Abstract
SARA (Saturates, Aromatics, Resins, Asphaltenes) fractionation is a common simplification technique used for decades in petrochemical analysis. A large number of studies are dealing with the different fractions, but overall, the saturate fraction is strongly neglected. Of the very few available studies on the saturates fraction, almost all have been performed using gas chromatographic (GC) techniques. This discriminates the results of the saturate fraction especially since non-volatile, high molecular weight and polar constituents are mostly excluded. Here, for the first time, saturate fractions of different crude oils from different origins are analyzed using direct infusion ultrahigh resolution mass spectrometry (UHRMS), to study the compositions on a molecular level. Electrospray (ESI), atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) are used in positive mode. The observed results show the presence of different heteroatom containing classes, with different chemical identities (i.e., presence of thiophenes, mercaptans and cyclic-sulfides in case of S-containing compounds). These results show the high affinity of some specific compounds towards different ionization techniques. Finally, the saturate fraction is shown to include much more than only volatile, saturated and aliphatic compounds. The detected compounds in this fraction present a very wide variety, not only in terms of their carbon atoms per molecule and their aromaticity, but also with regard to their functional groups and structural arrangements.
Highlights
A better understanding of the properties and behavior of crude oil during different processes, requires a thorough knowledge about its chemical composition [1]
SARA is the acronym for the different fractions resulting from this technique (Saturate, Aromatics, Resin and Asphaltene)
Saturate samples were diluted in a toluene: methanol (1:1, v/v) mixture to a Electrospray ionization (ESI): sample solutions with wereno introduced at a flow rate of 5 μl min−1
Summary
A better understanding of the properties and behavior of crude oil during different (petroleum) processes (i.e., extraction, transportation and refining), requires a thorough knowledge about its chemical composition [1]. Some reports indicate the presence of more than 106 chemical compounds in crude oil samples [2] These compounds include aliphatic and polyaromatic hydrocarbons, different types of hetero-cycles (N, O, S) as well as some trace amount of metals [3,4], which differ in molecular weight, and in polarity and structure. Due to this high complexity, there is no single analytical technique that can fully characterize a crude oil and provide detailed chemical information on the molecular level. SARA fractionation is based on the solubility, polarity and adsorption affinity of the different petroleum compounds [3,9]
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