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A comparison of high accuracy methods for the analysis of total sulfur in gas oils by three metrology institutes is described. The work contains a comparison of three different aspects of the analysis: sample digestion, instrumental measurement and isotope dilution strategies.
Thermal ionisation mass spectrometry (TIMS) in combination with high pressure asher (HPA) or Carius tube digestion has traditionally been used for such high accuracy analysis. However, this paper demonstrates that the faster, less laborious technique of ICP-MS in combination with microwave digestion is equally capable. Results from the comparison experiments show excellent agreement for fuel samples containing sulfur concentrations in the range 11200 µg g-1. This excellent agreement across the different sample preparation, measurement and isotope dilution techniques has been exploited in the certification of two new diesel fuel reference materials.
The present work describes the development of an existing TIMS procedure to a reference procedure for low sulfur concentration measurements in fossil fuels. With this enhanced procedure SI-traceable sulfur mass fractions below 10 mg kg-1 can be obtained. The achieved detection limit is approximately 0.2 mg kg-1. The procedure was validated by certified reference materials. The procedure was already applied to candidate reference materials and to samples analysed within projects of the Comité Consultatif pour la Quantité de Matière CCQM.
Additionally the influence of the isotopic composition on the results and its corresponding uncertainty was studied. Reference data published in the literature on the isotopic composition of sulfur were assessed.
A new method has been developed for the measurement of sulfur in gas oils using a high pressure asher (HPA) for sample digestion. This, combined with a calibrated enriched isotopic reference material, and high precision mass spectrometry, makes it a powerful tool for an accurate and precise determination of the sulfur content in fossil fuels. The method described here was one of the methods used in the certification of the candidate reference materials BCR-104R, BCR-671 and BCR-672. Approximately 0.2 g of the gas oil samples were spiked with the spike isotopic reference material IRMM-646, which is enriched in 34S, and combusted in an HPA using nitric acid. The isotopes of sulfur were measured as arsenic monosulfides (AsS+) using a thermal ionization mass spectrometer equipped with a Faraday detector. The total uncertainty of the sulfur content (k = 2) is about 1%. The certified reference materials NIST SRM-2724b and NIST SRM-1619b and BCR-105 were used as part of the method validation work. The mass content of sulfur in BCR-104R was measured to be 1014 µg g-1, BCR-671 has 450 µg g-1 sulfur and BCR-672 shows a sulfur mass content of 201 µg g-1. The sulfur content of the candidate reference materials is homogeneous for the sample size used.