2101 Atomic Spectroscopy

Transcription

1 2101 Atomic Spectroscopy Atomic identification Atomic spectroscopy refers to the absorption and emission of ultraviolet to visible light by atoms and monoatomic ions. It is best used to analyze metals. Atomic spectroscopy is closely related to other forms of spectroscopy. It can be divided by atomization source or by the type of spectroscopy used. The basic principle is that light is passed through a collection of atoms. If the wavelength of the light has energy corresponding to the energy difference between two electronic energy levels in the atoms, a portion of the light will be absorbed. The relationship between the concentration of atoms, the distance the light travels through the collection of atoms, and the portion of the light absorbed is given by the Beer-Lambert law. 1 AAtomic Spectroscopy

2 Difference between AAS and UV-Vis Analysis is limited to atoms or its ions. Best analysis for metals. Analysis prep. best in water solution since metals are ions in aqueous media. Sample container is solution. Technique is destructive. No cuvettes. Spectral lines source is the light source. Atomic absorption rather than continuous radiation is seen in UV-Vis region. Some elements simply require thermal energy others require plasma. Wavelength are well known for quantification since application is limited to elements and all atomic line spectrum for elements are known. 2 AAtomic Spectroscopy

3 Summary of Techniques Flame atomic absorption Large flame as the atomizer. Graphite furnace atomic absorption Graphite tube that is electrically heated to high temperature. Inductive coupled plasma atomic emission Emission technique (no light sources) that uses very hot plasma. Flame photometry Intensity of light emitted by analyte in flame. Flame emission and atomic fluorescence Similar to molecular emission. Cold vapor mercury system Hg vaporize and absorption recorded. Metal hydride generation For difficult elements, similar to cold vapor mercury. Elements converted to useful hydride and analyzed. Arc and spark emission High voltage excite solid sample to cause spark, emitted light is analyzed. 3 AAtomic Spectroscopy

4 Detection Limits AAS 4 AAtomic Spectroscopy

5 Resolution (Width) Lines Spectra Atomic spectral lines have finite widths, factors to line broadening due to: Natural Broadening - The lifetime of the excited states lead to uncertainty leading to broadening due to shorter excited state lifetimes. Lifetimes of 10-8 s lead to width of 10-5 nm. Collisional Broadening -Also referred to as Pressure Broadening is the result of collision of the excited state leads to shorter lifetimes and broadening of the spectral lines. Doppler Broadening - When molecules are moving towards a detector or away from a detector the frequency will be offset by the net speed the radiation hits the detector. This is also known as the Doppler effect and the true frequency will ether be red shifted (if the chemical is moving away from the detector) or blue shifted (if the chemical is moving towards the detector) Signal Doppler Broadening 5 AAtomic Spectroscopy

6 Natural linewidths Width of an atomic spectral line is determined by the lifetime of the excited state Consequence of the Heisenberg uncertainty principle For example, lifetime of 10-8 seconds (10 ns) yields peak widths of 10-5 nm 6 AAtomic Spectroscopy 6

7 Origins of Atomic Spectroscopy Spectroscopy of atoms or ions do not involve vibrations or rotation transitions. Atoms are in the gas state. Transition involves promoting an electron from a ground state to a higher empty atomic state orbital, this state is referred to as the excited state. Shown to the right is the three sodium absorption and emission process and the emission lines. Atomic p-orbitals are in fact split into two energy levels for the multiple spins of the electron. The energy level is so small however that a single line observed. A high resolution would show the line as a doublet. c 7 AAtomic Spectroscopy

8 Atomic Absorption Spectroscopy Flame atomic absorption spectroscopy (AAS) is the most used of atomic methods. Block diagram of a single-beam atomic absorption spectrometer. Radiation from a line source is focused on the atomic vapor in a flame or an electro-thermal atomizer. The attenuated source radiation then enters a monochromator, which isolates the line of interest. Next the radiant power from the source, attenuated by absorption, is measured by the photomultiplier tube (PMT). The signal is then processed and directed to a computer system for output. 8 AAtomic Spectroscopy

9 Instrumentation Atomic spectroscopy begins with atomizing the sample. Sample introduction - Atomizer devices are either continuous or discrete. Continuous are in the form of plasmas and flame. Discrete are in the form of electrothermal. Nebulizers (method to introduce samples as mist) are components to introduce samples into the atomizer. Direct nebulizers creates fine droplets by aerosol. Shown is the continuous sample method. Samples are frequently introduced into plasmas or flame by means of nebulizer which takes the sample and convert it to a spray or mist. 9 AAtomic Spectroscopy

