult to remove. Meinhard recommend 3-5% hydrofluoric acid, dripped into the tip from a plastic dropper, to loosen deposits of oxides and metal salts (95). Warning Hydrofluoric acid causes very severe burns which may not be noticed immediately. Extreme care must be taken to prevent contact of this acid with the human body. Strong rubber gloves and a face shield should ALWAYS be worn when using hydrofluoric acid. A special kit for treating hydrofluoric acid burns should be available in the laboratory, and its use understood by all personnel who work with this acid Cross-flow nebulizers Cross-flow nebulizers have been used in ICP spectrometers ever since the work of Fassel s group at Iowa State University in the early 1970s (98). A cross-flow nebulizer consists essentially of a gas jet arranged to pass a high velocity stream of gas across the end of a narrow tube, the other end of which is supplied with liquid to be nebulized. The gas flow over the outlet of the tube creates a lowpressure region, and the pressure of the atmosphere forces the liquid into the gas stream where it is broken up into small droplets. 4-8 Publication date: 10/99

69 Nebulizers Cross-flow nebulizers were developed for flame spectrometry before the Second World War, but they are now superseded in that field by concentric nebulizers. gas aerosol liquid Figure 4.2 A cross-flow nebulizer The relative ease with which the cross-flow design could be adapted to the low gas flow required by the ICP led to widespread use of nebulizers of this type in ICP spectrometry, and many different versions have been developed. The performance of a cross-flow nebulizer depends critically on the alignment of the gas jet and the liquid tube (99). If this alignment is correct, the sensitivity and precision achieved with cross-flow nebulizers are generally little different from those obtained with other pneumatic nebulizers (100,101). Two approaches have been taken to the problem of achieving alignment of cross-flow nebulizers. With adjustable cross-flow nebulizers, the two tubes have to be adjusted by the operator. The easiest way to find the correct position is to remove the nebulizer from the spray chamber and from the sample pump, then to set the tubes so that liquid is drawn into the nebulizer by the pressure drop created by the gas flow, and finally to make fine adjustments to achieve a uniform, dense spray of small droplets. With practice, this adjustment is not difficult but it is questionable how long alignment will be maintained when the nebulizer is in operation. Any small drift in the alignment of the nebulizer tubes will result in severe drift in the analytical signal. To avoid this problem, fixed cross-flow nebulizers are aligned at manufacture and rely on their rigid construction to maintain alignment (102,103). Publication date: 10/99 4-9

70 Nebulizers V-groove nebulizers In these nebulizers the sample liquid flows down a V-shaped groove or channel, where it is nebulized by a jet of gas emerging from a very small hole in the base of the channel. liquid aerosol gas Figure 4.3 A V-groove nebulizer Conceptually, V-groove nebulizers evolved from the Babington nebulizer (104,105), in which liquid flows in a thin film over a small hole or slot, to be nebulized by an emerging jet of gas. Many variations on this basic design have been described in the literature ( ), and several different V-groove nebulizers are available commercially (113,122,123). V-groove nebulizers have been made from glass, and from various plastics. Nebulizers made from inert engineering plastics can be used with any liquid samples, including hydrofluoric acid, provided of course that the spray chamber and torch are also compatible with the samples. For samples containing hydrofluoric acid, for example, a plastic spray chamber and a torch fitted with an alumina injector tube are recommended. These are available from Varian. Because V-groove nebulizers do not act as pumps, an external pump is needed to transport the sample to the nebulizer. As with other pneumatic nebulizers, care is required in selecting appropriate pumping rates and in adjusting the pump to avoid pulsations in the sample delivery rate. The great advantage of the V-groove nebulizer is that the sample does not have to pass through any extremely narrow passages. The sample duct is typically about 1 mm in diameter, so there is no risk of it becoming blocked by undissolved material present in analytical solutions. An additional advantage is that the gas orifice is continually washed with fresh solution, so there is less possibility of its becoming clogged with solids deposited from analytical solutions. The V-groove nebulizer is ideally suited for the analysis of samples containing high levels of dissolved or suspended solids. ICP analysis of solid samples introduced into the plasma by nebulization of slurries of finely powdered material with a V-groove nebulizer has been reported in several papers (26, ). To avoid blockages that might be caused by analytical solutions entering the gas orifice, it is good practice to rinse the nebulizer thoroughly between samples and before turning off the nebulizer gas Publication date: 10/99

