Electrothermal AAS

Schematic design of an electrothermal atomizer transversely heated graphite tube and graphite contacts (Courtesy of Perkin-Elmer). 

A common problem in ETAAS is that analyte volatility depends on the compound in which it is present—chlorides are usually much more volatile than sulfates or phosphates— and in general on the concomitants. A pyrolysis curve established for one sample type may therefore not be valid 

Pyrolysis-atomization curves for electrothermal A AS. A = integrated absorbance signal plotted against applied pyrolysis temperature (pyrolysis curve) B = integrated absorbance signal plotted against atomization temperature (atomization curve). 1 = maximum pyrolysis temperature 2 = lowest temperature of complete volatilization 3 = appearance temperature 4 — optimum atomization temperature. (From Ref. 23 by permission.) 

In contrast to FAAS not only liquid but also solid samples can be analyzed in ETAAS. However, one of the problems is the reliable introduction of solid samples into the atomizer. The high sensitivity of the technique makes it necessary to handle samples in the submilligram range. The most reliable procedure was shown to be the slurry technique [14] whereby a finely powdered and/or homogenized solid sample is kept in suspension by ultrasonic agitation so that it can be introduced into the atomizer with a conventional autosampler. This technique has the significant advantage that a solid sample can be treated like a liquid, i.e., it can not only be pipetted but can also be diluted, mixed with a modifier, and several aliquots can be taken reproducibly. 

The above-mentioned problem with the high sensitivity of ETAAS which makes it necessary to handle submilligram masses of sample can be of advantage if microsamples have to be analyzed. Typical examples are the analysis of hair segments [15], finger nails, or other forensic samples. 

There are many reasons for incorrect data, but many times the source of error is traceable to either standards, sample preparation, or contamination. 

Studies of variations of the time course of concentration for trace elements can also benefit from the high sensitivity of ETA techniques. Until recently, these applications were in the province of techniques such as neutron activation or X-ray fluorescence, which require the use of high-cost irradiation, counting and computing facilities. 

The new, commercially available heated graphite ETA devices are available at much lower cost than XRF devices, and will certainly encourage expansion of the application of low total mass collection type studies. These heated graphite rods or furnaces are the type of ETA devices which one is most likely to encounter in general air pollution applications and most of this chapter s comments will be directed toward them however, the comments will also be pertinent to special purpose devices such as heated foil and wire atomizers. The reader is directed to Chapters 2 and 3 and to Siemer [9] for general descriptions of ETA devices, their applications, their characteristics and their limitations. Bancroft [10] has reviewed ETA applications to the ultratrace determination of metals. 

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Method abbreviations D-AT-FAAS (derivative flame AAS with atom trapping), ETAAS (electrothermal AAS), GC (gas chromatography), HGAAS (hydride generation AAS), HR-ICP-MS (high resolution inductively coupled plasma mass spectrometry), ICP-AES (inductively coupled plasma atomic emission spectrometry), ICP-MS (inductively coupled plasma mass spectrometry), TXRF (total reflection X-ray fluorescence spectrometry), Q-ICP-MS (quadrapole inductively coupled plasma mass spectrometry)... 

Soares M, Bastos M and Ferreria M (1994) Determination of Total Chromium and Chromium (IV) in Animal Feeds by Electrothermal AAS. J Anal Atom Spectrom 9 1269-1272. 

An appropriate amount of hydrated iron (III) or bismuth oxide was added the oxide precipitates were prepared separately and washed thoroughly with distilled water before use [43]. After about 24 h, the samples were filtered on 0.4 jtm Nuclepore filters. The separated precipitates were dissolved with hydrochloric acid and the solutions obtained were used for /-activity measurements. In the examination of solvent extraction, chromium was measured by using 51Cr, while iron and bismuth were measured by electrothermal AAS (EAAS). The decomposition of organic complexes and other procedures were also examined by EAAS. 

Batley [780] found that in situ deposition of lead and cadmium on a mercury-coated tube was the more versatile technique. The mercury film, deposited in the laboratory, is stable on the dried tubes which are used later for field electrodeposition. The deposited metals were then determined by electrothermal AAS. 

Bismuth Liquid-liquid extraction into xylene as the ammonium pyrrolidine dithiocarbamate complex Electrothermal AAS 0.3 ppt or 0.0005 xg/l [95]... 

Indium Co-precipitation with gallium phosphate Electrothermal AAS 0.3 pg/1 (500 ml sample) [870]... 

