A novel isotope dilution mass spectrometry method using ICP-MS is presented for established high accuracy reference values for sulfur in fossil fuels. The European Community plans to reduce the permitted content of sulfur in fossil fuels. In the case of diesel fuel the 1993 limit of 200 mug g-1 is to be reduced to 50 mug g-1 by the year 2003. The new approach will readily meet the needs set by these new legal limits. Samples were prepared by closed vessel microwave digestion to ensure retention of the volatile sulfur compounds in solution and spike equilibration. The natural and spiked isotopic ratios of 32S/34S were then measured using a Finnigan MAT Element magnetic sector ICP-MS operating in medium resolution (R = 3000) mode to avoid the spectral interference of O2 on the sulfur isotopes. The natural isotopic composition of sulfur varies in nature and so must be measured in each sample. Here, we demonstrate a novel method in which silicon is used to correct for the effects of mass bias. The isotopic composition of silicon is well characterised and has minimal natural variation. The combined uncertainty of natural ratio measurements in sulfur solutions and sample digests is about 0.4%. The new ICP- IDMS method shows excellent agreement with the certified values for NIST SRM (SRM2724b) of 427 ± 7 mug g-1. Samples taken from UK retail outlets sold as low sulfur diesel (< 50 mug g-1) differ from the target sulfur content by up to 15 mug g-1. This deviation is comparable to the range in results for recognised routine methods and highlights how improved reference values at these concentrations will aid both the producer and legislator.
J-Anal-At-Spectrom. Sept 2001; 16(9) 964-969