Combination of Negative Electrospray Ionization and Positive Atmospheric Pressure Photoionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry as a Quantitative Approach of Acid Species in Crude Oils

Crude oils differ from one another in numerous chemical and physical properties, many of which play an important role in defining their quality and price. Generally, statistical analysis of price differentials has focused on two main properties: density and sulfur content. However, the growing significance of high total acid number (TAN) crude oils, especially from developing countries, has aroused the necessity for extending these models. Consequently, refineries must obtain real and exact information regarding crude oil quality to achieve optimal crude oil selection and processing decisions. This could be attained when a detailed molecular-level characterization is performed. The present work presents the combination of negative electrospray ionization [(−)­ESI] and positive atmospheric pressure photoionization [(+)­APPI] Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry, as a prominent approach to semi-quantify the acid species comprised in crude oils. A novel polarity index is proposed that corrects the relative abundances of (−)­ESI classes, where mainly acid species are detected. By consideration of different indexes, it was possible to enhance the correlation coefficients (<i>R</i>²) from 0.579 to 0.986 between the percentage of acid compounds and TAN of crude oils, where most of the samples stand close to a linear tendency. These results avoid the deviations observed in previous works on the correlations between relative abundances of the O2 class through (−)­ESI and TAN and could support achieving optimal crude oil selection and defining their quality and price

Characterization of Acid Species in Asphaltenic Fractions by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry and Infrared Spectroscopy

Despite significant advances in the characterization of asphaltenes, the study of this fraction of crude oil remains challenging for the scientific community and oil companies. It is well-known that asphaltenes are responsible for many of the difficulties found in the extraction, production, transportation, storage, and refining of petroleum. Although they are defined as the fraction of crude oil that is insoluble in paraffinic solvents of low-molecular-weight-type n-heptane (C7) or n-pentane (C5), asphaltenes present a great compositional variety, concentrate a variety of elements (among which nitrogen, oxygen, sulfur, vanadium, and nickel stand out), apart from hydrogen and carbon, and are distinguished by grouping the molecules with greater aromaticity of crude oil. Given its high compositional and structural complexity, associating operational problems with specific characteristics of asphaltenes is a task that is still in force. Asphaltenes, in addition to being distinguished by their high aromaticity, are also known for their acid–base properties. The acid character varies according to the origin of the asphaltenes and is related to the presence of carboxylic acids, phenols, carbazoles, and indoles. The basic character can be attributed to the presence of amines, amides, and other nitrogen-containing compounds. Progressing in the detailed compositional characterization of the polar compounds comprising the n-heptane-insoluble fraction (asphaltenes) is still a necessary task toward the understanding of asphaltene aggregation phenomena and the relationship with crude oil properties, such as interfacial tension and viscosity. In the present work, the naphthenic acids of the insoluble fractions of five Colombian crude oils were characterized. The n-heptane-insoluble fraction was subjected to a washing process affording four subfractions, named as extractable 48 h, extractable 72 h, and extractable 96 h, which correspond to the heptane-soluble fractions that were recovered after each indicated time, and asphaltene 96 h, which corresponds to the remaining heptane-insoluble compounds after 96 h of washing. The samples were chosen so that they had a wide range of asphaltene content that varied from 3.0 to 19.2%. The acid fraction was obtained from each sample by solid-phase extraction using aminopropyl silica as an adsorbent material