Spectrométrie de masse et techniques de purification
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Deep-UV Laser Ablation Technology [213 nm] Coupled with Plasma Quadrupole Mass Spectrometry for Rapid Determination of Nickel/ Vanadium Ratios in Asphaltenes

Pillay AE, Elhameed AA, Nunn R and Stephen S

Knowledge of nickel/vanadium (Ni/V) ratios in petroleum fuel is essential to source geochemical information and to limit catalytic poisoning and corrosive effects in chemical refining processes. However, these ratios have been found to vary widely and are affected by various factors, including the sulphur content of the petroleum feedstock. They are normally obtained by digesting and treating the feedstock in suitable acid or organic media followed by numerical analysis. Our research investigated a novel sample introduction technique for obtaining Ni/V ratios by ablating petrified asphaltene samples (following liquid nitrogen pre-treatment) with a 213 nm laser beam coupled to a hyphenated plasma mass spectroscopic system. Laser ablation is ideal for delving beneath the surface of a solid matrix to measure the composition of the substrate. Iterative scanning of the sample surface and subsequent depth-profiling was achieved at pre-determined surface points to depths of 50 μm at selected sub-surface intervals. Characteristic intensities originating from the metals of interest were measured. Laser results were compared with the ‘digested’ results obtained directly by ICP-MS. In the case of the digested crude oil samples, these ratios varied between 0.6 and 0.8. This was higher than corresponding literature values, which occurred within a range of 0.3-0.6. The laser ablation results for typical asphaltene samples showed that Ni/V ratios varied in the interval 0.2-0.8, which were considered to be elevated and in keeping with the elevated crude oil ratios. The digested asphaltenes also reflected higher ratios, suggesting the influence of sample inhomogeneity and nickel impurities. Factors affecting variations in this ratio are presented, and the advantages of a more stable ratio are discussed. The study is of definite interest to petroleum engineers, and has considerable scope for extended research.