COOKBOOK Book One AI 12 Atomic Absorption Spectrometer Updated: Jan 22
AI 12 Cookbook Table of Contents AI 12 COOKBOOK Table of Contents BOOK ONE- FAAS Chapter 1: Theory of AAS Introduction 2 Flame Atomic Absorption Spectrometry (FAAS) 3 Graphite Furnace Atomic Absorption Spectrometry (GFAAS) 4 Vapor Hydride Generation Atomic Absorption Spectrometry (VG AAS) 5 Chapter 2: AAS Instrumentation Fundamentals 9 Light Source Atomizer Optics Detector Optics 1 Lenses Mirrors Monochromator Diffraction Grating Slit Width Atomizer 15 Flame Graphite Furnace Detector 18 Chapter 3: Background Correction Fundamentals 22 The Frequency of Measurement The Interval between Measurements The Function used to Calculate Net Absorption Spectral and Structured Backgrounds The Effect on the Linear Working Range Deuterium (D2) Background Correction 24 Smith-Hieftje (S-H) Background Correction 27 Zeeman Background Correction 28 Comparison of Background Correction Methods 29 i
AI 12 Cookbook Table of Contents Chapter 4: Comparison of Analytical Techniques Things to Consider 34 Applications 34 Expected Concentration Ranges 34 Elements 34 Atomization Efficiency 34 Interferences 35 Spectral Background Matrix Detection Limits 35 Sensitivity 35 Precision 35 Linear Working Range 35 Minimum Sample Volume 36 Sample Throughput 36 Sample Usage 36 Total Dissolved Solids 36 Method Development 36 Ease of Use 36 Automation/Unattended Operation 36 Costs 37 Initial Investment Running Costs Chapter 5: Standard and Sample Preparation Apparatus 41 Water 41 Standard and Blank Solutions 41 Sample Solutions 42 Storage of Solutions 42 Calibrations 43 Matrix Effects 43 Chemical Interferences 43 Incomplete dissociation of analyte compounds Ionization ii
AI 12 Cookbook Table of Contents Chapter 6: FAAS Analytical Data Sheets Introduction 47 THE ELEMENTS Aluminum, Al 49 Antimony, Sb 5 Arsenic, As 51 Barium, Ba 52 Beryllium, Be 53 Bismuth, Bi 54 Boron, B 55 Cadmium, Cd 56 Calcium, Ca (air/acetylene) 57 Calcium, Ca (nitrous oxide/acetylene) 58 Cesium, Cs 59 Chromium, Cr (air/acetylene) 6 Chromium, Cr (nitrous oxide/acetylene) 61 Cobalt, Co 62 Copper, Cu 63 Dysprosium, Dy 64 Erbium, Er 65 Europium, Eu 66 Gadolinium, Gd 67 Gallium, Ga 68 Germanium, Ge 69 Gold, Au 7 Hafnium, Hf 71 Holmium, Ho 72 Indium, In 73 Iridium, Ir 74 Iron, Fe 75 Lanthanum, La 76 Lead, Pb 77 Lithium, Li 78 Lutetium, Lu 79 Magnesium, Mg 8 Manganese, Mn 81 Mercury, Hg 82 Molybdenum, Mo 83 Neodymium, Nd 84 Nickel, Ni 85 Niobium, Nb 86 Osmium, Os 87 Palladium, Pd 88 Phosphorous, P 89 Platinum, Pt 9 Potassium, K 91 Praseodymium, Pr 92 Rhenium, Re 93 Rhodium, Rh 94 Rubidium, Rb 95 Ruthenium, Ru 96 Samarium, Sm 97 iii
AI 12 Cookbook Table of Contents Scandium, Sc 98 Selenium, Se 99 Silicon, Si 1 Silver, Ag 11 Sodium, Na 12 Strontium, Sr 13 Tantalum, Ta 14 Tellurium, Te 15 Thallium, Tl 16 Tin, Sn 17 Titanium, Ti 18 Tungsten, W 19 Uranium, U 11 Vanadium, V 111 Ytterbium, Yb 112 Yttrium, Y 113 Zinc, Zn 114 Zirconium, Zr 115 Chapter 7: Practical Applications - FAAS Marine 12 Water 121 Biological 122 Food 123 Agricultural 124 Petroleum 127 Miscellaneous 129 References 132 iv
1 - Theory of AAS Chapter 1: Theory of AAS Introduction Flame Atomic Absorption Spectrometry (FAAS) Graphite Furnace Atomic Absorption Spectrometry (GFAAS) Vapor Hydride Generation Atomic Absorption Spectrometry (VG AAS) 1
1 - Theory of AAS Introduction The essential elements of the theory behind the analytical technique of atomic absorption spectroscopy (AAS) are compacted into the following paragraphs. Atomic absorption spectroscopy (AAS) relies on the fact that the light absorption of free atoms [1-7]. All atoms can absorb light, but only at discrete wavelengths corresponding to the energy requirements of the particular atom. In other words, each element absorbs light at some specific and unique wavelength and does not absorb light at all on other wavelengths. For example, in a sample with multiple elements (say, copper, lead, iron, and nickel), only copper will absorb light that is at the characteristic wavelength for copper. Furthermore, the amount of light absorbed depends on the number of absorbing atoms that are present in the light path. All these factors enable AAS to be used as a tool for quantitative analysis. In practice, measuring the amount of light absorbed by several known standards allows a calibration curve to be constructed. Then, the unknown concentration of a sample can easily be determined based on the amount of light it absorbs. The amount of light energy absorbed at this wavelength depends on the concentration of the atoms in the medium (as dictated by Lambert s law and Beer s law). Lambert s law states that the portion of light absorbed by a transparent medium is independent of the intensity of the incident light and each successive unit layer of the medium absorbs an equal fraction of the light passing through it. Beer s law states that the light absorbed is proportional to the number of absorbing atoms in the medium. Mathematically, when light of intensity I o passes through a medium of length x with atom concentration of C, the intensity I of the light beam emerging from the medium is given by: I = I o e -kcx where k is a proportionality constant (the absorption coefficient). The absorption of the medium, A, is defined to be: A = lg (I o /I) = kcx This equation states that the absorbance, A, of the medium is linearly proportional to the concentration of the absorbing atoms. The absorption coefficient (or absorptivity), k, can be determined by constructing a calibration curve (i.e. plotting the observed absorbance versus the known sample concentration). The slope of the calibration curve is kx, and x is easily measurable or already known. Unknown sample concentrations may be determined from the calibration curve based on their measured absorbances. Any way that you look at, every AAS experiment can be broken down to the following procedure: A sample has an unknown amount of a known element (e.g. the sample is known to contain lead, but not how much lead). The sample must be made into a homogeneous, liquid solution (if it is not already). 2