10 Type of Atomizer The technique typically makes use of a flame to atomize the sample, but other atomizers such as a graphite furnace or plasmas, primarily inductively coupled plasmas, are also used. When a flame is used it is laterally long (usually 10 cm) and not deep. The height of the flame above the burner head can be controlled by adjusting the flow of the fuel mixture. A beam of light passes through this flame at its longest axis (the lateral axis) and hits a detector. 10 AAtomic Spectroscopy

11 Radiation Source The radiation source chosen has a spectral width narrower than that of the atomic transitions. Hollow cathode lamps (HCL) are the most common radiation source in atomic absorption spectroscopy. Inside the lamp, filled with argon or neon gas, is a cylindrical metal cathode containing the metal for excitation, and an anode. When a high voltage is applied across the anode and cathode, gas particles are ionized. As voltage is increased, gaseous ions acquire enough energy to eject metal atoms from the cathode. Some of these atoms are in an excited states and emit light with the frequency characteristic to the metal. Many modern hollow cathode lamps are selective for several metals. Diode lasers can also be used in atomic absorption spectroscopy. The diode lasers have good properties for laser absorption spectrometry. The technique is then either referred to as diode laser atomic absorption spectrometry (DLAAS or DLAS), or, since wavelength modulation most often is employed, wavelength modulation absorption spectrometry. 11 AAtomic Spectroscopy

12 Hollow Cathode Lamp (HCL) An HCL (Hollow Cathode Lamp) usually consists of a glass tube containing a cathode, and anode, and a buffer gas (usually a noble gas). A large voltage across the anode and cathode will cause the buffer gas to ionize, creating a plasma. The buffer gas ions will then be accelerated into the cathode, sputtering off atoms from the cathode. Both the buffer gas and the sputtered cathode atoms will in turn be excited by collisions with other atoms/particles in the plasma. As these excited atoms decay to lower states, they will emit photos, which can then be detected and a spectrum can be determined. Either the spectrum from the buffer gas or the sputtered cathode material itself, or both, may be recorded. 12 AAtomic Spectroscopy

13 Hollow Cathode Tube Mechanism of Cathode Hollow Tube c 13 AAtomic Spectroscopy

14 Plasma Source Plasma is the phase of matter with its electrons are stripped. In argon plasma, argon ions and electrons act as the conducting species. Three power sources are dc-electric, radio and microwave frequency generators. The most advantageous is the radio or inductively coupled plasma (ICP) because of sensitivity and minimal interference. DC plasma source (DCP) are also advantageous and is also simple and less expensive. Inductive Coupled Plasma consist of three concentric quartz tubes in which streams of argon flow. Ionization of the argon is initiated by a spark from a Tesla coil. The geometries of CP source, in radial geometry or axial geometry. 14 AAtomic Spectroscopy

15 Electrothermal Atomizer (Graphite Furnace) Electrothermal atomizer deposit a few microliters of sample in the furnace with a syringe or an autosampler. This is followed by drying ashing and atomization steps that is carried out by instrument programming. There are other type of atomizers devices. Examples are the gas discharge which results in glow discharge. Early atomizers include dc and ac arcs which have been replaced almost entirely by ICP. Shown is the cross-sectional view of a graphite furnace atomizer. The L vov platform and its position in the graphite furnace. The L'vov platform isolates the sample from the tube walls to allow more reproducible atomization of the sample through indirect heating. The platform heats primarily by the radiation given off from the tube walls. 15 AAtomic Spectroscopy

16 Flame Atomizer Flame atomizers contains a pneumatic nebulizer, which converts the sample solution into a mists or aerosol. Shown is a diagram of a three electrode dc plasma jet. Two separate dc plasmas have a single common cathode. The overall plasma burns in the form of an inverted Y. Samples are introduced as aerosol from the arc between the two graphite anodes. Observation of emission in the region beneath the strongly emitting plasma core avoids much of the plasma background emission. When a nebulized sample is carried into a flame, desolvation of the droplets occurs in the primary combustion zone, located in the tip of the burner. The fine solid particles are carried to a region in the center of the the flame called the inner core. 16 AAtomic Spectroscopy