71 Nebulizers In addition to nebulizing liquids containing high levels of solids, the V- groove nebulizer performs very well with less demanding samples, giving precision and sensitivity similar or better to those achieved with glass concentric nebulizers. The V-groove nebulizer is a very versatile and useful nebulizer for ICP spectrochemical analysis. It accommodates the whole range of liquid samples, and is physically robust. The only delicate part is the gas orifice, which is protected from damage by its position at the base of the V-groove. Maintenance of V-groove nebulizers The only problem that may occur with a V-groove nebulizer is blockage of the gas orifice. With proper care to ensure that the nebulizer is clean before connecting it to the gas supply, and with routine flushing with clean rinse liquid between samples and at the end of an analysis, this problem should not arise. The gas orifice is easily damaged, so it is very risky indeed to attempt to remove any blockage by probing. To do so would almost certainly enlarge the gas orifice and ruin the nebulizer. Analytical performance is determined by the aerosol characteristics and the gas throughput. If the gas orifice becomes enlarged for any reason, the operating pressure will have to be reduced to achieve the correct gas throughput. Eventually it will no longer be possible to achieve efficient nebulization at a gas throughput suitable for the ICP. The best way to clear a blocked gas orifice is to apply vacuum to the gas inlet while immersing the V-groove in clean, particle-free water. If that is not successful, it may be possible to remove the blockage by dissolving it out with appropriate acids or other solvents. The precision and sensitivity of the V-groove nebulizer is critically dependent on the size of the gas orifice and the gas pressure. The optimum nebulizer gas pressure should be determined for each nebulizer. 4.4 Summary For routine analysis of most aqueous and organic samples, glass concentric nebulizers will give satisfactory sensitivity and precision. Care must be taken to ensure that the samples are free of particulate matter that might clog the nebulizer. Such nebulizers are not suitable for samples containing hydrofluoric acid, nor for samples containing undissolved material. Despite their extensive use in ICP spectrometry, cross-flow nebulizers offer no major advantages over modern glass concentric nebulizers. Their only advantage over V-groove nebulizers is their ability to operate without a sample pump. However, since sample pumps are Publication date: 10/

72 Nebulizers nearly always used in practice to avoid hydrostatic head effects, this is of little practical significance. Cross-flow nebulizers are not offered by Varian. The V-groove nebulizer is a universal nebulizer, giving good sensitivity and precision for dilute aqueous and organic samples, as well as handling samples with high levels of dissolved or suspended solids. This nebulizer is also suitable for samples containing hydrofluoric acid. An ICP torch with an alumina injector tube is recommended for samples containing this acid. It is the operator s responsibility to adjust the operating parameters to obtain the best conditions for each analysis. With both the glass concentric nebulizer and the V-groove nebulizer, the nebulizer pressure should be selected for the best precision and satisfactory sensitivity for most analyses. As there may be considerable variation from one nebulizer to the next, the optimum operating pressure should be determined for each nebulizer Publication date: 10/99

73 5 Optimization Optimization 5.1 Optimization with Liberty The Liberty ICP-AES system provides computer control of the full range of operating and measuring parameters, except the nebulizer pressure. The graphics display provided in the software can be used to monitor the intensity of individual emission lines as the various parameters are changed. This capability to define optimum settings for individual lines provides the flexibility to configure the instrument to suit the analytical requirements for all sample types. This chapter provides guidelines for establishing the optimum parameter settings for individual emission lines. The information presented in this chapter will help the analyst establish optimum conditions with minimal development time. The observed intensities of the spectral lines of an element are affected by the settings of a number of instrument parameters, which are not necessarily the same for all types of matrices. Therefore, it is important to establish the optimum operating conditions for the specific type of sample required to be analyzed. Important parameters in the optimization procedure include the RF power, the viewing or observation height, plasma gas flow and the nebulizer gas flow. The remaining parameters such as sample uptake rate and auxiliary gas flows all play a secondary role. However, the first step in establishing the conditions required for good analytical performance is to eliminate potential problems rather than attempt to compensate for them. For example, the potential problems caused by spectral interferences can be eliminated by selection of the appropriate emission line Emission line selection The primary emission line of an element is typically selected to provide maximum sensitivity. However, if spectral overlap is found on this line, it is possible in most cases to select an alternative interference-free line for that particular element. Many spectral problems encountered in ICP can be avoided by the selection of emission lines that are free from spectral interferences. Publication date: 10/99 5-1