1 - Theory of AAS A blank solution must be prepared. This blank must contain none of the element of interest. A series of standard solutions must be prepared. These standards have known (but varied) concentrations of the element of interest. These standards are used to prepare a calibration curve. Analyze the blank solution to determine the "blank" absorbance value. This is the absorbance value for a sample with a zero concentration of the element of interest. Individually analyze all of the standard solutions. Construct a calibration graph. For the blank and each standard solution, plot its absorbance value against its concentration. Analyze the unknown sample. The concentration of the unknown sample, based on its measured absorbance value, can be determined from the calibration curve. There are three main techniques of atomic absorption spectrometry: Flame atomic absorption spectrometry (FAAS), graphite furnace atomic absorption spectrometry (GFAAS), and vapor hydride generation atomic absorption spectrometry (VG AAS). Each has its own distinct advantages and disadvantages. Each has specific applications for which they are the superior AAS technique. For example, FAAS is suitable for analyses where the sample is above trace quantities, and where high sample throughput, ease of use, and low initial investment are required. GFAAS is ideal for samples that are in the parts per billion (ppb) range and where the sample volume is limited. VG AAS is useful for determining elements that form volatile hydrides at sub-trace levels. A brief outline of each technique is provided below. Flame Atomic Absorption Spectrometry (FAAS) The atomization process by which the atom population is generated is of primary importance in AAS because analysis depends entirely on the fact that free, uncombined atoms will absorb light of a particular wavelength. The key to successful operation of an atomic absorption spectrometer lies in generating a supply of free, uncombined atoms in the ground state and exposing this atom population to light at the characteristic absorption wavelength. The source of energy for free atom production is heat, most commonly in the form of an air-acetylene or nitrous oxideacetylene flame (Flame AAS). With this type of atomizer, the sample solution is introduced in the form of a spray of fine droplets. This is accomplished by a pneumatic nebulizer in most case. The spray of droplets is carried by a gas (usually the oxidant for the flame) through the spray chamber and burner head into the flame. The heat of the flame is sufficient to dry (desolvate) each of the sample droplets and (usually) to decompose chemical components from the resulting dried particles into their constituent atoms. Thus a population of ground state atoms is created in the flame and atomic absorption measurements can be made. Flame systems for AAS give excellent results, and they are simple, inexpensive, convenient and extremely useful. They permit rapid analytical measurements through a very simple sample introduction technique. The major limitation of flame AAS is that the burner-nebulizer system is a relatively inefficient sampling device. Only a small fraction of the sample that is taken up reaches the flame. Additionally, once atomized, the sample passes quickly through the light path. An improved sampling device would atomize the entire sample and retain it in the light path for an extended period of time to enhance the sensitivity of the 3
1 - Theory of AAS technique. Electrothermal atomization using a graphite furnace provides these features. Graphite Furnace Atomic Absorption Spectrometry (GFAAS) Graphite Furnace Atomic Absorption Spectrometry (GFAAS) has gained a reputation in the field of analytical chemistry as a routine technique for the determination of very low levels of trace metals in a variety of sample matrices. With GFAAS, the flame has been replaced by an electrothermally heated graphite tube. The sample is injected directly into the tube as a small liquid volume (5 to 1 µl), which is then heated in a programmed series of steps to remove the solvent and major matrix components. Free analyte atoms in the gaseous state are eventually produced inside the graphite furnace by rapidly heating with a strong electric current to temperatures between 15 and 3 C. All of the analyte is atomized, and the atoms are retained within the tube (and the light path, which passes through the tube) for an extended period (typically to.5 second). The performance of this technique relied on the stability of the temperature. The recently developed transversely heated integrated contact graphite furnace ensures the temperature over the entire length of the tube is very uniform. As a result, sensitivity and detection limits are significantly improved while matrix interferences and memory effects are reduced. After the measurement, the analyte vapor is then purged from the graphite furnace by helium or argon gas. The magnitude of the absorbance is recorded as a function of time by the readout system. The mechanism of atomization in a graphite furnace depends significantly on the chemical nature of the analyte element, the availability of active sites, the gaseous species within the graphite furnace, the atomization temperature, and the graphite furnace itself. After drying and pyrolysis the analyte atoms may be present in reduced, oxidized or complex form. Upon heating the graphite tube to the atomization temperature, free analyte atoms are generated by: (1) vaporization of the reduced form from the surface, (2) by dissociation of the oxide form into gaseous free analyte atoms as the graphite tube heats up, and (3) vaporization as oxides (or any other molecular species) and dissociation into free analyte atoms in the gaseous phase. Examples of these mechanisms are given below - where M is an analyte, C is carbon and O is oxygen and subscripts g and s refer to gaseous and solid phases: M (s) M (g) MO (s) + C (s) M (g) + CO (g) MO (s) MO (g) MO (g) M (g) + O (g) CO (g) + O (g) CO 2 (g). Analysis times for GFAAS are longer than those for FAAS, and fewer elements can be determined with this technique. Nonetheless, GFAAS s enhanced sensitivity, ability to analyze very small sample sizes, and ability to directly analyze certain types of solid samples significantly expand the capability of atomic absorption spectrometry. 4
1 - Theory of AAS Vapor Hydride Generation Atomic Absorption Spectrometry (VG AAS) For the determination of As, Bi, Ge, Pb, Sb, Se, Sn, Te and Hg with AAS, vapor/hydride generation (VG) techniques have been proven to provide very high sensitivities and reduced interferences. With VG AAS, analytes are first reduced to their corresponding volatile hydrides (or metallic form for Hg) by sodium borohydride in an acidic medium. The vapors are then transported by a carrier gas into the atomizer for atomization and AA measurement. Aurora Instrument s AI 12 uses an open ended, temperature controlled electrothermally heated quart tube for continuous flow vapor/hydride generation determinations. The heating unit can be installed and removed easily. 5
