Fractionation and speciation of nickel and vanadium in crude oils by size exclusion chromatography-ICP MS and normal phase HPLC-ICP MS

Abstract

The coupling of size exclusion chromatography (SEC) and normal phase (NP) HPLC using entirely organic mobile phases (tetrahydrofuran, xylene) with inductively coupled plasma mass spectrometry (ICP MS) were developed and investigated for the molecular distribution of nickel and vanadium in crude oils. The metal species were fractionated by SEC using three columns in series with the increasing porosity (100, 1000 and 100000 A) covering the molecular mass range (in eq. polystyrene) between 300 and 2 × 106 Da. The resolution achieved allowed the discrimination of at least three classes of Ni and V species with varying proportions of the metals as a function of the origin of crude oil, crude oil fraction (asphaltene, maltene) and dilution factor. Normal phase HPLC-ICP MS allowed the separation of the porphyrin-type fraction as well as separation of the remaining species into three distinct fractions. The metal species in the SEC fractions were found to be sufficiently stable to be collected and preconcentrated to allow the development of a bidimensional chromatography SEC-NP-HPLC-ICPMS for the probing of the metal distribution in crude oils in terms of molecular weight and polarity.