Revisitation of mineralization modes for arsenic and selenium determinations in environmental samples

Abstract

Mineralization procedures for arsenic and selenium analysis are usually limited to wet digestion methods owing to high volatility of these analytes. On the other hand, variable amounts of silicon in some types of samples imply elaborated mineralization procedures to liberate analytes which may be retained in an insoluble residue. Consequently, methods for such material generally include an hydrofluoric step followed by an evaporation to dryness. This type of mineralization is most easily accomplished using a dry ashing procedure. For plant analysis, a well validated and readily applicable dry ashing method is used for a long time in several laboratories but up today one could suppose that As and Se determinations cannot be performed after such a type of mineralization. Surprisingly, it has been observed that for plant samples these analytes are detected even after a calcination at 450°C. The general usefulness of a dry ashing method for analysis of all other analytes (main, minor and trace elements) incitates us to also verify As and Se recoveries. Results obtained in this work indicate clearly that plants of terrestrial origin may be mineralized using dry ashing procedure without As and Se losses. This statement was confirmed by analyses of several reference terrestrial plant samples (RMs) and laboratory control samples. Another confirmation was given by the direct graphite furnace analysis of the same plant samples but in slurried form (SS-ETAAS). As a direct consequence, As and Se analysis in terrestrial plants no more necessitates a separate preparation methodology. On the other hand, significant losses of As and Se were observed for aquatic plants, e.g. algaes. For the analysis of this type of samples, a separate wet digestion procedure remains unavoidable if the determination of As and Se has to be considered. Also some preparation procedures were tested for As and Se-analysis of soil and sediment reference samples. In these cases the wet digestion with a mixture of nitric, perchloric and hydrofluoric acids seems to remain the best alternative.