Comparison of LC-MS-based methods for the determination of carboxylic acids in animal matrices

Carboxylic acids (CAs) are key players in human and animal metabolism. As they are hardly retained under reversed-phase liquid chromatography (RP-LC) conditions in their native form, derivatization is an option to make them accessible to RP-LC and simultaneously increase their response for mass spectrometric detection. In this work, two RP-LC tandem mass spectrometry-based methods using aniline or 3-nitrophenylhydrazine (3-NPH) as derivatization agents were compared with respect to several factors including completeness of derivatization, apparent recoveries (R_As) in both cow feces and ruminal fluid, and concentrations obtained in feces and ruminal fluid of cows. Anion exchange chromatography coupled to high-resolution mass spectrometry (AIC-HR-MS) served as reference method. Derivatization efficiencies were close to 100% for 3-NPH derivatization but variable (20-100%) and different in solvent solutions and matrix extracts for aniline derivatization. Likewise, average R_As of ¹³C-labeled short-chain fatty acids as internal standards were around 100% for 3-NPH derivatization but only 45% for aniline derivatization. Quantification of CAs in feces and ruminal fluid of cows initially fed a forage-only diet and then transitioned to a 65% high-grain diet which yielded similar concentrations for 3-NPH derivatization and AIC-HR-MS, but concentrations determined by aniline derivatization were on average five times lower. For these reasons, derivatization with aniline is not recommended for the quantitative analysis of CAs in animal samples.