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2 - Instrumentation Chapter 2: AAS Instrumentation Fundamentals Light Source Atomizer Optics Detector Optics Lenses Mirrors Monochromator Diffraction Grating Slit Width Atomizer Flame Graphite Furnace Detector 8
2 - Instrumentation Fundamentals There are four components that are essential to every AAS instrument: a light source, an atomizer, an optics system, and a detector. Light Source Usually a hollow cathode lamp (HCL) is used as the light source in AAS. A less common light source is the electrodeless discharge lamp (EDL). An HCL produces an intense, narrow line emission of light at a wavelength that is specific to the element that the HCL cathode is coated with. Most elements emit at multiple wavelengths, but all emissions are intense, sharp lines (called resonance lines). For example, a copper HCL emits at 324.75 nm, 327.4 nm, 222.6 nm, 249.2 nm, and 244.2 nm. Generally, one line is more intense than the others and is therefore the most sensitive and useful line for AAS analysis. HCLs are coated with the element of interest to produce light of the resonance wavelength(s) that is/are specific to element. When this light is passed through a medium that contains atoms of the same element, the light will be partially absorbed. Atomizer The purpose of an atomizer is to create a population of free atoms that is suitable for absorption of light. The atomizer must have an energy source in order to do this. Usually the energy comes from heat, and the most common source of the heat is a flame (either an air/acetylene or a nitrous oxide/acetylene flame). An AAS instrument with a flame atomizer is called a flame atomic absorption spectrometer (FAAS). With a flame atomizer, the sample is introduced into the flame as an aerosol (a mist of tiny droplets). The flame burner head [8] is designed to be long, thin, and aligned with the light path. Such a design causes the aerosol atoms to be atomized in the flame while they are in the light path so that they can absorb the light. A very important part of the atomizer system is the nebulizer. The nebulizer is responsible for nebulizing a liquid sample into an aerosol. The sensitivity of a FAAS instrument depends heavily on how efficiently the nebulizer can convert a sample to an aerosol. Optics The optics system of an AAS instrument is responsible for getting the light from the light source to the detector. Along the way, the light must be passed through the atomized sample and through a monochromator. A monochromator is used to isolate specific wavelengths from the bulk light that it receives. For example, it may be necessary to isolate the analytical wavelength of interest from light that was emitted from the fill gas of the HCL, or from stray room light that entered the spectrometer. Detector The detector part of an AAS instrument measures how much light is transmitted through the spectrometer. Most commonly a photomultiplier tube (PMT) is employed for this purpose. While the above four components are the essential ones of an AAS instrument, there are still others that play important roles. 9
2 - Instrumentation There inevitably are electronic devices that convert the signals from the detector into something that is useful for the human researcher. Older instruments used to employ signal meters and plotters that would chart the strength of the absorbance signal on a moving strip of paper. Instruments of today have replaced the meter and chart recorder with computer software that has many more capabilities. Modern software provides real-time plots of absorbance versus time, constructs calibration curves, and calculates statistics such as RSD values. On the starting end of the sample analysis spectrum, computer software can keep track of samples that are running, that you will run, and that you did run. It can also be used to setup and run the instrument without operator intervention at all. Optics The ideal optics system will have the following characteristics: Have 1% efficient light throughput. In other words, if there is no atomized sample in the light path, then 1% of the light from the source will reach the detector. Allow zero stray light. Provide a high signal to noise ratio (S/N). Provide absolute selectivity and resolution for the wavelength being measured. Provide constant dispersion, regardless of wavelength. Have no optical aberrations. Provide unique performance over a wide wavelength range. Unfortunately, there is no such thing as an ideal optics system. The best that you can hope for is to have a system that is optimized for your needs. There are two ways to control the path of light within an optics system: with mirrors or with lenses. Most optics systems make use of both. Lenses Good quality lenses are made from silica glass and have good light transmission over a broad wavelength range (~19-9 nm). Transmission losses occur at both interfaces of the lenses (i.e. at the lens surface where the light enters and at the lens surface where the light exits). The losses are typically between 1-14% for each lens in the optical path. A feature of lenses that must be kept in mind is that the refractive index of the lens is dependent on the wavelength of the light being refracted. This means that the focal length of the lens will be different for every wavelength. Rather than moving a lens to keep the focal point in the same position for different wavelengths, optics systems will keep their lenses fixed and tolerate the relatively minor losses associated with the changing focal lengths. Most lenses used are designed for wavelengths in the UV region. This is because most analytical wavelengths are in this range and the median refractive index is about 25 nm. For an air/acetylene flame, where the path length is around 1 cm, the losses due to focal length differences are negligible. For a nitrous oxide/acetylene flame, however, the path length is only 5 cm and the losses can start to become noticeable. In GF AAS, where the sample atomization occurs 1
