An arsenic compound that once resisted analysis has now succumbed to a chemical trick that allows mass spectrometry to determination sample concentration.

Monomethylarsonous acid is a serious problem in groundwater at one former herbicide production site in the US and potentially elsewhere. To enable the electrospray ionization tandem mass spectrometry (with liquid chromatography separation), a North American team has developed a derivatization step using dimercaptosuccinic acid. This allowed structure-specific detection of monomethylarsonous acid, along with arsenite, arsenate, monomethylarsonic acid, and dimethylarsinic acid. Measured values agreed well with theory and revealed groundwater values to be the highest observed in the environment.

• New Method and Detection of High Concentrations of Monomethylarsonous Acid Detected in Contaminated Groundwater
  A. McKnight-Whitford, B. Chen, H. Naranmandura, C. Zhu, X. C. Le,
  Environ. Sci. Technol. 2010, 44.
  DOI: 10.1021/es100273b