| The presence of metallic species in samples like edible and lubricating oils, jet fuel, diesel or biodiesel may come from raw materials(crude oil, soils and crops), may be added on purpose to improve specific characteristics, may be present as a result of the wear of engine lubricated areas or may be transferred during processing, transportation or storage. Whatever the source, the quantification of trace metals is vital important either to evaluate the quality and physic-chemical properties of the oil products or to control environmental emissions further to protect human health. These determinations are mainly accomplished by atomic absorption spectrometry(AAS) with flame(FAAS) or electrothermal(ET-AAS) atomization, inductively coupled plasma optical emission spectrometry(ICP-OES) and ICP-mass spectrometry(ICP-MS). However, the determination of trace metals in oil samples is hampered due to its low concentration. Moreover, their high viscosity turns it difficult to introduce the sample in the instrument, and the high organic content of the oil matrix, increases the possibility of interference during analysis. The instability and high cost of the organic standards required for calibration and the lack of certified reference materials for most of the samples makes the analysis even more complicated. The paper proposes a new approach named Extraction Induced by Emulsion Breaking(EIEB) for the extraction of metals in edible vegetable oils and their determination using inductively coupled plasma mass spectrometry(ICP-MS). And the analysis and application of EIEB-ICP-MS method in oils were explored in the following aspects:(1) In this study, a novel method for the simultaneous ICP-MS determination of Mg, Cr, Mn, Fe, Ni, Cu, Zn, Ag, Cd, Ba and Pb in edible vegetable oils using extraction induced by emulsion breaking is proposed. In the method, each sample of oil was emulsified with a Triton X-114 solution containing HNO3 and the emulsion was broken by centrifugation, and then the acid aqueous phase deposited in the bottom of the centrifuge tube was collected for the determination of analytes. The matrix effects were corrected in the mixed standard solution of Sc, In and Bi, and a dynamic reaction cell was used to eliminate the interference of polyatomic ions. As there are no certified materials of edible oil with certified concentrations of the metals under study, two different strategies were employed to validate the accuracy of the method. In the first case, the results obtained by the proposed method were compared with those obtained using the microwave digestion extraction method. Secondly, a recovery test was carried out, by spiking the samples with known amounts of analytes, which were in the form of organometallic standards diluted in hexane. The result showed that the detection limits of the eleven metals were in the range of 0.004-0.271μg/L. This method was sensitive, precise and applicable for the analysis of trace metals in eight kinds of edible vegetable oils such as blend, corn, soybean, peanut, rapeseed, sunflower, camellia and olive oils. And the results suggested that there were rich micro-nutrients in edible vegetable oils. According to GB/T 2762-2012, all oils were very low in heavy metals, so with high edible value. The technique can be applied to the quality and safety evaluations of edible oils.(2) This work proposes a simple, fast and reliable method for the determination of trace concentrations of Mg, Ca, Cr, Mn, Fe, Ni, Cu, Zn and Pb in biodiesel samples by ICP-MS. In order to overcome problems related to the organic matrix in the direct introduction of the samples, a new extraction approach was investigated. The method was based on the extraction induced by emulsion breaking, in which metals were transferred from the biodiesel to an acid aqueous phase after formation and breaking of a water-in-oil emulsion prepared by mixing the biodiesel sample with an aqueous solution containing Triton X-114 and HNO3. The pretreatment conditions were studied in details. With DRC, the gas flow rate of reaction was changed, and the sensitivity of the method was ensured. Consequently, the spectral interferences caused by the overlaps of polyatomic molecules were effectively decreased. Rh as internal standard element was used to compensate matrix effect and signal drift. The detection limits of the nine metals were in the range of 0.008-0.189μg/L. The recovery was 91%-109% by adding organometallic oiled standard, and the relative standard deviation(RSD) was less than 4.23% for all analytes. This method turned out to be precise and reliable. Five different biodiesel samples were analyzed, and the results showed that the contents of different metals in the same biodiesel sample were greatly different. The concentrations of Ca, Cr and Fe were much higher than other metals. The sum contents of Ca and Mg in five sample were lower than the specified value(5mg/L) in the ASTM D 6751-10 and EN 14214-2008 standards. And the same metal content in different biodiesel samples also had big difference, which might be due to the difference of the raw material oils and additives. This method provides scientific rationale for the determination of trace metals from biodiesel.