Application of High-Performance Liquid Chromatography/Electrospray Mass Spectrometry for Identification of Carboxylic Acids Containing Several Carboxyl Groups from Aqueous Solutions

A high-performance liquid chromatography/electrospray ionization mass spectrometry (HPLC/ESI-MS) method has been developed for the separation and detection of a series of carboxylic acids with one or more carboxyl groups, which can be formed during air oxidation of lignite. Chromatographic separation used an RP-C18 column with a gradient of water [containing 0.01% (v/v) formic acid] and acetonitrile, at a flow rate of 0.3 mL min⁻¹. Detection by means of megaflow electrospray-MS was performed in the negative-ion mode [ESI(–)] at different cone voltages. The ESI(–) mass spectra of the carboxylic acids at a cone voltage of −20 V are characterized by the formation of [M – H]⁻ anions together with fragmentation by loss of CO₂ and sometimes H₂O. Increasing the cone voltage effects enhanced fragmentation and leads to further structural hints. Lignite-derived analytes used for these HPLC/ESI-MS investigations were enriched from aqueous solution by solid-phase extraction on ISOLUTE ENV+, a polystyrene copolymer. From fragmentation results obtained with standards, conclusions regarding the existence of a variety of carboxylic acids containing one or more carboxyl groups could be drawn. A more definitive structural elucidation is possible when these results are used in combination with data from GC/MS (after analyte derivatization).