Rapid On-Column Esterification and Separation of Carboxylic Acids in Petroleum Fractions

Carboxylic acids are frequently encountered in sediments and petroleum, and their individual molecular structures are of great significance for the study of geochemical significance. However, analyzing carboxylic acid individual compounds within complex organic mixtures, like petroleum, poses a challenge for gas chromatography–mass spectrometry (GC–MS) as a result of their high polarity and boiling points as well as potential interference from other matrix compounds. This study proposes a novel approach for separation and characterization of carboxylic acids in petroleum samples, employing the esterification reaction on a solid-phase extraction column packed with silver-ion silica gel. Carboxylic acids are promptly converted into the corresponding methyl esters during elution, and these resulting nonpolar methyl esters were subsequently subjected to GC–MS analysis. The conversion ratios of on-column esterification and the corresponding reaction conditions were validated using carboxylic acid model compounds. The on-column esterification process enables the rapid and selective esterification of carboxylic acid compounds in crude oil, facilitating their separation from the petroleum matrix. This method has been demonstrated to be highly valuable for elucidating the molecular structures of carboxylic acid isomers in petroleum. In addition, the method is both flexible and time-efficient, demonstrating its potential for analyzing the molecular structures of carboxylic acid markers in the fields of geochemistry and petrochemistry.