Speciation and Stability of Arsenic Compounds

All reagents were of analytical grade. Deionized (DI) water (18 MΩ Barnstead Nanopure Diamond) was used throughout the experiments. Sodium metaarsenite, 98% (As^III), sodium arsenate dibasic, 99% (As^V), cacodylic acid sodium salt, 98% (DMA^V), L-glutathione reduced, 98–100% (GSH), phosphorous pentoxide (P₂O₅), sulfur dioxide lecture bottle, diethylether anhydrous and deuterated water (D₂O) were purchased from Sigma-Aldrich, USA. Monosodium acid methane arsonate, 99.5% (MMA^V) was purchased from Chem Service, USA. Oxidized glutathione (GSSG) was purchased from MP Biomedicals, USA. Hydrochloric acid (HCl), trace metal grade, potassium iodide (KI), acetonitrile, methanol and optima LC/ MS-grade water were purchased from Fisher, USA. Calcium chloride anhydrous (CaCl₂) and formic acid, 99% were purchased from Acros Organics, USA. Phosphate buffer saline (PBS) (pH = 7.4), was prepared by dissolving 4.00 g of sodium chloride (NaCl) with 1.36 g of sodium phosphate, dibasic (Na₂HPO₄·7H₂O), 0.10 g of potassium chloride (KCl) and 0.12 g of potassium dihydrogen phosphate (KH₂PO₄) (Fisher, USA) in 500 mL of DI water.⁴⁵ For the study of pH effect on As species stability and half-life, pH adjustments were made with either ammonium hydroxide (NH₄OH) or nitric acid (HNO₃), trace metal grade both purchased from Fisher, USA.