74 Optimization If, on the other hand, an alternative interference-free line cannot be found, the capability to change to a higher grating order, and therefore higher resolution, is another option that Liberty offers for the elimination of a spectral interference Effect of resolution The first step in establishing the occurrence of spectral interference is to compare the spectrum of a pure solution of the analyte of interest with that of the sample. If spectral interference is established, select a higher grating order to try to resolve the overlapping peaks in the sample. Liberty s typical resolution and wavelength range for each grating order is shown in Table 5.1. Table 5.1 OrderRange Liberty Grating Orders & Resolution Typical Resolution 1st order nm nm 2nd order nm nm 3rd order nm nm 4th order nm nm The default grating orders automatically set by Liberty when an emission line is selected provide the best signal to background ratio. However, Liberty also provides the flexibility to change the grating order for a particular wavelength when higher resolution is necessary to avoid spectral interferences. 5.2 Predicting trends during optimization Having ensured that the selected emission lines are free from spectral interferences, either by appropriate choice of line or by using higher resolution, the next step is the optimization of the plasma operating parameters. Since Liberty provides the capability to optimize the full range of plasma conditions, understanding the effect of each parameter on a particular emission line will significantly reduce the time required to derive the optimum conditions The origin of emission lines Each element possesses a unique, characteristic set of electronic energy levels. An excited atom releases energy in the form of radiation at wavelengths corresponding to the transitions between the different energy levels as it returns to its stable state. Predicting the 5-2 Publication date: 10/99

75 Optimization intensity trends of the individual emission lines as different parameters are changed requires an understanding of the origin of the line because not every line for a particular element will display the same trend. Generally, because elements in the plasma exist in equilibrium between ionic and atomic species, it is possible to group lines into transitions originating from ATOMIC states (denoted by 'I', e.g. Pb I nm) and those originating from singly ionized states (denoted by 'II' e.g. Pb II nm). Further, because there can be a large spread in the transition energies for different emission lines, the lines can be characterized as HARD (i.e. lower wavelengths), or SOFT (i.e. higher wavelengths). Figure 5.1 shows how emission lines can be classified. I (atomic) I (atomic) Hard Low wavelength II (ionic) Soft High wavelength II (ionic) Figure 5.1 Wavelength groups Examples of HARD and SOFT emission lines: HARD SOFT ATOMIC P I nm Na I nm As I nm Li I nm Zn I nm IONIC Cd II nm Ba II nm Al II nm Nd II nm Pb II nm The significance of these groupings is that optimum conditions for a HARD ATOMIC line, for example, may represent poor conditions for a SOFT IONIC line. Therefore, it is necessary to optimize for each line on an individual basis. This can be readily performed with the Liberty software as it allows the analyst to store different conditions for every emission line. Publication date: 10/99 5-3

76 Optimization Once the selected analytical lines have been classified as described previously, decisions on which parameters will enhance the sensitivity of individual spectral lines can be made based on the guidelines in the remainder of this chapter Plasma viewing height The intensity of the emission at a particular wavelength varies according to the observed region in the plasma. The optimum viewing position is not necessarily the same for each element or emission line because each line has different characteristics. Further, the optimum viewing position varies for different sample types and also for different operating conditions. Figure 5.2 Viewing height profiles of different elements Since the viewing height is a critical parameter that can significantly enhance sensitivity, Liberty provides an automatic viewing height optimization package (radial instruments only). A mirror, under stepper motor control, is automatically driven to view different heights in the plasma, taking measurements along the way to derive the optimum viewing position. The viewing position can be derived automatically in one of two ways: by maximum intensity or by signal-to-background ratio (SBR). These may not necessarily define the same optimum viewing position, because the background levels also change depending on the viewing position. Therefore, it is best to optimize by intensity when analyzing high concentration levels because the background signal will be insignificant compared with the very high analyte signal. However, the viewing height should be optimized by SBR for the analysis of low concentration levels where the background signal will contribute a significant proportion to the total signal Minimizing the effect of easily ionizable elements The viewing height can also be adjusted to minimize the interferences caused by the presence of easily-ionizable elements in the sample matrix. This is particularly important for solutions containing high levels of Group I and II elements. 5-4 Publication date: 10/99

