Page 27 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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operators and through the instrumentation. We shall deal first with errors and then look at
interferences in some detail. Interferences specific to individual techniques are discussed in more detail
in the relevant chapters.

1.5.1.1 Sample Pre-treatment Errors

Obviously, the accuracy of an analysis depends critically on how representative the sample is of the
material from which it is taken. The more heterogeneous the material, the greater the care must be
taken with sampling. The analytical methods described in this book can typically be used on small
samples (100 mg of solid or 10 cm of liquid), and this again heightens the problem. Readers are
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referred to a general analytical text for details on sampling, but it should be stressed that if either the
concentration of the analyte in the sample does not represent that in the bulk material, or the
concentration of the analyte in the solution at the time it is presented to the instrument has changed, the
resultant error is likely to be greater than any other error discussed here. Regrettably, the supreme
importance of these points is not always recognized.

Usually, samples are presented for analysis as liquids. Thus, solid samples must be dissolved.
Analytical or ultra-high-purity grade reagents must be used for dissolution to prevent
contamination at trace levels. Certain volatile metals (e.g. cadmium, lead and zinc) may be lost when
dry ashing, and volatile chlorides (e.g. arsenic and chromium) lost upon wet digestion. It is
particularly easy to lose mercury during sample preparation. Appropriate steps must be taken in the
choice of method of dissolution, acids and conditions (e.g. whether to use reflux conditions) to prevent
such losses.
Another method that has become increasingly popular is microwave digestion. Sample may be placed
in a PTFE bomb and a suitable digestion mixture of acids added. The bomb is then placed in a
microwave oven and exposed to microwave radiation until dissolution is complete. This technique has
the advantage that digestion may be accomplished within a few minutes. Some bombs have a pressure
release valve. These valves become necessary when oxidizing acids, e.g nitric acid, that produce large
quantities of fumes are used. Other bombs do not have these valves, and care must be taken that
dangerous or damaging explosions do not occur.

Another method of bomb dissolution involves placing the sample and digestion mixture in a sealed
PTFE bomb and then encasing this in a stainless-steel jacket. This may then be placed in a conventional
oven for a period of several hours. This technique, although cheaper, takes substantially longer.

Trace metals may be lost by adsorption on precipitates, such as the silica formed on digestion using
oxidizing acids. This possibility should be investigated (e.g. by recovery tests).