Flame atomic absorption spectrometry interferences

Fang et al. [661] have described a flow injection system with online ion exchange preconcentration on dual columns for the determination of trace amounts of heavy metal at pg/1 and sub-pg/1 levels by flame atomic absorption spectrometry (Fig. 5.17). The degree of preconcentration ranges from a factor of 50 to 105 for different elements, at a sampling frequency of 60 samples per hour. The detection limits for copper, zinc, lead, and cadmium are 0.07, 0.03, 0.5, and 0.05 pg/1, respectively. Relative standard deviations are 1.2-3.2% at pg/1 levels. The behaviour of the various chelating exchangers used was studied with respect to their preconcentration characteristics, with special emphasis on interferences encountered in the analysis of seawater. 

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. 

To avoid problems previously encountered with flame atomic absorption spectrometry of arsenic, and also with flameless methods such as that in which the dementis converted to arsine, Ohta and Suzuki [25] proposed an alternative method based on electrothermal ionisation with a metal microtube atomiser. Effective atomisation can be achieved by the addition of thiourea to the arsenic solution or by preliminary extraction of the arsenic-thionalide complex. The second method is recommended for soil samples so as to avoid interference due to the presence of trace elements. 

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 analyte is accumulated on the filter as a solid compound which is further dissolved and directed towards the detector. Matrix interferences are circumvented and sensitivity is improved, as demonstrated in the determination of copper in silicate rocks by flame atomic absorption spectrometry [295]. The analyte was precipitated by rubeanic acid, the precipitate was separated by continuous filtration and the depleted sample zone was directed towards waste. After precipitate collection, a potassium dichromate stream was allowed to pass through the filtering unit, thus dissolving the retained precipitate and transporting it towards the detector. The dynamic concentration range was expanded (0.3—200 -igL 1 Cu) by passing different sample volumes through the... 

In-line filtration without a filtering element is also feasible. To this end, a three-dimensional reactor [299], also called a knitted or knotted reactor (see 6.2.3.4), can be used, as emphasised in the landmark article reporting the flow injection determination of lead in blood and bovine liver by flame atomic absorption spectrometry [300]. The analyte was co-precipitated the complex formed was retained on the inner walls of a knitted reactor and then released by isobutyl methyl ketone and transported to the detector. Interference from iron(III) at high concentrations was circumvented, sensitivity was markedly improved and precise results were obtained. This innovation was recently exploited to remove organic selenium and determine the speciation of inorganic selenium in a flow-injection system with atomic fluorescence spectrometric detection [301]. 

Spectrometric methods require a prior sampling preparation containing a separation step. The separation step is necessary especially to eliminate interference. Nonspectral interferences in flame atomic absorption spectrometry can be overcome by using a calibration model.221 The model uses two independent variables for analyte quantification (the amount of the sample and the amount of analyte added) the measured absorbance is the dependent variable. To control the matrix interferences without prior knowledge of the matrix composition, it is necessary to carry out nine calibration points to obtain accurate analytical information. This confers high reliability of the analytical information for determination of trace elements in complex matrices. 

Ihnat M (1978) Interferences in the determination of chromium by flame atomic absorption spectrometry. Can J Spectrosc 23 112-125. 

Limbeck A, Puls C, Handler M (2007) Platinum and palladium emissions from on-road vehicles in the Kaisermtihlen tunnel (Vienna, Austria). Env Sci Technol 41 4938 945 Liu P, Su Z, Wu X, Pu Q (2002) Application of isodiphenylthiourea immobilized silica gel to flow injection on-line microcolumn preconcentration and separation coupled with flame atomic absorption spectrometry for interference-free determination of trace silver, gold, palladium and platinum in geological and metallurgical samples. J Anal At Spectrom 17 125-130... 

AAS is split into two types according to the method by which the sample is atomized. In flame atomic absorption spectrometry (FAAS) the sample is aspirated into a flame which is placed in the path ofthe light. In electrothermal atomic absorption (EAAS) the sample is placed in a graphite tube and heated in a brief pulse by passing an electric current through the tube. Generally, EAAS is more sensitive, giving better detection limits, but suffers from more matrix interference effects than FAAS. 

See also Atomic Absorption Spectrometry Interferences and Background Correction Flame Electrothermal Vapor Generation. Atomic Spectrometry Overview. Flow Injection Analysis Principles. 

Flame atomic absorption was until recently the most widely used techniques for trace metal analysis, reflecting its ease of use and relative freedom from interferences. Although now superceded in many laboratories by inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry, flame atomic absorption spectrometry still is a very valid option for many applications. The sample, usually in solution, is sprayed into the flame following the generation of an aerosol by means of a nebulizer. The theory of atomic absorption spectrometry (AAS) and details of the basic instrumentation required are described in a previous article. This article briefly reviews the nature of the flames employed in AAS, the specific requirements of the instrumentation for use with flame AAS, and the atomization processes that take place within the flame. An overview is given of possible interferences and various modifications that may provide some practical advantage over conventional flame cells. Finally, a number of application notes for common matrices are given. 

Flame atomic absorption spectrometry The determination of magnesium by FAAS is performed in diluted samples using the resonance line 285.2 nm and a stoichiometric air-acetylene flame. The interference due to phosphate is eliminated by the addition of LaCls. FAAS is used for the most reliable determination of total magnesium in physiological samples and has been chosen as the basis for the reference method. 

See also Atomic Absorption Spectrometry Interferences and Background Correction. Atomic Emission Spectrometry Principles and Instrumentation Interferences and Background Correction Flame Photometry Inductively Coupled Plasma Microwave-Induced Plasma. Atomic Mass Spectrometry Inductively Coupled Plasma Laser Microprobe. Countercurrent Chromatography Solvent Extraction with a Helical Column. Derivatization of Analytes. Elemental Speciation Overview Practicalities and Instrumentation. Extraction Solvent Extraction Principles Solvent Extraction Multistage Countercurrent Distribution Microwave-Assisted Solvent Extraction Pressurized Fluid Extraction Solid-Phase Extraction Solid-Phase Microextraction. Gas Chromatography Ovenriew. Isotope Dilution Analysis. Liquid Chromatography Ovenriew. 

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). 

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. 

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 . 

The importance of catalysts in our energy and pollution conscious age is growing. Many catalysts depend for their activity on low levels of rather exotic metals, while even trace surface levels of elements such as lead may impair their activity. Thus there is plenty of scope for atomic absorption spectrometry. Many homogeneous catalysts are deposited on an alumina base, thus obtaining dissolution is not always easy and some interference in the air/acetylene flame may be encountered. A leaching procedure (e.g. with nitric acid) to dissolve adsorbed trace metals may be used to circumvent these problems. 

Clinical measurements of lithium may be performed using atomic absorption spectrometry (AAS) or flame emission spectrometry (FES) 64). AAS is regarded as the more reliable of the two techniques for blood lithium. FES is rather more sensitive, but suffers from interference from the high sodium and potassium concentrations in blood. Recent developments include inductively coupled plasma (ICP) emission spectrometry, electrothermal atomization atomic absorption spectrometry (ETAAS), and spectrofluorimetric methods. Spectrofluorime-try and ETAAS offer greater sensitivity than the traditional methods and are useful research tools 65, 66). 

For atomic absorption spectrometry, the selectivity refers to the reaction that takes place in the flame. Every element absorbs at a specific wavelength. Interference can be the result of anions from the matrix, which can produce a flame background or which can just mask the ions of interest. At a certain ratio, the influence of the anions is not significant. Furthermore, the influence depends on the composition of the matrix.323 325... 

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]. 

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