Complementary molecular and elemental detection of speciated thioarsenicals using ESI-MS in combination with a xenon-based collision-cell ICP-MS with application to fortified NIST freeze-dried urine

The simultaneous detection of arsenic and sulfur in thioarsenicals was achieved using xenon- based collision- cell inductively coupled plasma (ICP) mass spectrometry (MS) in combination with high- performance liquid chromatography. In an attempt to minimize the (OO+)-O-16-O-16 interference at m/ z 32, both sample introduction and collision- cell experimental parameters were optimized. Low flow rates (0.25 mL/ min) and a high methanol concentration (8%) in the mobile phase produced a fourfold decrease in the m/ z 32 background. A plasma sampling depth change from 3 to 7 mm produced a twofold decrease in background at m/ z 32, with a corresponding fourfold increase in the signal associated with a high ionization surrogate for sulfur. The quadrupole bias and the octopole bias were used as a kinetic energy discriminator between background and analyte ions, but a variety of tuning conditions produced similar (less than twofold change) detection limits for sulfur (S-32). A 34- fold improvement in the 32S detection limit was achieved using xenon instead of helium as a collision gas. The optimized xenon- based collision cell ICP mass spectrometer was then used with electrospray ionization MS to provide elemental and molecular- based information for the analysis of a fortified sample of NIST freeze- dried urine. The 3 sigma detection limits, based on peak height for dimethylthioarsinic acid (DMTA) and trimethylarsine sulfide (TMAS), were 15 and 12 ng/ g, respectively. Finally, the peak area reproducibilities (percentage relative standard deviation) of a 5- ppm fortified sample of NIST freeze dried urine for DMTA and TMAS were 7.4 and 5.4%, respectively.