Electrothermal atomization disadvantages

Q. What are the advantages and disadvantages of these alternative techniques when compared with conventional electrothermal atomization ... 

While it is to be expected that the effects of these disadvantages will continue to diminish as more becomes known about electrothermal atomization, currently it can be said that if there is sufficient sample for flame or ICP analysis, and that these techniques offer sufficient sensitivity, then they should be used in preference. Plasma techniques should be used in preference to the flame if more than one analyte is to be determined. Recently a multi-element, simultaneous electrothermal instrument has been developed. These spectrometers still use a suite of hollow cathode lamps as sources. At present, a maximum of six analytes can be determined simultaneously. This area is likely to expand very rapidly, which may lead to a resurgence in the technique. If the sensitivity of a flame or ICP-AES is insufficient, and ICP-MS cannot be afforded, electrothermal atomization comes into its own, and is invaluable when either high sensitivity is required or when only small amounts of sample are available. 

Solid samples can be introduced into electrothermal atomizers either as such or as slurries. Each insertion mode has its own features, advantages and disadvantages. 

The relative precision of electrothermal methods is generally in the range of 5% to 10%, compared with the 1% or better that can be expected for flame or plasma atomization. Furthermore, furnace methods are slow and typically require several minutes per element. Still another disadvantage is that chemical interference effects are often more severe with electrothermal atomization than with flame atomization. A final disadvantage is that the analytical range is low, usually less than two orders of magnitude. Consequently, electrothermal atomization is ordinarily applied only when flame or plasma atomization provides inadequate detection limits or when sample sizes are extremely limited. 

Laser-excited atomic fluorescence spectrometry is capable of extremely low detection limits, particularly when combined with electrothermal atomization. Detection limits in the femtogram (10 g) to attogram (10 g) range have been shown for many elements. Commercial instrumentation has not been developed for laser-based AFS, probably because of its expense and the nonroutine nature of high-powered lasers. Atomic fluorescence has the disadvantage of being a singleelement method unless tunable lasers with their inherent complexities are used. 

Electrothermal atomic absorption spectrometry (ETAAS) has been the single most important technique in advancing our knowledge of the transition metal distribution in seawater. The graphite-furnace mode is used most frequently. It has the advantage of high sensitivity and therefore small sample volume (e.g., 10-50//L). Major disadvantages are the matrix interferences which usually necessitate a pre-concentration and/or a separation step (see Sections 12.2.1 and 12.2.2). Another application of ETAAS is the cold-vapour technique for the determination of mercury (Section 12.2.4). 

The limited use of atomic fluorescence has not arisen so much from any inherent weakness of the procedure but rather because the advantages of atomic fluorescence have been small relative to the well-established absorption and emission methods. Thus, although fluorescence methods, particularly those based on electrothermal atomization, are somewhat more sensitive for several elements, the procedure is also less sensitive and appears to have a smaller useful concentration range for several others. Furthermore, dispersive fluorescence instruments are somewhat more complex and more expensive to purchase and maintain.- These disadvantages have been largely overcome in some special-purpose dedicated instruments such as the one described in the Instrumental Analysis in Action feature at the end of Section 2. 

However, furnace atomic absorption has certain disadvantages. The flame is more precise, faster and much less trouble and should always be used if sensitivity is adequate. The electrothermal method takes longer and requires more skill. Precision for comparable absorbances will generally be poorer. Moreover, contamination can be a real problem at these ultra-trace levels and all possible sources of analyte contamination have to be scrutinised. 

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. 

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