Atomic absorption spectrometry impurities

For example, the industrial preparation of mineral acids, such as sulfuric, hydrochloric and nitric, inevitably leads to them containing small concentrations of metals as impurities. If the acid is to be used purely as an acid in a simple reaction, the presence of small amounts of metals is probably unimportant. If, however, the acid is to be used to digest a sample for the determination of trace metals by atomic absorption spectrometry, then clearly the presence of metallic impurities in the acid may have a significant effect on the results. For this latter application, high-purity acids that are essentially metal-free are required. 

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 . 

If the reference materials are pure substances and can be specified on the microscopic level, then they represent the unit amount of substance. Because there are no absolute pure substances the representation is in all cases an approximation. The degree of approximation is given by the accuracy of the contents of the main component. In case of pure elements, e.g. metals Fe, Cu, Zn the determination of the main component by coulometry is limited by an uncertainty of 0.01%. The determination of all impurities needs completeness and requires a great deal of analytical equipment. However, a combination of inductively coupled plasma-mass spectrometry (ICP-MS), atomic absorption spectrometry (AAS) and isotope dilution mass spectrometry (ID-MS) covering all elements of the periodic table allows a decrease of total uncertainty to 0.0032% (Cu, see Fig. 8). 

Atomic absorption spectrometry has been applied to the analysis of over sixty elements. The technique combines speed, simplicity and versatility and has been applied to a very wide range of non-ferrous metal analyses. This review presents a cross section of applications. For the majority of applications flame atomisation is employed but where sensitivity is inadequate using direct aspiration of the sample solution a number of methods using a preconcentration stage have been described. Non-flame atomisation methods have been extensively applied to the analysis of ultra-trace levels of impurities in non-ferrous metals. The application of electrothermal atomisation, particularly to nickel-based alloys has enabled the determination of sub-part per million levels of impurities to be carried out in a fraction of the time required for the chemical separation and flame atomisation techniques. 

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. 

Catalysts for the chemical industry have to be characterized with respect to their trace impurities and major components. Not only is their composition when they are used initially in chemical reactors important, but also their alteration in the course of time. As carbide forming elements such as V and Ti are often used, atomic absorption spectrometry could be problematic. This also applies to catalysts for exhaust gas detoxification in cars. Noble metals such as Pt, Pd and Rh are fixed on alumina supports often also containing cerium compounds. Both for the determination of the stoichiometry but also for the monitoring of the noble metal contents in used catalysts, AAS suffers from problems because of the need for sample dissolution as well as for the requirement to determine refractory oxide forming elements. 

Docekal B. and Krivan V. (1992) Direct determination of impurities in powered silicon carbide by electrothermal atomic absorption spectrometry using the slurry sampling technique, J Anal At Spectrom 7 521-528. 

Since the most common method of production utilizes organomer-cury precursor, mercury is the major toxic impurity. It is determined by atomic absorption spectrometry and its USP limit is 0.5 pg/ml of L-dopa solution... 

Dulski, T.R. and Bixler, R.R. (1977) Determination of trace impurities in iron- and nickel-base alloys by graphite furnace atomic absorption spectrometry. Anal. Chim. Acta. 91. 199-209. 

Kumar, S.J., Meeravah, N.N., Arunachalam, J. (1998) Determination of trace impurities in high purity gaUium by inductively coupled plasma mass spectrometry and cross vaUdation of results by transverse heated graphite furnace atomic absorption spectrometry. Analytica Chimica Acta, 371,305-316. 

The need for the determination of metallic constituents or impurities in pharmaceutical products has, historically, been addressed by ion chromatographic methods or various wet-bench methods (e.g. the USP heavy metals test). As the popularity of atomic spectroscopy has increased, and the equipment has become more affordable, spectroscopy-based techniques have been routinely employed to solve analytical problems in the pharmaceutical industry. Table 1 provides examples of metal determinations in pharmaceutical matrices, using spectroscopic techniques, and the reasons why these analyses are important. Flame atomic absorption spectrometry (FAAS), graphite furnace atomic absorption spectrometry... 

The conventional method for quantitative analysis of galHum in aqueous media is atomic absorption spectroscopy (qv). High purity metallic galHum is characteri2ed by trace impurity analysis using spark source (15) or glow discharge mass spectrometry (qv) (16). 

The biggest group of inorganic impurities are metals, which are usually analyzed using atomic spectrometry techniques (e.g., atomic absorption spectroscopy, AAS ... 

There are many methods for metal determination (Standard Methods, 1998 section 3000). Some, for example as gravimetric, titrimetric or colorimetric methods, are most effective at high metal concentrations. Others, for example atomic absorption (AA), inductively coupled plasma (ICP) or inductively coupled plasma mass spectrometry (ICPMS) are far more sensitive. The latter are used for typical textile applications, such as compliance testing for water quality or detection of trace impurities in high-volume raw materials. 

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