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The potential of liquid chromatography-mass spectrometry (LC-MS) using electrospray ionization (ESI) was investigated for the identification and quantification of organoarsenic species excreted in rats urine chronically exposed to dimethylarsinic acid (DMAA). Quantification was performed by both LC-ESI-MS and LCinductively coupled plasma mass spectrometry (ICP-MS). The detection limits of organoarsenic species in LC-ESI-MS with cation-exchange chromatography were 75-200 pg as arsenic.Although there are about ten times higher than that of LC-ICP-MS, LC-ESI-MS had a low enough detection limit to determine major metabolic arsenic species in the urine. LC-ESI-MS was applied to the identification of organoarsenic species in the urine. Major arsenic peaks in urine were identified as DMAA and trimethylarsine oxide using agreement of the spectra and retention times. Three unidentified arsenic peaks were found in the urine; one of these was determined to be tetramethylarsonium ion by agreement of both the spectrum and the retention time. LC-ESI-MS and LC-ICP-MS were also used to quantify organoarsenic in urine: good agreement between LC-ESI-MS and LC-ICP-MS was obtained.

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Tools that provide absolute quantification of biomolecules, particularly of proteins and their post-translational modifications, without needing suitable specific standards, are urgently demanded nowadays. To this end, we have significantly improved the recently introduced strategy based on CH4 addition to the plasma for absolute quantification of biomolecules using HPLC-ICP-MS. Addition of CO2 has been optimized and finally selected as a safer, and more efficient quantitative strategy, able to provide constant (< 6% error) signal response factor for the six elements assayed (S, P, As, Se, Br, I) under compromise conditions. In the particular case of absolute protein quantification, accuracy and precision attainable for S-based absolute determination of intact proteins using internal and external S-generic standards were compared. Potential for real sample analysis was demonstrated by the high-sensitive analysis of toxins present in snake venoms. Finally, multi-elemental speciation capabilities of the approach have been also demonstrated through P and S simultaneous analysis in phosphoproteomics. Simultaneous accurate determination of both absolute protein amount and corresponding phosphorylation degree for intact β-Casein, and even impurity traces of κ and α-s1 isoforms present, have been successfully achieved using a simple mixture of inorganic P and S standards. Lowest detection limits (< 1 fmol protein) ever published for Sand P-based intact protein quantification with ICP-MS are reported. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Analysis of results on the correction of signal variations and signal enhancement for S, P, As, Se, Br, and I, under the addition of CH4:Ar and CO2:Ar. LC-MS/MS analysis of tryptic digestion of β-casein sample. capHPLC-ICP-MS/MS quantification of Pseudechis papuanus venom sample using CH4:Ar and CO2:Ar.

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