Measurements of sulfur in oil using a pressurised wet digestion technique in open vessels and isotope dilution mass spectrometry

Abstract

Graz University of Technology has developed a new technique for digesting samples using the well-established high-pressure asher (HPA) from Anton Paar GmbH (Graz, Austria). The digestion is performed in semi-open vessels inside a pressurised autoclave. The new HPA equipment consists of a liner for the autoclave, special sample racks and 30-mL digestion vessels made of quartz, covered with PTFE stoppers. The Laboratory for Isotope Dilution and Nuclear Analysis of the Federal Institute for Materials Research and Testing (BAM, Berlin) tested this new equipment in order to assess its usability for the decomposition of larger sample amounts of gas oils for the measurement of sulfur. Several experiments were carried out using the new sample decomposition technique. In order to test the recovery of the new digestion method, a gas oil material with known sulfur content was chosen, quantified by the validated conventional closed vessel HPA digestion in combination with thermal ionisation mass spectrometry. Isotope dilution mass spectrometry has been applied as analytical method in this investigation. The gas oil was spiked with an isotopic spike material, which is enriched in ³⁴S, and was then wet digested in the HPA. The oxidized sulfur of the dried samples was reduced to H₂S and precipitated as As₂S₃. The sulfur was measured as arsenic monosulfide (AsS⁺). The mass content of sulfur in the gas oil tested is 453.5 mg kg⁻¹. Recovery tests for increasing masses of gas oils indicate that the recovery using the new measurement technique decreases with increasing mass of gas oil. Results were obtained for approximately 0.3 g sample weight and had less overlap with the result of the old HPA method within the stated uncertainties. At approximately 0.5 g sample weight the yield decreases to about 97% and at approximately 1.0 g sample weight to about 87%. In comparison with the conventional closed vessel HPA digestion, the new technique shows no clear advantages for the certification of the sulfur content in gas oil other than a more convenient handling. The total uncertainty of the sulfur mass fractions (k=2) is about 1.5%. Repeated determination of the oil samples show a relative standard deviation of about 0.8% and indicate that the analytical procedure is robust and reproducible. The demonstrated reproducibility allows the establishment of correction factors for the yield, which in turn enables higher sample masses for routine work. The blank level (0.26×10^-6 g) was within the range of the conventional closed HPA digestion procedure·(0.28×10^-6 g). Cross contamination could not be detected. In terms of trace metal analysis a good applicability and more advantages over the conventional closed vessel HPA digestion can be assumed.