Atomic-absorption spectrometry

Detection limits achievable by various atomic spectroscopy techniques are reviewed in Table 11.1. 

Since shortly after its inception in 1955, AAS has been the standard tool employed by analysts for the determination of trace levels of metals. In this technique a fine spray of 

Element Wavelength (nm) AAS Lamp current (mA) Flame AAS Furnace AAS (IL/ Atomiser 55 CTF ICP 

FIGURE 1.17 (a) Schematic diagram of an atomic absorption spectrophotometer (AAS)  

The absorption of photons by the sample depends on the concentration of the analyte, the length of the optical path of the sample, and the cross section for absorption of photons of that wavelength. Qnantiflcation is based on comparison of the signal intensity of standard solutions that contain known (preferably certilied) concentrations of the analyte with that of the sample using Beer-Lambert law. The underlying principle is that each element has characteristic wavelengths of absorption and emission that depend on the electronic structure of its atoms that in turn represent the energy levels of the orbital that these electrons occupy. 

In the classic AAS instruments, the radiation source usually consisted of a lamp with a filament (or hollow cathode) made of, or coated with, the same element that is to be determined so that there is a resonant absorption of the emitted photons. The detector used in this system can be a simple photomultiplier and an optical filter is often used to improve specificity. There are other types of photon sources, especially in modem instruments, where several wavelengths can be simultaneously anitted and detected. A wavelength selector is positioned in front of the detector in order to distinguish the absorption of the different elements (wavelengths). 

The sensitivity of AAS techniques in liquid samples is usually in the parts-per-million (ppm or mg L ) range, but for some elements and instruments, parts-per-billion (ppb or pg L ) limits of detection are attainable. Modem AAS instrumentation deploys advanced background reduction and signal processing methods in order to improve the analytical performance. 

In principle, all elements can be determined by AAS, since the atoms of any element can be excited and are therefore capable of absorption. The limitations lie practically only in the field of instrumentation. Measurements below 200 mn in the vacuum UV range are difficult, owing to the incipient absorption of atmospheric oxygen. With modified instruments and a shielded flame or a graphite furnace, it is possible to determine such elements as iodine at 183.0 nm, sulfur at 180.7 nm, and phosphorous at 177.5 nm, 178.3 nm and 178.8 nm. 

Any difference in the behaviour of the analyte atoms in the sample and in the standard implies an interference. AAS using a line source for excitation suffers little spectral interference. Background interference in AAS is more important. This nonspecific absorption is caused by  

To overcome this problem, three different correction techniques have been proposed the Zeeman technique offers the most advantages. 

Standard deviations for fast sequential FAAS are about 2%. FAAS has had relatively little improvement in detection limits and performance over the last 15 years. 

Atomic absorption spectrometry is also being used in the hyphenated mode. Parris et al. [Ill] described 

As a method for elemental determinations, atomic absorption spectrometry (AAS) goes back to the work of Walsh in the mid-1950s [2]. In AAS, the absorption of resonant radiation by ground state atoms of the analyte is used as the analytical signal. This process is highly selective as well as very sensitive, and AAS is a powerful method of analysis, used in most analytical laboratories. Its methodological aspects and applications are treated in several textbooks [154]-[156]. 

A primary source is used which emits the element-specific radiation. In the beginning, continuous sources were used with a high-re.solution spectrometer to isolate the primary radiation. However, due to the low radiant densities of these sources, detector noise limitations occurred, or the spectral band width was too large for sufficiently high sensitivity. 

For AAS, the analyte must be present as an atomic vapor, i.e., an atomizer is required. Both flames and furnaces are used, and the corresponding methodologies are known as flame AAS and graphite furnace AAS. Special methods of atomization are based on volatile compound formation, as with the hydride technique (Sections 21.4.3 and 21.5.5). AAS is generally used for the analysis of 

Atomic absorption spectrometers contain a primary source, an atomizer with its sample introduction system, and a monochromator with a suitable detection and data acquisition system (Fig. 30). 

Radiation from the primary source (a) is led through the absorption volume (b) and subsequently into the monochromator (c). As a rule, radiation densities are measured with a photomultiplier and processed electronically. Czerny-Turner or Ebert monochromators with low focal length (0.3-0.4 m) and moderate spectral bandpass (normally not below 0.1 nm) are frequently used. 

