Analysis of Metals in Condensates and Naphtha by Inductively Coupled Plasma Mass Spectrometry

Abstract

Condensates and naphtha are petroleum samples with largely gasoline-range components (C₅–C₁₀). Metal organic complexes are source inherited components of oils which are associated with the polar components and asphaltenes of oils. Because of the very low levels of biomarkers such as hopanes and steranes in condensates, they present a special correlation problem. Geochemically significant metals, such as V and Ni, can be measured in some condensates using conventional Meinhardt–Scott chamber inductively coupled plasma mass spectrometry (ICP-MS). In order to increase the sensitivity and the range of condensates which can be analysed, the Cetac U-6000 AT Ultrasonic nebuliser–desolvation system was tested. The Cetac system improved the sensitivity for V and Ni by a factor of about 40. Valuable information for relating these difficult samples to each other and to heavier oils in a basin can thus be supplied to exploration geochemists. The influence of organic matrix on the analytical signal was investigated. Matrix effects are more pronounced for the Cetac system than for the conventional ICP-MS system. For both systems, the use of an In internal standard was found to be necessary for compensating for differences in nebulisation and combustion behaviour caused by variations in the nature of the sample. The use of ICP-MS for the analysis of toxic metals such as Hg and Pb in condensates and naphthas is also discussed. The results are compared with those obtained using electrothermal atomic absorption spectrometry and cold vapour atomic absorption spectrometry in an interlaboratory study of naphthas. Conventional Meinhardt–Scott chamber ICP-MS provided good accuracy and precision of analysis compared with the other techniques during this study. Volatile species such as Et₄Pb and Me₂Hg were lost in the desolvation unit when using the Cetac system. Volatilisation effects were not observed when using the conventional Meinhardt–Scott chamber ICP-MS system.

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