Chemical interferences atomic absorption spectrometry

Atomic absorption spectrometry is one of the most widely used techniques for the determination of metals at trace levels in solution. Its popularity as compared with that of flame emission is due to its relative freedom from interferences by inter-element effects and its relative insensitivity to variations in flame temperature. Only for the routine determination of alkali and alkaline earth metals, is flame photometry usually preferred. Over sixty elements can be determined in almost any matrix by atomic absorption. Examples include heavy metals in body fluids, polluted waters, foodstuffs, soft drinks and beer, the analysis of metallurgical and geochemical samples and the determination of many metals in soils, crude oils, petroleum products and plastics. Detection limits generally lie in the range 100-0.1 ppb (Table 8.4) but these can be improved by chemical pre-concentration procedures involving solvent extraction or ion exchange. 

Flame atomic absorption spectrometry can be used to determine trace levels of analyte in a wide range of sample types, with the proviso that the sample is first brought into solution. The methods described in Section 1.6 are all applicable to FAAS. Chemical interferences and ionization suppression cause the greatest problems, and steps must be taken to reduce these (e.g. the analysis of sea-water, refractory geological samples or metals). The analysis of oils and organic solvents is relatively easy since these samples actually provide fuel for the flame however, build-up of carbon in the burner slot must be avoided. Most biological samples can be analysed with ease provided that an appropriate digestion method is used which avoids analyte losses. 

The disadvantages of electrothermal atomisation (ETA) — atomic absorption spectrometry (AAS) are the physical, chemical and spectral interferences, these being more severe than with flame atomic absorption spectrometry (FAAS), and which depend critically upon the experimental and operational conditions within the atomiser and the nature of the chemical pretreatment used. It is not intended to discuss here the theoretical aspects of these interferences which have been reviewed excellently elsewhere [2], but it is pertinent to consider briefly how these interferences affect the various stages of the analysis and how they may be minimised. 

Radziuk, B. and Thomassen Y. (1992). Chemical modification and spectral interferences in selenium determination using Zeeman-effect electrothermal atomic absorption spectrometry. J. Anal. At. Spectrom., 7, 397. 

Table 6 Chemical interferences in the determination of some elements, and reagents for eliminating the interferences (R = releasing, S Shielding) (Source T.C. Rains, Chemical Interferences in Condensed Phase, in Flame Emission and Atomic Absorption Spectrometry, VoL V, ed.s J.A. Dean and T.C. Rains, Marcel Dekker, New York, 1%9)...

In addition to spectral interferences, chemical interferences are also significant in AA spectrometry. Although in many instances, they can be reduced by judicious optimisation of the operating conditions. Chemical interferences are observed in atomic absorption spectrometry as a consequence of (a) formation of compounds of low volatility, (b) influence on dissociation equilibria, and (c) ionisation of the analytes. 

Fiydride generation (and cold-vapor) techniques significantly improve atomic absorption spectrometry (AAS) concentration detection limits while offering several advantages (1) separation of the analyte from the matrix is achieved which invariably leads to improved accuracy of determination (2) preconcentration is easily implemented (3) simple chemical speciation may be discerned in many cases and (4) the procedures are amenable to automation. Disadvantages with the approach that are frequently cited include interferences from concomitant elements (notably transition metals), pH effects, oxidation state influences (which may be advantageously used for speciation) and gas-phase atomization interferences (mutual effect from other hydrides). 

Direct measurements of several trace metals by electrothermal atomic absorption spectrometry (ETAAS) have been reported. In general, sensitivities are inadequate for open-ocean waters, though in more metal-enriched environments (e.g., coastal waters and sediment pore waters) such analysis is possible careful corrections for the large and complex salt effects are necessary. The interferences can be minimized by the use of appropriate chemical modifiers, platforms in the graphite tubes, and sophisticated background correction schemes such as Zeeman. 

Several methods are available for the determination of total aluminum in biological and other materials. Chemical and physicochemical methods are in most practical situations insensitive and inaccurate X-ray fluorescence is specific but lacks sensitivity neutron activation analysis is complex and subject to interferences, although it is a very sensitive technique. Nuclear magnetic resonance spectroscopy is not very sensitive but useful to get information on speciation [33]. Graphite furnace atomic absorption spectrometry (GFAAS) is the most widely used technique and can produce reliable results, provided that the matrix effects are recognized and corrected. Savory and Wills [19] reviewed chemical and physicochemical methods for the determination of aluminum in biological materials, e.g. X-ray fluorescence, neutron activation analysis, atomic emission spectrometry, flame emission, inductively coupled plasma emission spectroscopy, and AAS. 

J.Y. Cabon and A.L. Bihan. Direct determination of zinc in seawater using electrothermal atomic absorption spectrometry with Zeeman-effect background correction effects of chemical and spectral interferences. Journal of Analytical Atomic Spectrometry 9 477-481,1994. 

Byrne,]. P, Chakrabarti, C. L., Gregoire, D. C., Lamoureux, M., and Ly,T. (1992). Mechanisms of chloride interferences in atomic absorption spectrometry using a graphite furnace atomizer investigated by electrothermal vaporization inductively coupled plasma mass spectrometry. Part 1. Effect of magnesium chloride matrix and ascorbic acid chemical modifier on manganese.J. A [Pg.199]

Atomic fluorescence flame spectrometry is receiving increased attention as a potential tool for the trace analysis of inorganic ions. Studies to date have indicated that limits of detection comparable or superior to those currently obtainable with atomic absorption or flame emission methods are frequently possible for elements whose emission lines are in the ultraviolet. The use of a continuum source, such as the high-pressure xenon arc, has been successful, although the limits of detection obtainable are not usually as low as those obtained with intense line sources. However, the xenon source can be used for the analysis of several elements either individually or by scanning a portion of the spectruin. Only chemical interferences are of concern they appear to be qualitatively similar for both atomic absorption and atomic fluorescence. With the current development of better sources and investigations into devices other than flames for sample introduction, further improvements in atomic fluorescence spectroscopy are to be expected. 

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