Since shortly after its inception in 1955, AAS has been the standard tool employed by analysts for the determination of trace levels of metals. In this technique a fine spray of the analyte is passed into a suitable flame, frequently oxygen-acetylene or nitrous oxide-acetylene, which converts the elements to an atomic vapour. Through this vapour is passed radiation at the right wavelength to excite the ground state atoms to the first excited electronic level. The amount of radiation absorbed can then be measured and directly related to the atom concentration a hollow cathode lamp is used to emit light with the characteristic narrow line spectrum of the analyte element. The detection system consists of a monochromator (to reject other lines produced by the lamp and background flame radiation) and a photomultiplier. Another key feature of the technique involves 

This technique can determine a particular element with little interference from other elements. It does, however, have two major limitations. One of these is that the technique does not have the highest sensitivity. The other is that only one element at a time can be determined. This has reduced the extent to which it is currently used. 

Increasingly, due to their superior intrinsic sensitivity, the AAS currently available are capable of implementing the graphite furnace techniques. Available suppliers of this equipment are listed in Appendix 1. 

Elmer 2280) and a double-beam instrument (Perkin Elmer 2380). 

A primary source is used which emits the element-specific radiation. Originally continuous sources were used and the primary radiation required was isolated with a high-resolution spectrometer. However, owing to the low radiant densities of these sources, detector noise limitations were encounterd or the spectral bandwidth was too large to obtain a sufficiently high sensitivity. Indeed, as the width of atomic spectral lines at atmospheric pressure is of the order of 2 pm, one would need for a spectral line with 2 = 400 nm a practical resolving power of 200 000 in order to obtain primary radiation that was as narrow as the absorption profile. This is absolutely necessary to realize the full sensitivity and power of detection of AAS. Therefore, it is generally more attractive to use a source which emits possibly only a few and usually narrow atomic spectral lines. Then low-cost monochromators can be used to isolate the radiation. 

For AAS the analyte must be present in the atomic vapor state. Therefore, the 

Accordingly, it was very soon found that using sources for which the physical widths of the emitted analyte lines are low is more attractive. This is necessary so as to obtain high absorbances, as can be understood from Fig. 76. Indeed, when the bandwidth of the primary radiation is low with respect to the absorption profile of the line, a higher absorption results from a specific amount of analyte as compared with that for a broad primary signal. Primary radiation where narrow atomic lines are emitted is obtained with low-pressure discharges as realized in hollow cathode lamps or low-pressure rf discharges. Recently, however, the availability of narrow-band and tunable laser sources, such as the diode lasers, has opened up new per- 

For AAS, the analyte must be present in the atomic vapor state. Therefore, the use of an atomizer is required. Both flames and furnaces are used and the appro- 

Department of Applied Research Bodenseewerk Perkin-Elmer GmbH Postfach 101164 D-88662 Uberlingen, Germany 

Atomic absorption obeys the Lambert-Beer law (1), which states that absorbance A (negative logarithm of the transmission factor) is proportional to the concentration c of the absorbing substance and to the thickness d of the absorbing layer  

The general construction of an AAS instrument is simple and is shown schematically in Fig. 

The most important components are a radiation source, which emits the spectrum of the analyte element, an atomizer in which the atoms of the analyte element are formed, a monochromator for the spectral dispersion of the radiation and separation of the analytical line from other radiation, a detector permitting measurement of radiation intensity, followed by an amplifier and a signal-processing unit with a readout device. The primary radiation is modulated either mechanically or electrically at a fixed frequency, and the amplifier electronics are turned to the same frequency. In such a system only the element-specific radiation having the modulation frequency is amplified while any other radiation emitted by the atomizer, which is not modulated, is neglected. 

Very many of the advantages of AAS can be directly or indirectly traced to the narrow half-intensity width of the resonance lines, i.e., the absorption of an element takes place within a very limited spectral range of about O.OOl-O.OOS nm. This advantage becomes very noticeable if the radiation sources used for excitation emit the spectrum of the analyte element in spectral lines that are narrower than the absorption lines. Hollow cathode lamps (HCLs) and electrodeless discharge lamps (EDLs) are particularly suitable as radiation sources. The latter typically provide a higher radiation intensity which results in a better signal-to-noise (S/N) ratio, particularly in the far-ultraviolet (UV) range of the spectrum. Radiation sources that emit a continuous spectrum are 

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Elemental Analysis Atomic absorption spectrometry X-Ray fluorescence spectrometry Plasma emission spectrometry... 

