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).