Selective profiling of carboxylic acid in crude oil by halogen-labeling combined with liquid chromatography and high-resolution mass spectrometry

Carboxylic acids are a small but important compound class within petroleum chemistry, contributing significantly to the behavior of crude oil, e.g., in production, processing, and its environmental impact. A more detailed structural information is fundamental to improve our understanding of their influence on petroleum properties. However, due to matrix effects, structural diversity and low abundance, selective characterization of carboxylic acids in crude oil remains challenging. In this work, we present a new methodology for profiling such compounds by liquid-liquid extraction and selective derivatization using 4-bromo-N-methylbenzylamine (4-BNMA) followed by liquid chromatography and electrospray ionization Orbitrap high-resolution mass spectrometry (LC-ESI-Orbitrap HRMS). The fragmentation of 4-BNMA derivatives produces a pair of unique product ions, m/z 169 and 171, which enables the identification of chromatographic sections containing carboxylic acids. The mass spectra of these fractions are extracted, with the acids further computationally isolated based on the unique isotope pattern from the incorporated bromine atom. The method was optimized and validated using seven acid standards and designs of experiments (DOEs), assuring robustness and sensitivity for non-target screening purposes. This method successfully detected up to 380 carboxylic acids in six Danish North Sea crude oil, with up to two carboxyl groups and other heteroatom functionalities. The results indicated that the most populated species are fatty acids (double bond equivalents, DBE, = 1) and naphthenic acids (DBE = 2 6). Besides classical carboxylic acids detected, we observed other acids containing multiple functional groups with nitrogen, oxygen, and sulfur atoms.