From J. A. Dean and T. C. Rains, Standard Solutions for Flame Spectrometry, in Flame Emission and Atomic Absorption Spectrometry, J. A. Dean and T. C. Rains (Eds.), Vol. 2, Chap. 13, Marcel Dekker, New York, 1971. 

Source Compiled from Parson, M. L. Major, S. Forster, A. R. Appl. Spectrosc. 1983,37, 411-418 Weltz, B. Atomic Absorption Spectrometry, VCH Deerfield Beach, FL, 1985. 

Trace metals in sea water are preconcentrated either by coprecipitating with Ee(OH)3 and recovering by dissolving the precipitate or by ion exchange. The concentrations of several trace metals are determined by standard additions using graphite furnace atomic absorption spectrometry. 

Rocha, E. R. P. Nobrega, J. A. Effects of Solution Physical Properties on Copper and Chromium Signals in Plame Atomic Absorption Spectrometry, /. Chem. Educ. 1996, 73, 982-984. 

L Vov, B. V. Graphite Furnace Atomic Absorption Spectrometry, AuflZ. Chem. 1991, 63, 924A-931A. 

Stolzberg, R. J. Screening and Sequential Experimentation Simulations and Elame Atomic Absorption Spectrometry Experiments, /. Chem. Educ. 1997, 74, 216-220. 

Highly sensitive iastmmental techniques, such as x-ray fluorescence, atomic absorption spectrometry, and iaductively coupled plasma optical emission spectrometry, have wide appHcation for the analysis of silver ia a multitude of materials. In order to minimize the effects of various matrices ia which silver may exist, samples are treated with perchloric or nitric acid. Direct-aspiration atomic absorption (25) and iaductively coupled plasma (26) have silver detection limits of 10 and 7 l-lg/L, respectively. The use of a graphic furnace ia an atomic absorption spectrograph lowers the silver detection limit to 0.2 l-ig/L. 

Z. Pang, Elowinjection Atomic Absorption Spectrometry,Wiley Sons, Inc., New York, 1995. 

Numerous methods have been pubUshed for the determination of trace amounts of tellurium (33—42). Instmmental analytical methods (qv) used to determine trace amounts of tellurium include atomic absorption spectrometry, flame, graphite furnace, and hydride generation inductively coupled argon plasma optical emission spectrometry inductively coupled plasma mass spectrometry neutron activation analysis and spectrophotometry (see Mass spectrometry Spectroscopy, optical). Other instmmental methods include polarography, potentiometry, emission spectroscopy, x-ray diffraction, and x-ray fluorescence. 

Miscellaneous. Trace analyses have been performed for a variety of other materials. Table 9 Hsts some uses of electrothermal atomic absorption spectrometry (etaas) for determination of trace amounts of elements in a variety of matrices. The appHcations of icp /ms to geological and biological materials include the following (165) ... 

For the deterrnination of trace amounts of bismuth, atomic absorption spectrometry is probably the most sensitive method. A procedure involving the generation of bismuthine by the use of sodium borohydride followed by flameless atomic absorption spectrometry has been described (6). The sensitivity of this method is given as 10 pg/0.0044M, where M is an absorbance unit the precision is 6.7% for 25 pg of bismuth. The low neutron cross section of bismuth virtually rules out any deterrnination of bismuth based on neutron absorption or neutron activation. 

I have carried out widespread studies on the application of a sensitive and selective preconcentration method for the determination of trace a mounts of nickel by atomic absorption spectrometry. The method is based on soi ption of Cu(II) ions on natural Analcime Zeolit column modified with a new Schiff base 5-((4-hexaoxyphenylazo)-N-(n-hexyl-aminophenyl)) Salicylaldimine and then eluted with O.IM EDTA and determination by EAAS. Various parameters such as the effect of pH, flow rate, type and minimum amount of stripping and the effects of various cationic interferences on the recovery of ions were studied in the present work. 