2 - Instrumentation only at a small point in the center of the graphite tube, the focal length from the light source is absolutely critical. As the light source wavelength is increased, the focal point will move away from its original position at the center of the graphite tube. Because of this, optics systems for GF AAS systems usually employ only mirrors, and not lenses. Mirrors Mirrors perform much better than lenses, both in terms of reflecting efficiency and focal length change. A mirror has a very thin (e.g. 2 µm) top coating of aluminum that reflects more than 9% of the light that strikes it (for the range 19-9 nm). Also, when light reflects off a mirror there is no change in the focal length. That is, the focal length of a mirror depends only on its shape and is independent of wavelength. Plane mirrors are used to fold light and curved mirrors (also called collimating mirrors) are used to focus light. For example, a plane mirror is needed to fold light around a 9 corner, and a curved mirror is needed to focus that light onto an entrance slit. While the use of mirrors does solve the problem of focal length differences, it raises the challenge of designing and manufacturing focusing mirrors that are free from other optical aberrations, such as astigmatism. Because of the thinness of the mirror coating, mirrors are extremely fragile and must be handled with care. Finger prints, and even soft tissues, can irreversibly damage a mirror s coating. Reactive liquids and gases can even cause harm. To increase their longevity, most optics mirrors are further coated with a transparent silica or magnesium fluoride film for protection. Even still, one should avoid any kind of direct contact with mirror surfaces. Monochromator There are several different designs of monochromators available. No matter which design is used in an AAS instrument, however, some fundamental principles remain the same. Light enters the monochromator through an entrance slit. The light is folded and focused in the monochromator by use of mirrors. The light is dispersed into its component wavelengths by some sort of diffracting element. The diffracted light then leaves the monochromator through an exit slit. The most common monochromator design is the Czerny-Turner design, which is used in the AI 12. A schematic of this type of monochromator is shown in Figure 2.1. The Czerny-Turner monochromator uses two separate mirrors to collimate and focus light. Mirror #1 receives the light that was focused through the entrance slit. The light that reflects off this mirror is collimated into parallel beams and then strikes the diffraction grating, which diffracts the light into a spectrum of wavelengths. This spectrum is dispersed at a variety of angles, depending on the wavelength of each component of the spectrum. The light then strikes Mirror #2, which focuses the light through the exit slit and into the detector. Some monochromators make use of only one mirror, but there is a definite advantage to using two mirrors. The two mirrors will each be smaller than a single mirror, so they are easier to manufacture. This means that there is less chance for surface aberrations and therefore allows optimum light throughput and resolution. 11
2 - Instrumentation Figure 2-1 Schematic diagram of a Czerny-Turner monochromator The detector is dumb it doesn t know whether the light it receives is an analytical signal or not. The detector merely counts the number of photons that it receives (the intensity of the light) and sends a signal to an amplifier then sends back to computer. So, it is the job of the monochromator to isolate a specific, narrow resonance line from the rest of the spectrum before the light is allowed to reach the detector. The monochromator ensures that only the analytical wavelength of light reaches the detector. This wavelength selectivity is achieved by rotation of the diffraction grating with respect to the incident light. Turning the grating moves the spectrum across the exit slit, and therefore changes the wavelength of light that passes through the slit. Another means of controlling the light that passes through the exit slit is by changing the width of the slit. A narrower slit allows two closely spaced wavelengths to be resolved, but it also decreases the light throughput of the optics system as a whole. If there are no interfering wavelengths close to the analytical wavelength, then a wider slit can be used to increase the light throughput, say, improve the signal noise ratio. Other, less common used monochromator designs are the Ebert-Fastie and the Littrow designs. The Ebert-Fastie monochromator design makes use of a single, large mirror to focus light. See Figure 2.2 for a schematic. One area of the mirror collimates incoming light onto the diffraction grating and then another area of the mirror focuses the light dispersed from the grating onto the exit slit. This single mirror does both of the jobs of the double mirrors in the Czerny-Turner design. A disadvantage of the Ebert-Fastie design is that the single mirror must be large. This increases the probability of surface imperfections during the manufacture of the mirror, which in turn impairs the light throughput and resolution of the monochromator. An Ebert-Fastie monochromator is, however, less expensive than a Czerny-Turner. 12
2 - Instrumentation Figure 2-2 Schematic diagram of an Ebert-Fastie monochromator The Littrow monochromator design is similar to the Ebert-Fastie design in that it also employs only one mirror. See Figure 2.3 for a schematic. The difference lies in that the Littrow monochromator uses the same area of the single mirror for collimating the light onto the grating as for focusing the dispersed light onto the exit slit. This fact increases the chances of optical aberrations (even more so than the Ebert-Fastie design). Figure 2-3 Schematic diagram of a Littrow monochromator Diffraction Grating There are two types of diffraction gratings that are commonly used today in AAS instruments: ruled gratings and holographic gratings. Ruled gratings have been around longer than holographic gratings, which were only introduced in the late 196s. A grating, in general, is a closely spaced series of grooves in a flat, reflecting surface. The grooves must be perfectly parallel and uniformly 13
