Page 87 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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3.9 The Relative Merits of Electrothermal Atomization

It is probably best to regard electrothermal and flame atomization and plasma-based instruments as
complementary techniques. Some factors which govern the choice of technique for a given application
are given below.

3.9.1 Advantages of Electrothermal Atomization

(i) Increased sensitivity: the theoretical improvement obtainable in electrothermal atomization in
comparison with flame atomization has been calculated by several workers. Such calculations are based
on the poor nebulization efficiency associated with flames (10%), the rapid dilution in the flame
with the expansion of the flame gases and the short residence time. Improvements in detection limits
of furnaces compared with flames range up to 4000-fold for zinc and are typically in the range 100-
1000-fold. For ICP-AES instruments, the nebulization efficiency is even worse (2%) and the other
shortcomings listed above are equally applicable. However, the technique cannot compete with ICP-MS
in terms of sensitivity.

(ii) Decreased sample size: the minimum requirement of a flame or a plasma instrument is 500 mm ,
3
except where flow injection or pulse nebulization is used. For electrothermal atomization, sample sizes
3
3
of 1-100 mm , typically 20-30 mm , are used. This means that dramatic improvements in absolute
sensitivities are obtained and measurements of picogram amounts of analyte are possible. Thus,
electrothermal atomization offers absolute analytical sensitivity comparable to that associated with
neutron activation analysis. Electrothermal atomization has particular advantages in situations where
sample size is limited. Table 3.1 shows typical characteristic masses obtainable in electrothermal
atomization. The use of limit of detection is rarely used for this technique because it depends so heavily
on the experimental conditions and on the injection volume.

(iii) In situ sample treatment: often tedious ashing procedures can be avoided by judicious choice of
acids and ashing temperature in the furnace.

(iv) Direct analysis of solid samples: solid samples can be placed directly in or on electrothermal
atomizers, often using purpose-made accessories. The only ways to analyse solids in plasma
instruments is