Determination of naphthenic acids in crude oil by chemical ionization mass spectrometry

Abstract

Naphthenic acids in petroleum are considered a class of biological markers. Their potential use in source correlation and as an indicator of biodegradation was reported in the past (Dzidic et al., 1988; Behar and Albrecht, 1984). Due to their highly complicated properties, detailed characterization of the acids is difficult.

A method based on positive ion CI (chemical ionization) mass spectrometry using isobutane reagent gas to produce (M+15)⁺ ions was applied to the analysis of naphthenic acid esters. Since the complex mixture of naphthenic acids cannot be separated into individual components, only the determination of relative distribution of acids classified in terms of hydrogen deficiency was possible. The identities and relative distribution of fatty and mono-, di-, tri-, and higher polycyclic acids were obtained from the intensities of the (M+15)⁺ ions according to z-series formula C _n H_2n+z O₂ of naphthenic acids. The components are characterized on the basis of group type and carbon number distributions. A comparison of the FAB and CI results showed that the group type distributions obtained by both methods agree surprisingly well. The results indicated this method is simple, rapid and easy to operate. The geochemical implication of naphthenic acids was investigated by using a set of well-characterized crude oil samples. It is found that the naphthenic acid distribution can be used as a fingerprint for oil-oil and oil-source correlations.