Atomic absorption spectrometry spectral interferences

Spectral interference is rarely encountered in atomic absorption spectrometry. Spectral interferences in the past were experienced typically if, in a given solution, element A was being determined in the presence of element... 

Spectral overlap of emission and absorption wavelengths Is a potential cause of Interference In atomic absorption spectrometry (57) Thus, (a) the emission line of Fe at 352.424 nm Is close to the resonance line of N1 at 352.454, (b) the emission line of Sb at 217.023 nm Is close to the resonance line of Pb at 216.999 nm, and (c) the emission line of As at 228.812 nm Is close to the resonance line of Cd at 228.802 (57). To date, these practically coincident spectral lines have not been reported to be of practical Importance as sources of analytical Interference In atomic absorption analyses of biological materials. 

Flame emission spectrometry is used extensively for the determination of trace metals in solution and in particular the alkali and alkaline earth metals. The most notable applications are the determinations of Na, K, Ca and Mg in body fluids and other biological samples for clinical diagnosis. Simple filter instruments generally provide adequate resolution for this type of analysis. The same elements, together with B, Fe, Cu and Mn, are important constituents of soils and fertilizers and the technique is therefore also useful for the analysis of agricultural materials. Although many other trace metals can be determined in a variety of matrices, there has been a preference for the use of atomic absorption spectrometry because variations in flame temperature are much less critical and spectral interference is negligible. Detection limits for flame emission techniques are comparable to those for atomic absorption, i.e. from < 0.01 to 10 ppm (Table 8.6). Flame emission spectrometry complements atomic absorption spectrometry because it operates most effectively for elements which are easily ionized, whilst atomic absorption methods demand a minimum of ionization (Table 8.7). 

A convenient method is the spectrometric determination of Li in aqueous solution by atomic absorption spectrometry (AAS), using an acetylene flame—the most common technique for this analyte. The instrument has an emission lamp containing Li, and one of the spectral lines of the emission spectrum is chosen, according to the concentration of the sample, as shown in Table 2. The solution is fed by a nebuhzer into the flame and the absorption caused by the Li atoms in the sample is recorded and converted to a concentration aided by a calibration standard. Possible interference can be expected from alkali metal atoms, for example, airborne trace impurities, that ionize in the flame. These effects are canceled by adding 2000 mg of K per hter of sample matrix. The method covers a wide range of concentrations, from trace analysis at about 20 xg L to brines at about 32 g L as summarized in Table 2. Organic samples have to be mineralized and the inorganic residue dissolved in water. The AAS method for determination of Li in biomedical applications has been reviewed . 

D. C. Baxter and J. Ohman, Multi-component standard additions and partial least squares modelling, a multivariate calibration approach to the resolution of spectral interferences in graphite furnace atomic absorption spectrometry, Spectrochim. Acta, Part B, 45(4 5), 1990, 481 491. 

Graphite Furnace Atomic Absorption Spectrometry Graphite furnace atomic absorption spectrometry (GFAAS), the most popular form of ET-AAS, is today a common technique widely used in routine laboratories and has become a powerful tool for the analysis of trace and ultratrace elements in clinical and biological samples [61]. The main advantages of this technique are low cost, simplicity, excellent detection power, and the fact that it allows very low sample volumes to be used (5-20 p,L). In this sense, this technique allows LoDs for many elements in the order of 0.01 pgl-1 in solution or 1 pg g-1 in solid samples to be achieved [62]. However, the technique is prone to spectral and matrix interferences. 

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. 

The general application of LEI spectrometry to the determination of trace metals has been somewhat limited but progress in this area should continue. Alloy analyses are particularly amenable to LEI spectrometry because of the absence of an ionizable sample matrix. Indium (303.9 nm) has been determined in a nickel-based high temperature alloy31. Atomic absorption spectrometry of this sample requires time-consuming extraction procedures to remove concomitant metals which contribute to spectral interferences. After dissolution of the alloy sample with acids, the concentration of indium was determined to be 35 pg/g by LEI spectrometry. 

