Preparation steps in environmental trace element analysis - facts and traps

The laboratory of CERVA is for several decades involved in the Belgian environmental research. The activity was associated to national monitoring programs dealing with trace metal pollution of all compartments of the environment (sea and river waters, sediments and organisms but also soils, plants, animal and food samples). Such a monitoring dealing with the total analyte contents in samples needed a comprehensive development of the whole methodology associated to the analyses using atomic absorption and emission spectroscopy techniques. This includes measurement but also preparation steps. The latter is the subject of this work. Long-term experience has shown that precisely sample preparation is the most critical part of the analysis because it is responsible for the largest and often hidden sources of errors. Errors due to contaminations may be usually overcome if necessary precautions are taken concerning reagents, tools and the manner of working. The problem is different for analyte losses: in this case, the responsible factor is an inappropriate methodology. This is particularly true for preparation of solid samples that have to be brought in a solution in order to satisfy needs of introduction systems of most spectroscopic techniques utilized in routine laboratories. For some types of samples (e.g. animal tissues), the dissolution is not a problem: it may be readily achieved by several procedures. This is not the case for samples that contain silicates in their matrix (e.g. soils, sediments, plants) because their complete dissolution cannot be ensured by a simple procedure. This review describes the present knowledge regarding possibilities and errors that concern preparation steps. In addition, possible effects of the preparation procedure on the quality of measurement are also systematically discussed.