Electrospray Mass Spectrometry of Arsenic Compounds and Thiol–Arsenic Complexes

This article reviews the identification and quantification of arsenic species and their complexes using electrospray ionization mass spectrometry (ESI-MS). Topics covered include accurate mass determination, select ion monitoring (SIM) and multiple reaction monitoring (MRM), isotopic ratios, common fragmentation patterns, transition ratios, wrong-way-round ionization, crosstalk, ion interference, and matrix effects. The analytical methods range from direct-infusion ESI-MS and tandem mass spectrometry (MS/MS), to the coupling of the ESI-MS to high-performance liquid chromatography (HPLC) separation, and finally to combined analysis using HPLC separation with both inductively coupled plasma mass spectrometry (ICP-MS) and HPLC/ESI-MS detections. The combination of both ICP-MS and ESI-MS is especially powerful, benefiting from the established, robust, and sensitive element-specific detection of ICP-MS, and the molecular detection of ESI-MS. Using the various mass spectrometry (MS) techniques, a variety of biological and environmental samples have been studied, including the urine of humans and animals, food and plants grown on arsenic-contaminated soil, surface water, and contaminated groundwater. This article also discusses recent studies on the formation of thiol-arsenic complexes between select arsenic species and thiols such as glutathione (GSH) and metallothionein (MT). The use of ESI-MS contributes to determining the stoichiometry and binding location, monitoring the reaction in real time, and evaluating binding constants. Information from binding studies has helped the development of improved ESI-MS methods through derivatization of arsenic species with thiols.