Atomic absorption spectrophotometry limitation

Theory. Conventional anion and cation exchange resins appear to be of limited use for concentrating trace metals from saline solutions such as sea water. The introduction of chelating resins, particularly those based on iminodiacetic acid, makes it possible to concentrate trace metals from brine solutions and separate them from the major components of the solution. Thus the elements cadmium, copper, cobalt, nickel and zinc are selectively retained by the resin Chelex-100 and can be recovered subsequently for determination by atomic absorption spectrophotometry.45 To enhance the sensitivity of the AAS procedure the eluate is evaporated to dryness and the residue dissolved in 90 per cent aqueous acetone. The use of the chelating resin offers the advantage over concentration by solvent extraction that, in principle, there is no limit to the volume of sample which can be used. 

A method has been developed for differentiating hexavalent from trivalent chromium [33]. The metal is electrodeposited with mercury on pyrolytic graphite-coated tubular furnaces in the temperature range 1000-3000 °C, using a flow-through assembly. Both the hexa- and trivalent forms are deposited as the metal at pH 4.7 and a potential at -1.8 V against the standard calomel electrode, while at pH 4.7, but at -0.3 V, the hexavalent form is selectively reduced to the trivalent form and accumulated by adsorption. This method was applied to the analysis of chromium species in samples of different salinity, in conjunction with atomic absorption spectrophotometry. The limit of detection was 0.05 xg/l chromium and relative standard deviation from replicate measurements of 0.4 xg chromium (VI) was 13%. Matrix interference was largely overcome in this procedure. 

Electrothermal atomic absorption spectrophotometry with Zeeman background correction was used by Zhang et al. [141] for the determination of cadmium in seawater. Citric acid was used as an organic matrix modifier and was found to be more effective than EDTA or ascorbic acid. The organic matrix modifier reduced the interferences from salts and other trace metals and gave a linear calibration curve for cadmium at concentrations < 1.6 pg/1. The method has a limit of detection of 0.019 pg/1 of cadmium and recoveries of 95-105% at the 0.2 pg of cadmium level. 

Different methods have different detection limits. For example, the flame atomic absorption spectrophotometry (AAS) method for aluminum has a detection limit of 30 parts per million, while the inductively coupled plasma... 

Lei et al. reported a method for the indirect determination of trace amounts of procaine in human serum by atomic absorption spectrophotometry [54], The sample was mixed with HCIO4, heated at 85°C for 30 minutes, diluted to a known volume with water, and centrifuged. 1 mL of the supernatant solution was buffered with 0.1 M sodium acetate-acetic acid to pH 3.86, and mixed with 0.2 M Zn(SCN)j reagent to a final concentration of 0.1 M. After dilution to 50 mL with water, the solution was shaken for 1 minute with 10 mL of 1,2-dichloroethane, whereupon the zinc extracted into the organic phase was determined by air-acetylene flame atomic absorption spectrometry for the indirect determination of procaine. The detection limit was found to be 0.1 pg/g, with a recovery of 89-98% and a coefficient of variation (n = 10) equal to 3.2%. 

Lead likewise should seldom if ever be found in wines approaching the suggested international limit of 0.6 mg/liter. The procedure by ashing and color development with dithiozone (or other reagent) is also time consuming and meticulous. Even with atomic absorption spectrophotometry there are losses in ashing and laboratory lead pick-up. Probably the best new attachments are those where the wine can be ashed directly in the apparatus. 

The exploitation of atomic-absorption spectrophotometry for monitoring HPLC column effluents has been recently examined by Funasaka et al. [46]. An eluent-vaporizing system was designed which introduced the effluent into the atomic-absorption unit. The limit of detection of compounds such as ethylmercury chloride was ca. 10 ng compared to 30 jug for a UV detector at 210 nm. The extreme selectivity of atomic absorption could make this technique of great value for the analysis of trace amounts of organometallic compounds and metal chelates. 

One limitation of atomic absorption spectrophotometry is that the samples generally have to be in solution, preferably aqueous. Thus, either the sample must be directly soluble in a suitable solvent or some type of pretreatment, such as acid digestion, is necessary. One exception is that some instruments using a graphite furnace can be modified for direct injection of solids. Another limitation is that only one metal can be analyzed at a time. There are four primary methods of accomplishing this ... 

