Using modern spectroscopic techniques to advance trace element speciation - case study thioarsenic species

Eingeladen vom Kolloquiums-Team.

The introduction of hydride generation in the mid 1970s enabled a first look at what lies beyond “total arsenic” concentrations in environmental samples - arsenite was identified to predominate under reducing, arsenate under oxidizing conditions; numerous organic arsenic species were later detected by chromatographic separation and retention time comparison to known standards.
This presentation will show how information from different modern spectroscopic techniques can be combined to identify and quantify previously unknown species that are lost or co-determined during routine species preservation and analysis. In samples from arsenic-sulfidic environments, anion exchange chromatography inductively-coupled plasma-mass spectrometry (AEC-ICP-MS) showed the formation of four compounds containing both arsenic and sulfur in significant amounts (up to 80% of total arsenic) and the S:As ratios were quantified as 1:1, 2:1, 3:1, and 4:1. Using electrospray ionization mass spectrometry and time-of-flight mass spectrometry (ESI-MS, ESI-TOF-MS) we were able to determine the molecular masses of these As-S-compounds, identify them as thioarsenates (AsSxO4-x3-) and distinguish apparently identical molecular masses of e.g. arsenate (H2AsO4-, m/z 141) and monothioarsenite (H2AsSO2-, m/z 141) by high-resolution mass-detection and characteristic fractionation patterns.
Previous identification of arsenic-sulfur compounds as thioarsenites based on data from the currently most advanced X-ray absorption spectroscopic methods (XANES and EXAFS) are likely misinterpreted because absorption edges of thioarsenates are significantly different from that of arsenate but coincide with that of arsenite and can thus be misidentified as As(III)-species. A brief introduction to the principle of each spectroscopic method will be given for non-users.

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