Manganese Manganese complexed with ammonium and diethyl am- Electrothermal AAS of Freon - [65,448]... 

Silver Silver complexed with ammonium dithiophosphorate and o, o diethylester then adsorbed onto carbon, silver desorbed with nitric acid Electrothermal AAS 0.3 ng/1 0.0003 ng/1 [908]... 

Burns et al. [106] used electrothermal AAS to determine inorganic and butyltin in seawater. The butyltin is extracted into toluene and the inorganic tin extracted as its Sn(IV) 8-hydroxyquinoline chelate into chloroform. The detection limit was 0.7 ng of tin. 

Chemistry (Brown et al. 1981). Direct aspiration into a flame and atomization in an electrically heated graphite furnace or carbon rod are the two variants of atomic absorption. The latter is sometimes referred to as electrothermal AAS. Typical detection limits for electrothermal AAS are <0.3 pg/L, while the limit for flame AAS and ICP-AES is 3. 0 pg/L (Stoeppler 1984). The precision of analytical techniques for elemental determinations in blood, muscles, and various biological materials has been investigated (Iyengar 1989). Good precision was obtained with flame AAS after preconcentration and separation, electrothermal AAS, and ICP-AES. 

Water Acid digestion in mixture of nitric, sulfuric, and perchloric acids Electrothermal AAS 0.2 pg Ni/L fluids 98% at 5 pg Ni/L 97%at8pg Ni/L lARC 1986 (Method 11)... 

Electrothermal AAS (i) Lower detection limits. Less precision, slower. 

Cobalt, Nickel, Copper Plants Ultrasonic slurry sampling, electrothermal AAS [85]... 

H. Matusiewicz, M. Mikolajczank, Determination of As, Sb, Sn, and Hg in beer and wort by direct hydride generation sample introduction - electrothermal AAS, J. Anal. Atom Spectrom., 482 (2001), 652-657. 

Operating parameters for electrothermal AAS determination of the rare-earth elements [194],... 

While Cr(III) is considered to be essential in nutrition and for the maintenance of normal glucose tolerance, Cr(VI) can have acute and chronic toxic effects, including carcinogenicity. For this reason, the determination of Cr(III) and Cr(VI) in environmental samples has become very important and has led to a variety of approaches to differentiate between these species. A speciation method for Cr by electrothermal AAS (ETAAS) was developed, whereby tfacFI reacts selectively with Cr(III) to form a chelate, which is volatilized at 140 °C and an aliquot of the recovered residue is placed in the graphite furnace for atomization of Cr(VI). The LOD and LOQ of the method are 0.15 and 0.52 p.gL Cr(IV), respectively. The Cr(III) concentration was established by difference from total... 

The methods officially used in the wine trade transactions are summarized in Table 8.1. Generally, the OIV methods are officially adopted in the European Union without significant technical changes. The methods reported are mainly colorimetric, titrimetric, or use Atomic Emission Spectroscopy (AES, e.g. Flame Spectrophotometry), Atomic Absorption Spectroscopy (AAS), Hydride Generation-AAS (HG-AAS), Electrothermal-AAS (ET-AAS) and Vapour Atomic Flourescence Spectrophotometry (VAF). 

Element Flame AA Electrothermal AA Flame Emission ICP Emission ICP-MS... 

The use of furnaces as atomizers for quantitative AAS goes back to the work of L vov and led to the breakthrough of atomic absorption spectrometry towards very low absolute detection limits. In electrothermal AAS graphite or metallic tube or cup furnaces are used, and through resistive heating temperatures are achieved at which samples can be completely atomized. For volatile elements this can be accomplished at temperatures of 1000 K whereas for more refractory elements the temperatures should be up to 3000 K. 

The high absolute power of detection of electrothermal AAS is due to the fact that the sample is completely atomized and brought in the vapor phase as well as to the fact that the free atoms are kept in the atom reservoir for a long time. The signals obtained are transient, as discussed earlier. 

Apart from graphite tube furnaces, both cups and filaments are used as atomizers in electrothermal AAS [271]. The models originally proposed by L vov et al. [171] and by Massmann [172] were described in Section 3.4. In the case of the latter, which is most widely used, the optical beam is led centrally through the graphite tube, which is closed at both ends with quartz viewing ports mounted in the cooled tube holders. Sample aliquots are introduced with the aid of a micropipette or a computer controlled dispenser through a sampling hole in the middle of the tube. 

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