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Abstract

Determination of As by ICP-MS is aggravated by the presence of Cl^- ions, forming in the plasma the ⁴⁰Ar³⁵Cl⁺ ion from which the ⁷⁵As⁺ ion cannot be discriminated. The presence of 1 000 mg l^-1 Cl^- gives rise to a signal corresponding approximately to 3 μg l^-1 As. Interferences can be reduced by about 70% by the addition of 4 vol.% methanol, the remaining fraction has to be corrected mathematically. A treatment based on the measured intensities of the ⁴⁰Ar³⁷Cl⁺ or ³⁵Cl¹⁶O⁺ ions can result in overcorrection; the signal of the ³⁵Cl⁺ ion was found more suitable for the purpose. The ³⁵Cl⁺/⁴⁰Ar³⁵Cl⁺ signal ratio is constant in the concentration range of 400-1 000 mg l^-1 Cl^- at least. The detection limit (3σ) found by measuring solutions which contained various amounts of Cl^- ions was 0.03 μg l^-1 As. This two-step correction for interferences was applied to the analysis of a mineral water with a chloride concentration of 1 400 mg l^-1. The results obtained by the calibration graph method and by the standard addition method were in a good agreement (1.50 μg l^-1 vs 1.44 μg l^-1 As, respectively), whereas a high overestimation (4.2 μg l^-1 As) resulted if the correction was omitted. Relative standard deviation of the determination was 3.2%.

Keywords: Arsenic; Mineral water; Inductively-coupled plasma mass spectrometry.