Electrothermal atomisation

The main advantages of electrothermal atomisers are that (a) very small samples (as low as 0.5 pL) can be analysed (b) often very little or no sample preparation is needed, in fact certain solid samples can be analysed without prior dissolution (c) there is enhanced sensitivity, particularly with elements with a short-wavelength resonance line in practice there is an improvement of between 102- and 103-fold in the detection limits for furnace AAS compared with flame AAS. 

Although electrothermal atomisation methods can be applied to the determination of arsenic, antimony, and selenium, the alternative approach of hydride generation is often preferred. Compounds of the above three elements may be converted to their volatile hydrides by the use of sodium borohydride as reducing agent. The hydride can then be dissociated into an atomic vapour by the relatively moderate temperatures of an argon-hydrogen flame. 

Carrion N, De Behzo ZA, Moreno B, Fernandez EJ, Flores D (1988) Determination of copper, chromium, iron and lead in pine needles by electrothermal atomisation spectrometry with slurry sample introduction. J Anal At Spectrom 3 479-483. 

Plasmas compare favourably with both the chemical combustion flame and the electrothermal atomiser with respect to the efficiency of the excitation of elements. The higher temperatures obtained in the plasma result in increased sensitivity, and a large number of elements can be efficiently determined. Common plasma sources are essentially He MIP, Ar MIP and Ar ICP. Helium has a much higher ionisation potential than argon (24.5 eV vs. 15.8 eV), and thus is a more efficient ionisation source for many nonmetals, thereby resulting in improved sensitivity. Both ICPs and He MIPs are utilised as emission detectors for GC. Plasma-source mass spectrometry offers selective detection with excellent sensitivity. When coupled to chromatographic techniques such as GC, SFC or HPLC, it provides a method for elemental speciation. Plasma-source detection in GC is dominated by GC-MIP-AES... 

Principles and Characteristics Flame emission instruments are similar to flame absorption instruments, except that the flame is the excitation source. Many modem instruments are adaptable for either emission or absorption measurements. Graphite furnaces are in use as excitation sources for AES, giving rise to a technique called electrothermal atomisation atomic emission spectrometry (ETA AES) or graphite furnace atomic emission spectrometry (GFAES). In flame emission spectrometry, the same kind of interferences are encountered as in atomic absorption methods. As flame emission spectra are simple, interferences between overlapping lines occur only occasionally. 

EDI-CI Electric discharge-induced chemical ionisation ETAAS, ET-AAS Electrothermal (atomisation) atomic absorption spectrometry... 

EDL Electrodeless discharge lamp ETA AES Electrothermal atomisation atomic... 

Electron diffraction spectroscopy ETA LEAFS Electrothermal atomisation laser-excited atomic fluorescence... 

Yuzefovsky et al. [241] used Cis resin to preconcentrate cobalt from seawater prior to determination at the ppt level by laser-excited atomic fluorescence spectrometry with graphite electrothermal atomiser. 

Ohta and Suzuki [397] investigated the electrothermal atomisation of lead for accurate determination of lead in water samples. Thiourea served to lower the atomisation temperature of lead and to eliminate the interferences from chloride matrix. The addition of thiourea also allowed the accurate determination of lead irrespective of its chemical form. The absolute sensitivity (1% absorption) was 1.1 x 10 12 g of lead. The method permits the direct rapid determination of lead in water samples including seawater. 

Bruland et al. [122] have shown that seawater samples collected by a variety of clean sampling techniques yielded consistent results for copper, cadmium, zinc, and nickel, which implies that representative uncontaminated samples were obtained. A dithiocarbamate extraction method coupled with atomic absorption spectrometry and flameless graphite furnace electrothermal atomisation is described which is essentially 100% quantitative for each of the four metals studied, has lower blanks and detection Emits, and yields better precision than previously published techniques. A more precise and accurate determination of these metals in seawater at their natural ng/1 concentration levels is therefore possible. Samples analysed by this procedure and by concentration on Chelex 100 showed similar results for cadmium and zinc. Both copper and nickel appeared to be inefficiently removed from seawater by Chelex 100. Comparison of the organic extraction results with other pertinent investigations showed excellent agreement. 

Batley and Matousek [390,778] examined the electrodeposition of the irreversibly reduced metals cobalt, nickel, and chromium on graphite tubes for measurement by electrothermal atomisation. This method offered considerable potential for contamination-free preconcentration of heavy metals from seawater. Although only labile metal species will electrodeposit, it is likely that this fraction of the total metal could yet prove to be the most biologically important at the natural pH [779]. 

Copper Adsorption on Qg resin Laser excited atomic fluorescence spectrometry with a graphite electrothermal atomiser 0.001 pg/1 [241]... 

Based on the configurations in Figure 1.5, many analytical techniques have been developed employing different atomisation/excitation sources. For example, two powerful AAS techniques are widespread one uses the flame as atomiser (FAAS) whereas the other is based on electrothermal atomisation (ETAAS) in a graphite furnace. Although the flame has limited application in OES, many other analytical emission techniques have evolved in recent decades based on dilTerent atomisation/excitation plasma sources. 