The resolution and selectivity in ICP emission comes primarily from the monochromator. As a result, a high-resolution monochromator can isolate the analyte spectral line from lines of concomitants and background emission. It can thus reduce spectral interferences. In atomic absorption spectrometry, the resolution comes primarily from the very narrow hollow cathode lamp emission. The monochromator must only isolate the emission line of the analyte element from lines of impurities and the fill gas, and from background emission from the atomizer. A much lower resolution is needed for this puipose. 

A modification of the GFAAS method for determining selenium levels in human urine was described by Saeed (1986). In this electrothermal atomic absorption spectrometry (EAAS) method, nitric acid, nickel, and platinum are added to the graphite cell. The addition of nickel helps to mask the spectral interference from phosphates in urine. EAAS has been used to determine selenium levels in human spermatozoa (Suistomaa et al. 1987). 

The determination of technetium by atomic absorption spectrophotometry was studied with a Tc hollow-cathode lamp as a spectral line source. The sensitivity for technetium in aqueous solution was 3-10 g/ml in a fuel-rich acetylene-air flame for the unresolved 2614.23-2615.87 A doublet. Cationic interferences were eliminated by adding aluminum to the sample solutions. The applicability of atomic absorption spectrophotometry to the determination of technetium in uranium and a uranium alloy was demonstrated [42]. A detection limit of 6 10 g w as achieved for measuring technetium by graphite furnace atomic absorption spectrometry. In using the same doublet and both argon and neon as fill gases for the lamp, 6-10 to 3 10 g of technetium was found to be the range of applicability [43]. 

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. 

Saeed, K. and Thomassen, Y. (1981). Spectral interferences from phosphate matrices in the determination of arsenic, antimony, selenium and tellurium by electrothermal atomic absorption spectrometry. Anal. Chim. Acta 130, 281. 

Initially hydride generation and cold vapour techniques were developed for the quantitative determination of the hydride-forming elements and mercury by atomic absorption spectrometry (Chapters, Sections 6.2 and 6.3), but nowadays these methods are also widely used in plasma atomic emission spectrometry. In the hydride generation technique, hydride-forming elements are more efficiently transported to the plasma than by conventional solution nebulization, and the production and excitation of free atoms and ions in the hot plasma is therefore more efficient. Spectral interferences are also reduced when the analyte is separated from the elements in the sample matrix. Both continuous (FIA) and batch approaches have been used for hydride generation. The continuous method is more frequently used in plasma AES than in AAS. Commercial hydride generation systems are available for various plasma spectrometers. 

A more recent addition to the AES family uses high temperature plasma to promote emission. Inductively coupled plasma atomic absorption spectrometry (ICP-AES) has the advantage that a wide range of elements may be measured simultaneously. However, great problems with spectral interferences restrict the value of this technique for complex mixture such as sediments. 

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. 

AAS is the most widely used analytical technique for the determination of lead in biological materials [57,58], The majority of AAS methods employ the electrothermal atomic absorption spectrometry (ETAAS) technique, using either Zeeman background correction or deuterium background correction for the determination of lead in biological fluids [55,59-65], Urine is less often employed as an indicator of exposure however, similar problems associated with AAS determination of lead exist for blood as well as urine (1) incomplete atomization (2) volatile lead salts (3) spectral interferences (4) buildup of carbonaceous residue reducing sensitivity and precision. These analytical problems are eliminated by optimal sample preparation, e,g., dilution, addition of matrix modifiers, deproteinization, and background correction and calibration by matrix-matched standards [66],... 

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. 

D. C. Baxter, W. Freeh and I. Berglund, Use of partial least squares modelling to compesate for spectral interferences in electrothermal atomic absorption spectrometry with continuum source background correction,... 

Spectral lines of molecular bands emitted by molecules and radicals present in a plasma often interfere with the atomic spectral lines in atomic emission spectrometry. However, in atomic absorption spectrometry the absorption by molecular bands stemming from undissodated molecules in the atom reservoir also leads to systematic errors and requires correction. Furthermore, in mass spectrometry molecular fragments give rise to signals, which can also interfere with the signals... 

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