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

Sulfur isotope ratios are difficult to determine by quadrupole ICP-MS due to interfering O-2(+) and NO+ molecular ions of high signal intensity at isotopes S-32 and S-34. 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, H-2 and Xe) on the transfer of sulfur ions through the hexapole and the breakdown of interfering O-2(+) and NO+ molecular ions at m/z = 32 and m/z = 34 were investigated. A rapid charge transfer reaction between O-2(+) and Xe gives at least a factor of 10 improvement in the S+ /O-2(+) ratio. A further reduction in O-2(+) is achieved by the addition of H-2.delta(34)S 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 S-34/S-32 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 (delta(34)S = 20.5 parts per thousand). 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 delta(34)S variation (+/-20 parts per thousand) encountered in nature and the accuracy and reproducibility of the technique (+/-3-5 parts per thousand).