Table 1 shows the detection limits of atomic absorption spectrophotometry for various metals. In general, flame atomic absorption spectrophotometry is quantitative in the lower parts-per-million levels and is readily automated for routine, high-volume samples. The other three techniques are used primarily for trace analysis and are quantitative to the lower parts-per-million levels for many elements. 

Reference 9 gives a review of applications of atomic absorption spectrophotometry to biological samples. Tiiis technique is widely used for metal analysis in biological fluids and tissues, in environmental samples such as air and water, and in occupational health and safety areas. Routine applications of flame emission spectrometry to biological samples are generally limited to the alkali and alkaline earth metals. Ion-selective electrode measurements (Chapter 13) have largely replaced the flame emission measurements in the clinical chemistry laboratory. 

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]. 

Analytical Methods and Speclatlon Electrothermal atomic absorption spectrophotometry (ETAAS), differential pulse adsorption voltammetry (DPAV), isotope-dilution mass spectrometry (ID-MS), and inductively coupled plasma mass spectrometry (ICP-MS) furnish the requisite sensitivity for measurements of nickel concentrations in biological, technical and environmental samples (Aggarwal et al. 1989, Case et al. 2001, Stoeppler and Ostapczuk 1992, Templeton 1994, Todorovska et al. 2002, Vaughan and Templeton 1990, Welz and Sperling 1999). The detection limits for nickel determinations by ETAAS analysis with Zeeman background correction are approximately 0.45 jg for urine,... 

For the determination of trace elements in the soil, atomic absorption spectrophotometry is also very suitable. A solution should be obtained from the sample to be analysed, most frequently by a decomposition with hydrofluoric acid extracts can also be prepared from the samples to be analysed. The detection limits and sensitivities for this technique are shown in Table 7.7. 

The literature on procedures for PbB determination is abundant. Those techniques that have been shown to provide accurate and precise PbB determinations in routine use include anodic stripping voltammetry (ASV), flame atomic absorption spectrophotometry (FAAS), discrete sampling FAAS, and graphite furnace AAS (GF-AAS). The method most widely used for routine determination is AAS in its various modifications. The relatively slow analysis rate of ASV tends to limit the application of this technique to that of a backup or reference method. Whatever the technique which is applied, it should be emphasized that avoidance of contamination, careful handling of the blood samples and frequent intra- and interlaboratory checks are more important for ensuring precision and reliability than the method itself. 

For flame atomic absorption spectrophotometry, the detection limit Is defined as the concentration that produces absorption equivalent to twice the magnitude of the background fluctuation. No mention is made of the blank or blank correction. This definition implies an instrument detection limit rather than a detection limit of a complete analytical procedure. Finally, no mention Is made of the need to determine the variability of responses. 

Collect the reaction solutions containing mercury ions which remain following completion of the manganese determination. Per 500 ml of solution treat with 30 g of sodium thiosulphate and then with 300 ml of 30 % sodium hydroxide solution. In this process the mercury is removed from the solution as sulphide. Hg2+ ions are no longer detectable in the filtrate of the supernatant colourless liquid (flameless atomic-absorption spectrophotometry detection limit 0.001 mg/1). 

Among the common metal ions, only aluminum and cobalt gave peaks when complexed with 8-quinolinol and eluted with SDS-acetonitrile mobile phases. However, the peaks appeared very close to each other with spectrophotometric detection (Fig. 12.2). The selective determination of aluminum was only possible with fluorimetric detection. The addition of SDS as well as several other surfactants to the aluminum complex solution, increased the fluorescence intensity. The procedure did not require deproteinization prior to analysis. The most commonly used technique for aluminum in human serum is graphite-furnace atomic absorption spectrophotometry, which is often limited due to serum matrix interference. 

The determination of germanium by means of atomic absorption spectrophotometry at the end of the 1960s was replaced by the introduction of the hydride technique with sodium tetrahydroborate as means of reduction. The detection limit is indicated with 2 x lOr g and 0.01 xg Ge/liter [44]. 

We have seen the relationship between absorption spectrophotometry and spectrofluorometry. A similar relationship exists between atomic absorption spectrophotometry and atomic fluorescence spectrophotometry. In atomic fluorescence, the flame retains its role as a source of atoms these atoms, however, are excited by an intense source of radiation and their fluorescent emission is assayed at an angle of 90° in a manner similar to that of spectrofluorimetry. Lack of sufficiently intense source for many elements has been the limitation of this technique, however, with time instrumental developments are overcoming this problem. High intensity hollow-cathode lamps, or xenon or mercury discharge lamps are used. 

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