2 - Instrumentation spaced from each other. The closer the grooves are spaced, the better the resolving capability of the grating. Gratings are produced with groove densities from 5 to 6 grooves/mm. The blaze wavelength of a diffraction grating is the wavelength of light that will be most efficiently diffracted by the grating. Generally, gratings can be used to diffract light that is 2/3 to 3/2 of the blaze wavelength. For example, consider a grating blazed at 4 nm. The grating can be used for wavelengths from 27 nm to 6 nm, but will diffract most efficiently wavelengths of 4 nm. Ruled grating are physically etched, groove-by-groove, by a machine. Basically, a mirror is mounted on a grooving machine, a diamond bit etches a straight groove, the mirror is moved a short distance and then the bit etches another straight groove parallel to the previous one. Holographic gratings, on the other hand, are manufactured with light, not a physical machine. A piece of glass is coated with a light-sensitive material, which is then exposed to two parallel beams of coherent light that produce an interference pattern on the coated glass. The bright areas of the interference pattern (where the light beams add constructively) form the grooves in the developed photoresist. A thin layer of aluminum is then applied onto the etched glass piece to form a mirrored, grooved surface a diffraction grating. The advantage of the holographic technique over the machine etching technique is that the holographic technique produces no systematic errors, since the grooves are the result of a perfect optical phenomenon. The machined technique, on the other hand, is only as good as the quality of the etching machine itself (which, for many purposes, is excellent, but will never be truly perfect ). Gratings that are used in monochromators are always copies of master gratings. A master grating is the original grating that was manufactured (either holographically or physically). Subsequent gratings can be made from the master by a process that essentially makes a molded copy of the master. There are advantages and disadvantages to both types of gratings. The appropriate one to use in a monochromator depends on several factors. Ruled gratings produce significant more stray light than holographic gratings, and this is especially true when groove density increases. For this reason, the maximum groove density of rules gratings is around 36 grooves/mm. Holographic gratings can have up to 6 grooves/mm. So, based on this factor, a holographic grating is better to use if a higher groove density is required to achieve a higher resolution and maintain a high signal to noise ratio. Ruled gratings do exhibit significantly better efficiency than holographic gratings. So, based on this factor, a ruled grating is more appropriate to use if light throughput is critical (for example, if the light source is very weak). If the light source is intense (as is the case in AAS instruments), then using a holographic grating is more advantageous than using a ruled grating. Slit Width The slit width affects how much light enters and exits the monochromator, and so is very important for light throughput. A wide slit width will allow more light to reach the detector and will improve the signal strength. But, if nonanalytical lines also reach the detector, then this will increase the noise and decrease the signal to noise ratio. Conversely, a narrow slit width will block out all non-analytical wavelengths, but may reduce the light throughput so much to make the signal to noise ratio unsatisfactory. So, the best slit width is the one that allows the most light to reach the detector and blocks out most of the noise. 14
2 - Instrumentation In other words, finding the optimum slit width is a compromise between maintaining high light throughput and maintaining a high signal to noise ratio. The maximum allowable slit width is generally determined by how closely spaced the analytical line of interest and its nearest neighbor in the spectrum are. The slit width must be narrow enough to block out any non-analytical lines, since they would increase the noise that the detector would see. Every spectrum is different, so the optimum slit width must be determined for each analysis. For elements whose analytical wavelengths are high (for example, rubidium at 78 nm), the spectra are usually not as dense as for elements with shorter analytical wavelengths (for example, zinc at 214 nm). Therefore, a larger slit width is usually permissible for elements like rubidium, whereas elements like zinc usually necessitate narrower slit widths. Atomizer The atomizer is arguably the most important part of an AAS instrument, since this is where the sample is converted into atoms that can absorb light. Having an efficient atomizer is essential. The absorbance signal is completely dependent on how many atoms there are to absorb the source light. So, a good atomizer will display good sensitivity to the sample being analyzed, and a poor atomizer will display poor sensitivity. The ideal atomizer has the following characteristics: Atomizes 1% of the sample delivered to it. No ionization occurs. No sample is left in the molecular, complexed form. Of course, no real atomizer is ideal, and the degree to which any sample is atomized depends to a large extent on the element being analyzed. Atomization is achieved by heating the sample to an extent where free ground state atoms are formed. In FAAS, atomization is done with a flame. In GFAAS, atomization is done with an electrically heated graphite tube furnace. In VG-AAS, atomization is done with an electrothermally heated furnace or a flame. Flame The atomizer in a FAAS instrument uses a nebulizer to convert a liquid sample into an aerosol. The sample is introduced into the nebulizer by aspiration though capillary tubing. The aspiration occurs pneumatically from the flow of fuel and oxidant gases through the nebulizer chamber. After the sample has traveled through the capillary tubing, it strikes a glass impact bead. This impact bead is designed so that when a stream of liquid strikes it, the liquid breaks apart into a mist of drops (an aerosol). This aerosol invariably contains drops of many sizes. The larger drops fall out, but the smaller drops remain suspended and are thoroughly mixed with the fuel and oxidant gases as the mixture is carried into the spray chamber. As the sample mist gets mixed with the gases, it moves along through the spray chamber and up towards the burner head. The sample entering the burner head is a uniform mixture of fuel gas, oxidant gas, and tiny sample droplets. Once the mixture enters the flame, the process of atomization by heat begins. The heat from the flame is usually sufficient to desolvate the sample droplets. Then, the solid particles that were formed (e.g. salts) are broken down, melted, or volatilized into gases. Finally, the molecules are thermally dissociated into atoms that are capable of absorbing their characteristic wavelength of light. Of course, the heat from the flame may be excessive and 15
