High accuracy analysis of low level sulfur in diesel fuel by isotope dilution high resolution ICP-MS, using silicon for mass bias correction of natural isotope ratios

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Abstract
A novel isotope dilution mass spectrometry method using ICP-MS is presented for establishing 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 2000 µg g−1 is to be reduced to 50 µg 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 µg g−1 compared to reference values of 428 ± 4 µg g−1. There is also good agreement with industry standard methods of analyses for low sulfur diesel. The combined uncertainty of the new method is 1–2% at −1. Samples taken from UK retail outlets sold as low sulfur diesel (−1) differ from the target sulfur content by up to 15 µg 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.

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