Multi-element organometal(loid) speciation by hydride generation-GC-ICP-MS: overcoming the problem of species-specific optima by using a pH-gradient during derivatisation

Abstract

Though a number of elements can be methylated in the environment, most studies focus on single elements, mainly due to analytical restrictions. The main problem of multi-element speciation of non-volatile methylated metal(loid) species is the heterogeneity of physico-chemical properties of these compounds. A common approach to analyse organometal(loid) species of different elements is the derivatisation by hydride generation (HG). However, derivatisation efficiencies are crucially dependent on pH and reaction optima differ significantly between species. In order to solve this problem we developed a semi-automated hydride generating procedure applying a pH-gradient. As at neutral pH artefact formation is generally lower but some species show poor derivatisation yields, we lower the pH-value from 7 to 1, while NaBH₄ is added continuously, thereby obtaining quantitative yields and low artefact formation for species with different pH-optima. After cryotrapping, the analytes are separated by gas chromatography (GC) and analyzed by inductively-coupled plasma mass spectrometry (ICP-MS). For the methylated compounds investigated recoveries of more than 90% and relative standard deviations below 3% were achieved. By pH-gradient-HG-GC-ICP-MS methylated Ge-, As-, Sn-, Sb-, Te- and Hg-species can be analysed simultaneously. Detection limits of pH-gradient HG-GC-ICP-MS for methylated metal(loid) species range from the sub to the mid ng/kg range depending on the blank concentration of the compound. The method is comparatively fast and appropriate for a high sample throughput and has been applied to soils, sediments, organic waste and compost samples.