Page 29 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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Readings should only be taken when the signal has reached equilibrium. When a new sample is
presented, several seconds will elapse before the system reaches equilibrium. Particular care must be
taken when the concentration of the samples or standards changes markedly, especially if the new
solution is more dilute.

Q. How can poor standards introduce analytical errors?

Q. Why should the integration button not be pressed until several seconds after a sample change?

1.6 Applications

Several texts have been published that contain information on applications. In addition, the Atomic
Spectrometry Updates published in the Journal of Analytical Atomic Spectrometry offer invaluable help
in the development of new applications in the laboratory. Nearly all the applications of analytical
atomic spectrometry require the sample to be in solution. Where possible, samples should be brought
into solution to give analyte levels of at least 10 times the limit of detection, known as the limit of
determination.

1.6.1 Clinical, Food and Organic Samples.

It is usually necessary to destroy the organic material before introducing the sample. Care must be
taken to avoid losses of volatile elements (an oxidizing wet ashing procedure is preferred for elements
such as lead, cadmium and zinc) and contamination from reagents. Various mixtures have been used
for wet ashing, including hydrogen peroxide-sulphuric acid (1:1) and hydrogen peroxide-nitric acid
(1:1) followed by the addition of perchloric acid. Such mixtures should be treated with care owing to
the possibility of explosions occurring on the addition of perchloric acid, which can only be used in a
stainless-steel fume hood. Beverages must be degassed before spraying. In serum analyses, the protein
is often precipitated with trichloroacetic acid before analysis, but only if the analyte is not likely to be
coprecipitated. Direct aspiration of diluted samples is to be preferred.

1.6.2 Petrochemicals

As organic solvents have different physical characteristics, aqueous standards cannot be used for
calibration when determining trace metals in oils or petroleum fractions. The sample can either be ashed
or diluted in a