2 - Instrumentation may ionize some atoms (remove an outer electron from the atom), thus decreasing the absorption signal. As well, the heat from the flame may be insufficient and not atomize enough of the sample molecules. This also decreases the absorption signal. The process of thermally atomizing the sample in the flame is a complex equilibrium, and the actual chemistry inside the flame at any stage (especially the atomization stage) is not clear. There are often numerous side reactions that occur simultaneously. The degree to which a sample may be ionized in a flame depends on the element, since each element has different energy requirements for ionization. To reduce ionization of atoms, easily ionizable elements (EIEs), such as the Group I elements (Li, Na, K, Cs) can be added to the original sample solution. If the EIEs are much more easily ionized than the sample atoms, then they will create a large population of electrons in the flame and shift the atomization/ionization equilibrium of the sample atoms towards the atomization side. There are two types of flames that are commonly used for FAAS: air/acetylene and nitrous oxide/acetylene. The air/acetylene flame (air being the oxidant, acetylene being the reductant, or fuel) burns at around 23 C. The nitrous oxide/acetylene flame burns much hotter at around 3 C. So, the flame temperature is a factor when determining which type of flame is best suited for atomization of a given element. Because of its cooler burning temperature, the air/acetylene flame works well for elements that are relatively easily atomized, such as copper, iron, nickel, and gold. The nitrous oxide/acetylene flame, with its higher burning temperature, is needed to atomize elements that require more energy to atomize, such as aluminum, silicon, titanium, and tungsten. Another important factor in optimizing the atomization of an element in a flame is the stoichiometry of the oxidant and reductant gases. A lean flame is fuel poor, and is therefore an oxidizing flame. A rich flame has excess fuel, and is therefore a reducing flame. For each type of flame, certain elements are atomized best in reducing flames, and certain elements are atomized best in oxidizing flames. There is extensive data on the absorbance characteristics of all the elements in flames. For example, in a reducing flame there are excess carbon and hydrogen atoms present in the flame (from the acetylene molecules). These excess atoms help break down the strong oxide bonds that form with some elements, such as chromium. Other elements that are best atomized in a reducing flame are tin and molybdenum. On the other end of the spectrum, elements like silver, cadmium, gold, and nickel are best atomized in an oxidizing flame. Some elements, like iron and gallium, are best atomized in a stoichiometric flame (i.e. neither rich nor lean). Furthermore, some elements are satisfactorily atomized over a wide range of flame gas mixtures. Copper, for example, is atomized in both rich and lean flames. For this reason, copper is often used to test or validate the sensitivity of an AAS instrument. The major disadvantage of the flame atomization technique is that it is very inefficient at converting the original sample to atoms. Overall, the atomizer system of a FAAS instrument can convert less than.1% of the original sample to absorbing atoms. The nebulizer component usually transports only less than 1% of the aspirated sample into the aerosol, and the other 9% is lost as waste in the spray chamber and nebulizer chamber. Furthermore, the sample that does make it into the flame (in aerosol form) is already greatly diluted from its mixing with the flame gases. And once the sample gets atomized in the flame, the atoms residence times in the light path are extremely short. Atoms travel through the light path at great speeds (at least 1 cm/1 ms) as they exit the slit in the burner head and travel up the flame. 16
2 - Instrumentation Graphite Furnace The graphite furnace (GF) atomizer solves the two major problems of the flame atomizer: poor atomization efficiency and short residence times. A graphite furnace is a tube that is connected to two low voltage electrodes. When a current is forced through the tube, the tube heats up. The amount of heating caused by the current flow can be accurately controlled, and the atomization of samples in a graphite furnace is usually performed over several heating steps. The graphite furnace is mounted in the electrodes so that one open end of the tube faces the light source and the other open end faces the entrance to the optics. This allows the light to pass freely through the graphite furnace along the axis of the tube. The tube is aligned so that the light path travels down the graphite tube axis and right through the center of the tube. With a GF atomizer, a very small amount of liquid sample (between 5 and 1 µl) is placed inside the center of the graphite tube. A heating program to atomize the sample consists of at least three principal steps: 1. Drying Step The graphite tube is quickly heated to a temperature just below the boiling point of the solvent. Then the temperature is slowly ramped past the boiling point. This step gently evaporates the solvent (without causing splattering or sample ejection) and leaves the dried sample inside the tube. 2. Ashing Step (or Charring) This step removes any dry or semi-dry matrix that is left over from the drying step. Matrix modifiers can be added to the sample before the heating program to stabilize the sample during the ashing step. Modifier gases, such as oxygen or hydrogen, can be added to the graphite furnace workhead during the ashing step to help remove the matrix. Ashing temperatures depend on the element being analyzed in its matrix. For example, cadmium ashes at 3 C, arsenic at 14 C, iron at 6 C, and lead at 48 C. 3. Atomization Step At this stage, the sample is a dry solid at the bottom of the graphite tube. The sample is atomized by rapidly increasing the temperature of the tube. The AI 12 can heat at a rate upto 38 K/s. The required atomization temperature depends on the element being analyzed. For example, cadmium atomizes at 125 C, arsenic at 225 C, nickel at 225 C, and lead at 14 C. There are distinct advantages to the graphite furnace atomizer when its performance is compared to the flame atomizer. The graphite furnace atomizes 1% of the sample (compared to less than.1% for the flame). Also, the residence time of the atomized sample in the light path is much longer in the graphite furnace than in the flame. The residence time in the graphite tube can range from to.5 second, whereas in the flame it is only milliseconds. Both of these factors increase the sensitivity of the graphite furnace atomizer, and detection limits with GFAAS are typically one to two orders of magnitude better than with FAAS. 17
