Determination of sulfur isotope ratios and concentrations in water samples using ICP-MS incorporating hexapole ion optics

Abstract

Sulfur isotope ratios are difficult to determine by quadrupole ICP-MS due to interfering O₂⁺ and NO⁺ molecular ions of high signal intensity at isotopes ³²S and ³⁴S. Rf-only hexapole devices have recently been introduced into ICP-MS instrumentation to facilitate ion transfer from interface to analyser. By introducing a mixture of ‘reactive’ gases into the hexapole, a series of ion-molecule reactions can be induced to reduce or remove interfering polyatomic species. The effects of various gas mixtures (He, H₂ and Xe) on the transfer of sulfur ions through the hexapole and the breakdown of interfering O₂⁺ and NO⁺ molecular ions at m/z=32 and m/z=34 were investigated. A rapid charge transfer reaction between O₂⁺ and Xe gives at least a factor of 10 improvement in the S⁺/O₂⁺ ratio. A further reduction in O₂⁺ is achieved by the addition of H₂. δ³⁴S variations were investigated in crater-lake waters and waters obtained from springs and rivers on the flanks of volcanoes in Java, Indonesia. Under optimum conditions (S=10-50 mg l^–1), the ³⁴S/³²S measurement precision for standards and samples was <0.3% RSD. Mass bias errors were corrected by using a concentration-matched in-house standard of average North Atlantic sea-water (δ³⁴S=20.5). Results compare favorably against published data measured by standard gas source mass spectrometric techniques. The proposed technique is potentially useful as a survey tool due to the large δ³⁴S variation (±20) encountered in nature and the accuracy and reproducibility of the technique (±3-5).

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