Analytical in source pyrolytic methylation electron impact mass spectrometry of phenolic acids in biological matrices

Phenolic acids in complex biological matrices are easily decarboxylated under pyrolysis electron impact (EI) conditions depending on their binding to other plant cell wall compounds. Pyrolysis gas chromatography/mass spectrometry (Py-GC/MS) of phenolic acids reveals only the decarboxylated compound but this product may also be derived from the alcoholic lignin monomers. Discrimination between the acidic and alcoholic phenol can be obtained by stabilization of the molecular ion of the two compounds. Samples of p-coumaric acid, of sporopollenin from Pinus pollen wings, of bamboo milled wood lignin (MWL) and of tobacco leaf nerve MWL were analysed by analytical Py-EIMS. Mass peaks for the molecular ion of p-coumaric acid and of the decarboxylated product may be observed. Results are compared with Py-MS data of samples treated with tetramethylammonium hydroxide (TMAH), a compound used to methylate the phenolic hydroxyl group and the carboxyl group of the phenolic acid in the ion source of the mass spectrometer. Mass spectra of samples treated with this agent show a high content of double methylated p-coumaric acid (m/z 192). TMAH treatment allows discrimination between phenolic acids and alcohols under Py-MS and Py-GC/MS conditions by producing more stable molecular ions.