Chemical Speciation of Naphthenic Acids in Brazilian Crude Oils by Advanced Analytical Techniques

The characterization of naphthenic acids (NAs) in petroleum samples has received particular attention and is still an analytical challenge as a result of the complexity of the matrix. This study aims to individually and comprehensively characterize NAs in real Brazilian crude oils by Orbitrap high-resolution mass spectrometry (Orbitrap-HRMS) and comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry (GC x GC-TOFMS). Linear, branched, and cyclic carboxylic acids were identified and semi-quantified by GC x GC-TOFMS in four crude oils with a distinct total acid number (TAN) and American Petroleum Institute (API) gravity values. The P1 sample, a crude oil with an intermediate TAN value (1.25 mg(KOH) g(-1) and 22.2 degrees), presented a total acid quantification of 16.6 mg goil(-1) by GC x GC-TOFMS and a higher total concentration of identified linear acids. The P2 crude oil, which has a lower API gravity value and a higher TAN value (19.8 degrees and 2.39 mgKOH g(-1), respectively), resulted in a higher total acid quantification of 35.2 mg goil(-1) and expresses the highest amount of identified cyclic acids, in particular, the tricyclic terpane acid homologous series. However, the P3 and P4 crude oils had similar API gravity and TAN values (29.9 degrees and 29.8 degrees and 0.22 and 0.24 mgKOH g(-1), respectively), and total acid quantification by GC x GC-TOFMS resulted in different values (2.185 and 9.636 mg goil(-1), respectively). These results indicate that the TAN value does not sufficiently express the actual quantification of acids when compared to the total acid quantification by GC x GC-TOFMS. The individual identification of the acidic species that comprise the oil can help to understand the real acidity of the samples. The data obtained via Orbitrap-HRMS show that a higher variability of molecular formulas with a large range of double bond equivalents (DBEs) was detected for the P1 and P2 oils, while the P3 and P4 oils showed a high percentage of linear carboxylic acids. Detailed information on the individual chemical structure of the NAs is very important for evaluating the physicochemical properties of the oil (e.g., TAN), using advanced analytical techniques and offering a new perspective for crude oil fingerprinting with greater analytical reliability.

Automated summary

This study characterized naphthenic acids in Brazilian crude oils using high-resolution mass spectrometry and comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry. The results showed variations in the types and concentrations of naphthenic acids in different crude oil samples, indicating that TAN values cannot accurately represent the actual quantification of acids.