Silicon speciation in light petroleum products using gas chromatography coupled to ICP-MS/MS

Silicon speciation in light petroleum products through the hyphenation of gas chromatography with ICP-tandem mass spectrometry (GC-ICP-MS/MS) is described in the present work. Eleven silicon compounds (nine siloxanes, trimethylsilanol and triethylsilane) have been taken as model molecules. The carrier and optional gas flow rates as well as the hydrogen gas flow rate in the octopole reaction cell (ORC) were critical variables. They precluded both the sensitivity and the extent of the effects caused by the sample matrix and silicon nature. The optimization of the latter variable mitigated the m/z 28 interference (14N14N+ and 12C16O+). Moreover, under the optimized conditions, a universal response was found irrespective of the silicon chemical form and sample matrix. The analytical performances of the method have been evaluated. Thus, the ICP-MS/MS response was linear between 0 and 500 μg kg−1 with correlation coefficients up to 0.999, whereas the limit of quantification (LOQ) ranged from 8 to 60 μg kg−1. Moreover, no drift was found for quality control samples analysed along a given analytical run. Only, the most volatile compound, i.e., trimethylsilanol (TMSOH), induced a drop in sensitivity caused by its partial evaporation from the GC vial. The method was validated by the analysis of real samples and the results were compared with those obtained by inductively coupled plasma optical emission spectrometry (ICP-OES) and X-ray fluorescence. No significant differences in Si total content provided by the three methods were found. These results demonstrated that all silicon species were taken into account using GC-ICP-MS/MS as the speciation method. Real light petroleum products, especially coker naphtha samples, were analyzed and it was verified that they only contained cyclic siloxanes (D3–D6), mainly hexamethylcyclotrisiloxane (D3) and octamethylcyclotetrasiloxane (D4).The authors wish to thank the Spanish Ministry of Science, Innovation and Universities for the financial support (Project Ref. PGC2018-100711-B-I00)

Investigation of the potential of the ICP-MS/MS for total and speciation analysis in petroleum fractions

International audienceThe capability of inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) to achieve total concentration and speciation using direct injection of petroleum products after solvent dilution for severely interfered isotopes was demonstrated here with different applications cases. For the direct determination of heavy elements (Z > 70) in organic matrices, the ICP-MS/MS was less sensitive than the ICP-HRMS. For light elements (Z < 40), the sensitivity was similar or better using ICP-MS/MS and for interfered elements (Si, S, Ca, Fe), the use of the two quadrupoles combined to the octopole reaction/collision cell (ORC) with He, O2 or H2 gave similar or better detection limits (LOD) than the ICP-HRMS in medium resolution. Comparable or better sensitivity were obtained replacing the 1.5 mm by the 1 mm injector diameter and especially for lighter elements (Z < 30) using the ICP-MS/MS. LOD in xylene were ranging from 0.004 μg/kg (V) to 0.9 μg/kg (Al) and appeared in the lowest values published in the literature using ICP-MS/MS in hydrocarbons. To demonstrate the performance of the ICP-MS/MS using direct injection of petroleum products after dilution in hydrocarbon solvent, three application cases were presented.Sulfur at very low levels in reformates was successfully monitored in oxygen mode using the oxide ion (32S → 48SO). The background equivalent concentration (BEC) origin was attributed to solvent contamination by sulfur and was confirmed by ultra-violet fluorescence (UVF) method. Using H2 for Ni (58Ni → 58Ni) and O2 for V (51V → 67VO) as reactant gas, the direct injection ICP-MS/MS method easily confirmed Ni and V concentrations measured using wavelength dispersive X-rays fluorescence (WDXRF) and allowed the determination of 14 elements in the asphaltene fraction, with concentrations ranging from 0.3 mg/kg (Al and Pb) to 37.4 mg/kg for Fe.For speciation of Ni, V and S, gel permeation chromatography (GPC) hyphenated to ICP-MS/MS is particularly powerful using O2 (2.5 mL/min) both for vacuum residue and hydrotreated (HDT) vacuum residue. Contrary to GPC-ICP-HRMS where two injections of sample were required (medium resolution for S and V and low resolution for Ni), GPC-ICP-MS/MS easily allowed the acquisition of the 3 elements in one mode during the same run and considerably reduce the analysis cost and time. Both for total and speciation analysis, the direct injection of petroleum products after solvent dilution ICP-MS/MS method is a significant advantage and appealing in high-volume petroleum laboratories