2 - Instrumentation Detector The detector is the part of the AAS instrument that receives the light output from the monochromator. The detector quantifies how much light it receives and creates an electrical current. That current is then amplified and converted into a digital signal that is recorded by a data acquisition system. By far the most common detector used in AAS instruments is the photomultiplier tube (PMT). Essentially, a PMT is a photon counter. Light from the monochromator enters the PMT through a quartz window. Photons (the quantum packets that make up light) strike the photocathode of the PMT. This converts the photon to a photoelectron via the photoelectric effect. However, the production of a single electron from a single photon won t generate a very strong signal, so an amplification 5 or 6 orders of magnitude is required. This amplification is achieved through the use of a series of 8 to 12 dynode plates. A voltage is applied between the photocathode and first dynode (on the order of 1 V). This voltage difference causes the photoelectron to accelerate from the photocathode to the dynode. When it strikes the dynode, several more electrons are produced. These electrons are in turn accelerated towards the second dynode, since there is also a voltage difference across the first and second dynodes. Each of the electrons striking the second dynode creates several more electrons. For example, if the electron collision into the first dynode created 5 electrons, then the collision of those 5 electrons into the second dynode will create 25 electrons. This chain reaction of electron production continues along the series of dynodes. If there are 12 dynodes in the chain, then the original single photon will produce 244,14,625 electrons from the final twelfth dynode. These final electrons are then collected by the PMT anode. The current that can result from the collection due to a single photon by the PMT can be as high as 1 ma! Clearly, PMTs are extremely sensitive detectors. Furthermore, the large amplification of the signal that is achieved by the PMT is achieved with very little increase in noise. 18
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3 - Background Correction Methods Chapter 3: Background Correction Methods Fundamentals The Frequency of Measurement The Interval between Measurements The Function used to Calculate Net Absorption Spectral and Structure Backgrounds The Effect on Linear Working Range Deuterium (D2) Background Correction Smith-Hieftje (S-H) Background Correction Zeeman Background Correction Comparison of Background Correction Methods 21
3 - Background Correction Methods Fundamentals Background correction is a necessary part of any good AAS instrument. Basically, there are two types of backgrounds that need to be corrected for: non-specific radiation and non-specific absorption. Non-specific radiation was dealt with in Chapter 2. Non-specific radiation is the extra light (non-analytical wavelengths) that can pass through the optics of an AAS instrument, enter the monochromator, and have the chance of reaching the detector. If this non-specific radiation reaches the detector, it will result in a falsely signal. Nonspecific radiation can come from many sources, including the HCL fill gas, sunlight, room light, and the light emitted by the flame itself. As was discussed in Chapter 2 (Monochromator section), it is the job of the monochromator to effectively filter all entering light and allow only a specific, variable wavelength to exit and reach the detector. So, the monochromator ensures that the only light that reaches the detector is the wavelength of light that is being absorbed by the sample being analyzed. The other type of background that must be corrected for is non-specific absorption (and will be discussed in this chapter). This correction cannot be accomplished by the monochromator, since it involves the same analytical wavelength that is selected by the monochromator. Non-specific absorption (also called background absorption) has a broadband effect and occurs when the source light is prevented from reaching the detector by means other than absorption by analyte atoms, such as scattering and blocking of light by other species in the light path. Molecular species and solid particles present in the flame are the major causes of non-specific absorption. When the heat of the flame is not sufficient to fully break down all molecular species (for example, matrix compounds), there can be sufficient remaining molecules to absorb, block, or scatter the source light. These molecules can be thought of as causing the same interference as putting one s hand in the light path: the light gets blocked, less light reaches the detector, and a falsely high absorbance signal results (because the signal analysis software thinks that less light is reaching the detector because more light is being absorbed by the analyte atoms). In FAAS, the background absorption is relatively minor (usually less than.5 absorbance units). In GFAAS, on the other hand, background absorption is severe and can reach levels of 2. absorbance units. Therefore, effective background correction methods become essential for accurate GF analyses. There are three background correction methods currently being used by AAS instrument manufacturers: Deuterium (D2), Smith-Hieftje (S-H), and Zeeman. For all three methods, there are several common factors that determine the effectiveness of the method: The frequency at which the peaks are measured; The interval between total absorbance and background absorbance measurements; The mathematical function used to calculate net atomic absorption; The ability to correct for spectral or structured background; The effect of the method on the linear working range. The Frequency of Measurement How rapid and transient the absorption peaks are dictates how fast absorption measurements must be made [9, 1]. In GF analyses, signal durations are typically to.5 seconds. The temporally uniform, isothermal atomization of the AI 12 provides peak durations that are in the low end of this range. The non-specific absorbance normally exhibits similar peak durations. Furthermore, the change in the absorbance values of these peaks can often be as high as 1 absorbance units per second. Therefore, a very high sampling frequency is required to accurately measure the very rapid and narrow peaks and to determine peak shapes. 22
