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For all preparation steps, the sample material comes into contact with other materials as well as with the laboratory environment. This can cause contaminations. These should, however, be avoided as far as possible in order not to influence the analytical result. Contaminations by the laboratory environment are usually in the range of trace amounts, in case of observing good laboratory practice. Another source of impurities is the contaminations by the sample preparation tools (e.g. mills, crushing tools, melting crucibles, etc.). Furthermore, if the preparation tools are not sufficiently cleaned, cross-contamination can occur as a result of transfers between the individual samples.

Table 3.5 Contaminations in the range of traces by preparation tools (main contaminations in bold).

Because contamination cannot be completely avoided, such as by abrasion of the preparation tools, it is important that these contaminations do not affect the analyte. This is especially the case when the analytes are present in trace amounts. Changes in the contents of the main components due to traces can be neglected as well as any noticeable influence on the matrix.

Table 3.5 summarizes some typical impurity elements that can be introduced during the different steps of sample preparation.