Herein, we describe a new methodology for profiling carboxylic acids in crude oil using a halogenated amine tag with subsequent LC separation and precursor ion search by Orbitrap HRMS. The acidic fraction was first isolated by LLE using a basic aqueous-solvent solution. The carboxylic acids in the extractant were then activated and derivatized with 1-ethyl-3-dimethylaminopropyl carbodiimide (EDC) and 4-bromo-Nmethylbenzylamine (4-BNMA), respectively (Figure ). The method is based on Marquis et al. who observed that 4-BNMA can (i) facilitate complete derivatization of small mono-and poly-carboxylic acids, (ii) feature a favorable fragmentation characteristic -bromine isotopic pattern, for unknown acid identification, and (iii) improve selectivity in reversed-phase LC. The fragmentation of derivatives to phenyl bromine ion pair (m/z 169 and 171) is an indicator to profile unknown carboxylic acid molecules. Thus, the selectivity of the derivatization is combined with computational isolation of the unique resulting fragmentation pattern. This approach can significantly simplify the complex crude oil matrix and data processing while increasing confidence in the formula assignments. The method performance was comprehensively optimized and evaluated, focusing on extraction and derivatization, assessing method sensitivity, and matrix effects. The applicability was demonstrated by the analysis of six North Sea crude oils. Up to 380 carboxylic acid constituents were identified, primarily aliphatic and naphthenic acids with a smaller fraction of molecules containing additional heteroatoms (i.e., nitrogen and sulfur).