77 Optimization Certain conditions, mainly determined by the plasma viewing height, can affect the accuracy of results. Very low viewing heights can result in an interference which is reversed at higher viewing heights. Figure 5.3 shows this effect. The interference can be minimized by setting the viewing height to the point where the two curves intersect. Figure 5.3 The effect of an easily ionizable element (Na) on the signal for 5 ppm Sr Minimizing the effects of molecular bands Molecular bands can pose a problem in certain parts of the spectrum, particularly in the nm wavelength range. The bands can appear as individual lines and can cause spectral interference or, at low analyte levels, can be mistaken for the analytical peak if the analytical search window has not been properly set. Lines such as Zn I nm, Cd II nm, Pb II nm and Be II nm are examples of lines for which molecular band lines can be troublesome with fixed viewing height ICP spectrometers. Optimizing the viewing height on Liberty can minimize and even eliminate these molecular band lines. Figure 5.4 shows that interfering band lines significantly reduce the detectability of Cd when the viewing height is set at 16 mm. However, setting the viewing height to 3 mm completely eliminates the band lines, resulting in an enhancement in the detectability of Cd. Figure 5.4 Molecular bands near the Cd II nm line can be eliminated by setting the viewing height to 3 mm Publication date: 10/99 5-5

78 Optimization Effect of RF power The RF power is another critical parameter that can significantly effect the signal of selected emission lines. High power settings will increase the intensity of the HARD lines because the subsequent higher energy in the plasma will promote the high energy transitions. The intensity of SOFT lines on the other hand, generally do not increase significantly with higher powers because they correspond to low energy transitions and therefore are already near their maximum sensitivity at lower powers. The background also tends to rise with higher power. The degree the background is raised will depend on the wavelength. Generally, the higher the wavelength the more the background is raised. It is important to monitor the effect on background because, depending on the wavelength, an increase in the measured signal with higher power may only be due to an increase in the background and not necessarily to a net increase in analyte signal. Figure 5.5 shows the effect of power on the Pb II line at nm compared to the trend observed for the Pb I atomic line at nm. While increasing the power increases the total intensity of both lines, only the Pb II line shows a significant increase in the net intensity. a) b) Figure 5.5 Effect of power on Pb II & I lines a) Pb220 ionic b) Pb405 atomic Organics When organic solvents are nebulized into the ICP, a green bulletshaped region can be seen in the central channel of the plasma. This results from the emission of light by C 2 molecules produced as the solvent molecules break down. It is important that this region not 5-6 Publication date: 10/99

79 Optimization extend into the observed part of the plasma. Higher power reduces the size of the bullet, and helps to keep it out of the observed region Nebulizer pressure The nebulizer pressure is another critical parameter that can significantly enhance the intensity of selected lines. The intensity of HARD lines can be increased significantly by reducing the nebulizer pressure. Reducing the nebulizer pressure effectively increases the residence time of the analyte in the plasma, giving the analyte a longer time to acquire the energy for the high energy transitions. The Pb II nm line is an example of a high energy transition that shows maximum sensitivity at low nebulizer pressures. Typical nebulizer pressures can be as low as 100 KPa, but note that this depends on other plasma operating parameters, including the type of nebulizer used. The optimum intensity of SOFT lines, on the other hand, is achieved at the low energy conditions resulting from the higher nebulizer pressures. The Pb I nm line will require a nebulizer pressure of typically 180 KPa. A compromise nebulizer pressure, weighted in favor of the lines requiring the maximum intensity, should be set for an analytical method that contains both HARD and SOFT lines. Figure 5.6 shows the effect of nebulizer pressure on both the nm and nm Pb lines. Figure 5.6 The effect of nebulizer pressure on signal intensity at the Pb nm and Pb nm lines Publication date: 10/99 5-7