The organic reagents are used extensively for determinations series of elements by different methods of analysis. We carry out the systematical investigation of organic derivatives of hydrazine as a reagent for determinations ion of metals by photometric and extractive-photometric methods or analysis, as well as methods of atomic absorption spectrometry. Series procedure determinations ion of metals in technical and environmental objects have been developed. 

MODERN ATOMIC ABSORPTION SPECTROMETRY ACHIEVEMENTS AND FUTURE PROSPECTS... 

Atomic absorption spectrometry (AAS) stalled its cai eer 50 years ago. During this time fundamentals of the method have been mostly discovered thus transforming AAS to very powerful but relatively simple method of analytical chemistry. Nowadays it is one of the most widespread methods in analytical labs. 

A NEW WAY TO CORRECT A NON-SELECTIVE LIGHT ABSORBANCE IN ATOMIC ABSORPTION SPECTROMETRY, BASING ON PRELIMINARY REGISTRATION OF MOLECULAR... 

FLOW INJECTION ELECTROCHEMICAL HYDRIDE GENERATION ATOMIC ABSORPTION SPECTROMETRY EOR THE DETERMINATION OE ARSENIC... 

INDIRECT DETERMINATION OF ASCORBIC ACID BY ELECTROTHERMAL ATOMIC ABSORPTION SPECTROMETRY... 

In this work, a method based on the reduction potential of ascorbic acid was developed for the sensitive detennination of trace of this compound. In this method ascorbic acid was added on the Cr(VI) solution to reduced that to Cr(III). Cr(III) produced in solution was quantitatively separated from the remainder of Cr(VI). The conditions were optimized for efficient extraction of Cr(III). The extracted Cr(III) was finally mineralized with nitric acid and sensitively analyzed by electro-thermal atomic absorption spectrometry. The determinations were carried out on a Varian AA-220 atomic absolution equipped with a GTA-110 graphite atomizer. The results obtained by this method were compared with those obtained by the other reported methods and it was cleared that the proposed method is more precise and able to determine the trace of ascorbic acid. Table shows the results obtained from the determination of ascorbic acid in two real samples by the proposed method and the spectrometric method based on reduction of Fe(III). 

The complex of the following destmctive and nondestmctive analytical methods was used for studying the composition of sponges inductively coupled plasma mass-spectrometry (ICP-MS), X-ray fluorescence (XRF), electron probe microanalysis (EPMA), and atomic absorption spectrometry (AAS). Techniques of sample preparation were developed for each method and their metrological characteristics were defined. Relative standard deviations for all the elements did not exceed 0.25 within detection limit. The accuracy of techniques elaborated was checked with the method of additions and control methods of analysis. 

COMPARISON OF MICROWAVE ASSISTED EXTRACTION METHODS FOR THE DETERMINATION OF PLATINUM GROUP ELEMENTS IN SOIL SAMPLES BY ELECTROTHERMAL ATOMIC ABSORPTION SPECTROMETRY AFTER PHASE SEPARATION-EXTRACTION... 

Direct atomic absorption spectrometry (AAS) analysis of increasing (e 0,10 g) mass of solid samples is the great practical interest since in a number of cases it allows to eliminate a long-time and labor consuming pretreatment dissolution procedure of materials and preconcentration of elements to be determined. Nevertheless at prevalent analytical practice iS iO based materials direct AAS are not practically used. 

B. Welz (translated by C. Skegg), Atomic Absorption Spectrometry, VCH, Weinheim, 1985. ISBN 0895734184. 

Electrothermal vaporization can be used for 5-100 )iL sample solution volumes or for small amounts of some solids. A graphite furnace similar to those used for graphite-furnace atomic absorption spectrometry can be used to vaporize the sample. Other devices including boats, ribbons, rods, and filaments, also can be used. The chosen device is heated in a series of steps to temperatures as high as 3000 K to produce a dry vapor and an aerosol, which are transported into the center of the plasma. A transient signal is produced due to matrix and element-dependent volatilization, so the detection system must be capable of time resolution better than 0.25 s. Concentration detection limits are typically 1-2 orders of magnitude better than those obtained via nebulization. Mass detection limits are typically in the range of tens of pg to ng, with a precision of 10% to 15%. 

Alkaline earth metals and transition metals Elame atomic absorption spectrometry ... 

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