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The stability of arsenate, monomethylarsonate, dimethylarsinate (DMA), arsenobetaine (AsB) and arsenocholine (AsC) at a concentration of 200 μg l⁻¹ in deionized water, urine and dry clean-up residue of urine, stored in dark at −20 °C, 4 °C and ambient temperature, without the addition of any stabilizer reagent was evaluated. The five species were determined independently by liquid chromatography with microwave-assisted oxidation-hydride generation atomic absorption spectrometric detection. At −20 °C, all species were stable in water and untreated urine; at 4 °C and ambient temperature, they were stable during the 67 days of testing in the urine dry residue after the clean-up procedure. In untreated urine samples at 4 °C and ambient temperature, AsC is unstable and easily transformed to the more oxidized species, AsB. In deionized water, AsB and AsC are transformed to other species such as DMA. The dry …