3 - Background Correction Methods For a GFAAS system, a 1 Hz sampling frequency can produce significant errors in the measurement of peak height and/or peak area. While the performance can be improved if a 3 Hz frequency is used, the errors still remain significant. An increase to a 6 Hz sampling frequency will provide acceptable results in a GFAAS system. The AI 12 utilizes a sampling frequency as high as 1Hz in single beam mode and 12 Hz in double mode, giving excellent peak definition and negligible errors in peak area and peak height measurements. The Interval between Measurements The Net Signal (NAS) is the analytical signal of interest. NAS is the absorption of the light that is attributed only to the analyte atoms. It is obtained by subtracting the Background Signal (BAS) from the Total Signal (TAS). Ideally, if BAS and TAS were measured simultaneously, then there would be no error in the NAS calculation. In reality, however, only one measurement can be made at a time. The best thing to do, then, is to make the time interval between successive measurements as short as possible, to approach simultaneous measurements. The shorter the time interval between a BAS measurement and a TAS measurement, the less error there will be in the calculation of NAS. This factor is especially important for GFAAS, where the background signal can change quite rapidly compared to the analyte signal. The AI12, which uses the D2 background correction method, uses a D2/HCL modulation frequency of 1 KHz, resulting in a time interval between successive measurements of less than.5 ms. With this system, the HCL and D2 lamps are alternately pulsed so that only one light source is passing through the sample at a time. When the HCL is pulsed, the D2 lamp is turned off and only the HCL light passes through the sample. The signal measured is the TAS. When the D2 lamp is pulsed, the HCL is turned off and only the D2 light passes through the sample. The signal measured is the BAS. The Function used to Calculate Net Absorption If a sufficiently fast switching frequency between the BAS and TAS measurements is employed (such as in the AI 12), then a simple subtraction of the BAS from the TAS can be performed to obtain the NAS. If relatively long time periods are elapsed between the BAS and TAS measurements, then interpolation techniques will be needed to approximate the NAS values. Furthermore, there are increased chances of significant changes in the background signals occurring over the elapsed time periods. Therefore, these interpolation techniques are much more susceptible to errors and inaccurate NAS values. Spectral and Structured Background Less than 1% of the samples encountered in the real world exhibit spectral or structured background interferences that cannot be overcome by optimizing the atomizer (for GFAAS, in particular, by optimizing the furnace heating program) and/or using an appropriate chemical modifier. The Effect on the Linear Working Range Due to complex splitting patterns, the TAS response of an instrument equipped with Zeeman background correction can be non-linear. As a result, when the BAS is subtracted from the TAS, a roll over point on a calibration curve can occur. That is, two different concentration values could correspond to a single NAS value. In such a case, the linear dynamic range has been reduced by the effects of the background correction method. In comparison, the D2 background correction method does not produce a roll over point in calibration curves and so has no negative effects on the linear dynamic range. 23
3 - Background Correction Methods Deuterium (D2) Background Correction The light source used in the Deuterium (D2) background correction method is the D2 lamp. The D2 lamp is a continuum source, rather than a sharp line emitter. That is, it emits a spectrum of radiation covering 18 nm to 425 nm. Even though this means that the use of the D2 method is limited to the UV range, it is not much of a detriment since the most significant background absorptions occur at low wavelengths anyway. See figure 3.1 for a graphical explanation of how D2 background correction works. The key thing about D2 background correction is the assumption that the absorption of radiation by the analyte atoms alone has a negligible effect on the D2 spectrum. The monochromator exit slit is relatively wide (at least nm), so a spectrum of D2 light of that width is allowed through. Compare this to the tiny width of spectrum that the analyte absorption occurs in (approximately.2 nm), and it s easy to see how absorption occurring in such small region of a wide spectrum will have a negligible effect on that spectrum s overall intensity. In other words, only a small fraction of the D2 spectrum is attenuated by the analyte absorption, and the rest is completely unaffected. Conversely, the atomic absorption due the analyte atoms has a significant effect on the intensity of the HCL radiation, since the HCL line is very sharp and narrow to begin with. But the absorption that occurs in real-life experiments is not due to analyte atoms alone. It is always a combination of analyte absorption and background absorption. Background absorption is broadband and so has an equal effect on the intensity of both the HCL line and the D2 band. So, when the individual effects of the analyte absorption and the background absorption are added together, the result is that the HCL line is attenuated cumulatively by both and the D2 band is only attenuated by the broadband background absorption. This fact forms the foundation of the D2 background correction method. When the D2 lamp is pulsed, the absorption of the D2 band is measured (the BAS). When the HCL lamp is pulsed, the absorption of the HCL line is measured (the TAS). The difference between the two (TAS - BAS) is the absorption due to the analyte atoms alone (the NAS), and this is the figure of analytical significance. 24