Accurate determination of sulfur in gasoline and related fuel samples using isotope dilution ICP–MS with direct sample injection and microwave-assisted digestion

Abstract

Inductively coupled plasma isotope-dilution mass spectrometry (ICP–IDMS) with direct injection of isotope-diluted samples into the plasma, using a direct injection high-efficiency nebulizer (DIHEN), was applied for accurate sulfur determinations in sulfur-free premium gasoline, gas oil, diesel fuel, and heating oil. For direct injection a micro-emulsion consisting of the corresponding organic sample and an aqueous ³⁴S-enriched spike solution with additions of tetrahydronaphthalene and Triton X-100, was prepared. The ICP–MS parameters were optimized with respect to high sulfur ion intensities, low mass-bias values, and high precision of ³²S/³⁴S ratio measurements. For validation of the DIHEN–ICP–IDMS method two certified gas oil reference materials (BCR 107 and BCR 672) were analyzed. For comparison a wet-chemical ICP–IDMS method was applied with microwave-assisted digestion using decomposition of samples in a closed quartz vessel inserted into a normal microwave system. The results from both ICP–IDMS methods agree well with the certified values of the reference materials and also with each other for analyses of other samples. However, the standard deviation of DIHEN–ICP–IDMS was about a factor of two higher (5–6% RSD at concentration levels above 100 μg g⁻¹) compared with those of wet-chemical ICP–IDMS, mainly due to inhomogeneities of the micro-emulsion, which causes additional plasma instabilities. Detection limits of 4 and 18 μg g⁻¹ were obtained for ICP–IDMS in connection with microwave-assisted digestion and DIHEN–ICP–IDMS, respectively, with a sulfur background of the used Milli-Q water as the main limiting factor for both methods.