An electrothermal atomiser, where a drop of the liquid sample is placed in an electrically heated graphite tube which consists of a cylinder (3-5 cm in length and a few millimetres in diameter) with a tiny hole in the centre of the tube wall for sample introduction (see Figure 1.7a). Both ends of the... 

Figure 1.7 Electrothermal atomisation atomic absorption spectrometry, (a) Photograph of a graphite tube, (b) Photograph of a L vov platform, (c) Schematic front and side-on views of a graphite tube with a L vov platform.

M. Grotti, P. Rivaro and R. Frache, Determination of butyltin compounds by high-performance liquid chromatography-hydride generation-electrothermal atomisation atomic absorption spectrometry, J. Anal. At. Spectrom., 16(3), 2001, 270-274. 

The determination of arsenic by atomic absorption spectrometry with thermal atomisation and with hydride generation using sodium borohydride has been described by Thompson and Thomerson [29], and it was evident that this method couldbe modified for the analysis of soil. Thompson and Thoresby [30] have described a method for the determination of arsenic in soil by hydride generation and atomic absorption spectrophotometry using electrothermal atomisation. Soils are decomposed by leaching with a mixture of nitric and sulfuric acids or fusion with pyrosulfate. The resultant acidic sample solution is made to react with sodium borohydride, and the liberated arsenic hydride is swept into an electrically heated tube mounted on the optical axis of a simple, lab oratory-constructed absorption apparatus. 

Thomas et al. [22] used chelation followed by AAS with electrothermal atomisation to determine down to 2.5 mg/kg lead in fruit and vegetables and in apples [23]. Stafilov and Rizova [24] used AAS to determine lead in cereals. 

Olayinka, K.O., Haswell, S.J. and Grzeskowiak, R. (1989) Speciation of cadmium in crab-meat by reversed-phase high-performance liquid chromatography with electrothermal atomisation atomic absorptive spectrophotometric detection in a model gut digestive system. J. Anal. At. Spectrom., 4, 171-175. 

Graphite Furnace Atomic Absorption Spectrometry (GFAAS) or Atomic Absorption with Electrothermal Atomisation (ETAAS)... 

D. L. Tsalev, T. A. Dimitrov, P. B. Mandjukov, Study of vanadium)V) as a chemical modiPer in electrothermal atomisation atomic absorption spectrometry, J. Anal. Atom. Spectrom., 5 (1990), 189D194. 

The technology is now available for many more instrument functions to be selected both on the main instrument and peripheral devices such as an electrothermal atomiser or autosampler. Programmes for instrument setting and data processing can be stored, for example, on magnetic cards. Although, as already indicated, the actual speed of analysis may not be vastly improved, the advantages lie in the better reliability and accuracy obtainable and in the possibility of more efficient use of the time of a skilled analyst. 

Where the concentration of an element in a sample falls below the detection limit for that element or is low enough to make a precise direct measurement impossible, other techniques must be used to pre-concentrate the element or remove the matrix. The possibilities given below are an alternative to using an electrothermal atomiser where the sensitivity is of the order of 100 to 1000 times greater see section VI. 

The increased sensitivity which is the main feature of electrothermal atomisation methods introduces a number of difficulties connected with the handling and preparation of samples. Some practical guidance on the avoidance of errors through contamination and on the choice and use of micropipettes is set forth in the following subsections. The analyst must appreciate, however, that we are dealing with a technique of ultramicroanalysis, and any advice or experience that he can make use of on that subject will be entirely relevant here. 

The precautions recommended to avoid contamination are detailed below and are divided into two sections, those considered essential to enable electrothermal atomisation to be carried out successfully, and those considered desirable. 

All glass or plastic vessels to be used for electrothermal atomisation work should be washed, rinsed and then soaked in 2% v/v nitric acid for at least 24 h and then thoroughly rinsed in high purity deionised or double-distilled water. 

The volumetric and storage ware used for solutions for electrothermal atomisation should be kept separate from apparatus used for conventional laboratory work. 

The complete electrothermal atomisation system should, preferably, be in a room separate from the general laboratory, and well away from sample preparation procedures. 

Unless a special type of autosampler can be used, micropipettes are an essential part of electrothermal atomisation techniques, as they give one of the most reliable methods of introducing small volumes of liquid samples into the graphite atomizer. 

The dead-space above the sample in the dart-like tip is much reduced by the use of the type of micropipette which uses fine capillary tips. The Oxford Ultramicro-sampler is an example. Use of this device completely overcomes the solvent expulsion problem. The principal disadvantage from the point of view of electrothermal atomisation with a graphite furnace is that its maximum capacity is 5 pi. This may be too little for the sensitivity of some elements. The tip material provided with this syringe is still prone to droplet formation, but this can be replaced by PTFE capillary tubing of the correct dimensions, e.g. Polypenco size TW 24, which appears to overcome the problem completely. Good precision should be attainable with tips made from this material, provided the tips are cut across at 90° to the tube axis. Chamfered tips give rise to a variable position of the meniscus with consequent loss of reproducibility. 

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