(3) This study describes the extraction/pre-concentration of Al, V, Mn, Fe, Ni, Cu, Zn, Sn and Pb from diesel oil and its determination by ICP-MS, proposed as a novel approach for the simultaneous determination of these kinds of analytes. The optimization of the method was performed by studying the effect of several parameters that could affect the extraction efficiency such as the concentrations of HNO3 and the emulsifying agents(Triton X-114 and Triton X-100) in the extractant solution, the centrifugal speed, the extraction time and collection time. The change of the background equivalent concentration(BEC) was studied in the standard model and DRC mode for all analytes. The results showed that in the DRC mode, the BEC of the interference elements were significantly reduced, and the interferences of polyatomic molecules ions were effectively overcome. Sc, Ge and Bi as internal standard elements were used to compensate matrix effect and signal drift. The optimum conditions for the determination were tested and discussed. Under the optimal conditions, the linear correlation coefficient of the nine metals were greater than 0.9993, the detection limits were less than 0.1μg/L, the recovery of the samples was between 85%-111% and the relative standard deviation(RSD) was less than 3.89%, which showed that the method was very sensitive and precise. Five different diesel oil samples were analyzed by EIEB-ICP-MS. It turned out that the contents of the heavy metals in the five diesel oils were all below 0.2mg/L. In addition, the concentrations of Al, Mn, Fe, Ni, Cu and Sn in different kinds of diesel oils were of great difference.(4) An analytical method of EIEB and ICP-MS with DRC were carried out for the determination of Mn, Fe, Cu and Zn in jet fuel. In the present case, the emulsions were broken by heating instead of centrifugation, which greatly reduced the emulsion breaking time. The determination of iron was under the DRC mode, selecting methane as the chemical reaction gas, and the interference of polyatomic ions was eliminated effectively. The developed method was successfully applied in the analysis of real samples of jet fuel. The obtained results were not different from those obtained by a reference method, based on the introduction of the samples as detergent emulsions into ETAAS. Also, recovery percentages in the range of 90-107% were observed when the analytes were added to the samples as organometallic standards. The detection limits of the four metals were lower than 0.12μg/L. The precision was satisfactory for this kind of procedure, being situated in the range of 1.98-3.94% for repeatability analysis and 5.22-6.70% for reproducibility analysis. Six different jet fuel samples were analyzed by the proposed method, and the results showed that the four heavy metals in jet fuel were all detected, but the content of metals were low, where the highest was no more than 45μg/L.The proposed method is simple, fast, precise and provides good limits of detection and quantification, which shows that it is an excellent alternative to other sample preparation methods such as total digestion of the samples, sample dilution with an appropriate solvent or direct injection of the oil as an emulsion or microemulsion.(5) A simple and fast method has been established for simultaneous determination of Al, Cr, Fe, Ni, Cu and Pb in fresh and used lubricating oils by EIEB-ICP-MS. In this work, all the samples were diluted with an organic solvent before the formation of emulsions due to higher viscosity of lubricating oils, when compared to other kinds of oils. Several parameters affecting the extraction efficiency of the procedure were investigated including the nature and concentration of the solvent used for sample dilution, the type and concentration of the surfactant, the concentration of HNO3, and the other operational conditions(extraction time, collection time and centrifugal speed). With Rh as internal standard element, the level of stability and precision of ICP-MS was significantly improved. DRC-ICP-MS was applied to eliminate the interference of polyatomic ions. The accuracy of the method was checked by an analysis of standard reference materials(SRM 1084 a and 1085 b, Wear-Metals in Lubricating Oil). The limits of detection for the six wear metals were in the range of 0.007-0.137ng/g. The developed method was applied in the analysis of six samples of fresh lubricating oils and six used lubricating oils. The results indicated that the contents in used lubricating oils were much higher than fresh lubricating oils. The content of iron has the largest added value, which increased by 42.66μg/g in used lubricating oils. While the smallest added value was 0.23μg/g of nickel. |
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