17 Nebulizer The solution is aspirated to the premix burner to the flame. The nebulizer accomplish this by aspiration and conversion to an aerosol at the head of the mixing chamber. There are two inlet to the nebulizer. One is the garden hose that feeds sample in to the mixing chamber. 17 AAtomic Spectroscopy

18 Premix Burner The burner for flame AA is a premix burner. All the components of the flame (fuel, oxidant and sample solution) are premixed on the way to the flame. 18 AAtomic Spectroscopy

19 The Burner Head The unexcited atom in the flame are available to be excited by a light beam. The light source is used and a light beam is directed through the flame. 19 AAtomic Spectroscopy

20 Analysis of Sample A liquid sample is normally turned into an atomic gas in three steps: Drying the liquid solvent is evaporated, and the dry sample remains. Vaporization (Charring) the solid sample vaporizes to a gas Atomization (pyrolysis) the compounds making up the sample are broken into free atoms. 20 AAtomic Spectroscopy

21 Interferences in AAS Broadening of a spectral line, which can occur due to a number of factors (Physical) Spectral: emission line of another element or compound, or general background radiation from the flame, solvent, or analytical sample. Background correction can be applied Chemical formation of compounds that do not dissociate in the flame Ionization of the analyte can reduce the signal Matrix interferences due to differences between surface tension and viscosity of test solutions and standards Another caveat: Non-linear response common in AAS 21 AAtomic Spectroscopy

22 Calibration in AAS In theory, Beer s law applies for dilute solutions. In practice, deviation from linearity is usual. Linear range Small dynamic range. Possible to use non-linear curve fitting for calibration. Reasons: 1 Self-absorption: excited atoms emit light that can also be absorbed instead of that of source: è on average, less light per number of atoms is absorbed. 2 Detector: oversaturation of detector. 22 AAtomic Spectroscopy

23 Alternative to matrix-matching: Method of standard additions Extensively used in absorption spectroscopy, accounts for matrix effects Techniques: Several aliquots of sample Sample (1): just the solvent and all other components except sample. Sample (2): diluted to volume directly Samples (3,4,5 ): known amounts of analyte added before dilution to volume, this is the addition Only makes sense if the added standard closely matches the analyte present in the samples chemically and physically. ü if simple, dissolved ions are analysed 23 AAtomic Spectroscopy

24 Method of standard additions If linear relationship exists between measured quantity and concentration (must be verified experimentally) then: A T = kv x c x V T + kv s c s V T V x, C x : volume and concentration of analyte V s : variable volume of added standard C s : concentration of added standard V T : total volume of volumetric flask k: proportionality constant (= єl) A x, A T : absorbances of standard alone and sample + standard addition, respectively. 24 AAtomic Spectroscopy

25 Method of standard additions Graphical evaluation slope = m = (єlc s ) / V T intercept = b = (єlv x c x ) / V T Limitations: The calibration graph must be substantially linear since accurate regression cannot be obtained from non-linear calibration points. Caution: The fact that the measured part of the graph is linear does not always mean that linear extrapolation will produce the correct results. It is also essential to obtain an accurate baseline from a suitable reagent blank 25 AAtomic Spectroscopy

26 Most simple version of standard addition: Spiking Spiking; deliberately adding analyte to an unknown sample. Technique: Preparation of sample and measurement of absorbance Addition of standard with known concentration, then measurement absorbance. From difference in the absorbance, calculate e From reading of sample alone, calculate amount of analyte (use Beer s law for calculations) 26 AAtomic Spectroscopy

27 Other uses for spiking Add spike at beginning of sample preparation. Process sample with and without spike. Difference should correspond to amount spiked. Deviation allows to calculate recovery factor. 27 AAtomic Spectroscopy

28 Atomic Absorption Spectroscopy Flame atomic absorption spectroscopy (FAAS). 28 AAtomic Spectroscopy

29 Flame Atomic Absorption Spectroscopy Atomic absorption spectroscopy (AAS) determines the presence of metals in liquid samples. It also measures the concentrations of metals in the samples, with concentrations range in the low mg/l range (ppm). In their elemental form, metals will absorb ultraviolet light when they are excited by heat. Each metal has a characteristic wavelength that will be absorbed. The AAS instrument looks for a particular metal by focusing a beam of UV light at a specific wavelength through a flame and into a detector. The sample of interest is aspirated into the flame. If that metal is present in the sample, it will absorb some of the light, thus reducing its intensity. The instrument measures the change in intensity. A computer data system converts the change in intensity into an absorbance. 29 AAtomic Spectroscopy