Abstract

The determination of S by solution nebulization quadrupole inductively coupled plasma mass spectrometry (ICP-MS) is difficult because of interferences from oxygen dimer ions. The large ¹⁶O₂⁺ ion current from the solvent water is a serious interference at ³²S, the most abundant of the four isotopes, and precludes its measurement. The isotopic composition of S varies in nature as a consequence of natural mass fractionation; therefore, high accuracy isotope dilution mass spectrometric (IDMS) determination of sulfur requires that the ratio of ³²S/³⁴S be measured, for the two isotopes represent over 99% of natural sulfur. In this work, electrothermal vaporization was used to generate a water-free aerosol of the sample. Non-solvent sources of oxygen were investigated and the spectral background minimized. Further reduction of the oxygen dimer was achieved by using nitrogen as an oxygen-scavenger in the argon plasma. The isotope ratio of ³²S/³⁴S was used for the determination of S by IDMS. The repeatability of the ³²S/³⁴S ratio in terms of the relative standard error (95% confidence level) of 6 replicate measurements of the spiked and the unspiked samples was about 0.3% and 0.7%, respectively. The detection limit of the method was 4 ng g⁻¹. Sulfur in two fossil fuel reference materials was measured and the results were in good agreement (within 0.3%) with those obtained by the more precise thermal ionization mass spectrometry (TIMS) method.