80 Optimization Organics Lower nebulizer pressures increase the residence time of the nebulized material in the plasma. This promotes the decomposition of molecules such as C 2 that cause the green bullet when organic solvents are nebulized into the ICP. A reduced nebulizer alleviates the problems of poor signal, increased background and carbon deposition on the torch that are associated with a large bullet region Gas flows Plasma gas Increasing the plasma gas increases the size of the hot, analytical zone within the plasma. The intensity of HARD lines tends to increase with the higher plasma gas flows because of the resultant increase in energy available for the high energy transitions. Lower plasma gas flows can reduce background levels and therefore increase the signal to background ratio for the SOFT lines. It is recommended that a wavelength scan be done to show the effect of changing the plasma gas flow, particularly if there are molecular emission band lines near the analyte line. Band lines can appear or become a significant problem with reduced plasma gas flows. Organics By increasing the temperature of the central channel, higher plasma gas flows promote the decomposition of molecules such as C 2 that form the green bullet when organics are introduced into the ICP. Higher plasma gas flows can reduce the problems of poor sensitivity, increased background and carbon deposition on the torch that are associated with a large bullet region. Auxiliary gas The main effect of the auxiliary gas is to lift the plasma away from the torch. This is especially important for the analysis of organics to minimize carbon build up on the injector tube. It is also significant in the analysis of solutions with high dissolved solids to minimize salting up of the injector tube. Higher auxiliary gas flows can also reduce molecular band lines. Organics Higher auxiliary gas flows prevent carbon build up on the injector tube. 5-8 Publication date: 10/99

81 Optimization Peristaltic pump speed Liberty s computer-controlled variable speed pump can be set to different flow rates depending on the sample type, thus reducing problems associated with variations in solution viscosity. The capability to control the rate at which the solution is pumped through the nebulizer is particularly important with organic samples, where the nebulization efficiency is higher than for aqueous solutions. Increasing the sample flow rate generally increases the signal and, because of thermal quenching, reduces the background levels. However, along with the benefits, there may also be an increase in noise levels, and more sample is consumed. Typical sample uptake rates are 1 ml/min (corresponding to rpm using blue pump tubing). If a lower sample uptake rate is required (for example, for organics) smaller internal diameter pump tubing is recommended instead of reducing the pump rate. This removes one source of imprecision by reducing pulsations, which become significant at lower pump rates. Organics Organic solvents have a high transport efficiency compared to aqueous solutions. Since the plasma will remain stable provided only 1-2% of the original sample reaches the plasma, it is necessary to reduce the sample uptake rate for organic solvents. As described previously, to maintain precision of analysis, it is recommended that smaller internal diameter pump tubing be used to reduce sample uptake rates as an alternative to reducing the pump speed Filters Optical filters can be selected either to reduce stray light or to eliminate possible spectral interferences resulting from higher order overlap. Liberty automatically sets the optimum filter depending on the wavelength and the grating order selected. However, Liberty also provides the option to select alternative filters (or no filter) to overcome the spectral and stray light problems arising from different sample matrices. Selecting no filter results in an increase in light throughput, and therefore sensitivity, but care must be taken to ensure that there is no spectral overlap due to wavelengths from other orders. This can occur particularly for wavelengths above 300 nm where Liberty uses the wide range PMT. For example, the interference caused by 1000 ppm Fe on the Na I nm line can be eliminated by selecting the appropriate filter. Publication date: 10/99 5-9

82 Optimization a) b) Figure 5.7 Eliminating higher order overlap a) without filter b) with filter Effect of the analytical search window The analytical search window is the wavelength region within the scan window where the peak search routine is performed to locate the analytical peak before replicate measurements are performed. The size of the analytical search window plays an important role, particularly when more than one peak is observed in a scan. Being able to determine the true analytical peak can become a significant problem when the size of the search window is set so that more than one peak appears within the window. This becomes even more significant when the analytical peak is smaller than the neighboring peaks, or if there is no analyte peak at all. In these cases, it is possible that a neighboring peak will incorrectly be identified as the analytical peak. The above problems can be eliminated by reducing the size of the analytical search window so that the required analytical peak is the only peak that appears within the window. Further, Liberty provides a peak tracking routine for the accurate measurement of samples with no detectable analyte. The peak tracking routine stores the position of the analyte peaks whenever a calibration or a reslope is performed. When Liberty s peak search routine determines that no analyte peak is present within the analytical search window, replicate measurements are performed at the position within the search window stored by the peak tracking routine Publication date: 10/99