Analysis of naphthenic acids by matrix assisted laser desorption ionization time of flight mass spectrometry

We report on a matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF) method to analyze naphthenic acids (NAs), from petrochemical samples, using 9-aminoacridine (9-AA) and a proton sponge 1,8-bis(dimethylamino) naphthalene (DMAN), as matrices. Singly charged deprotonated molecules of NAs, [M−H]−, dominated the negative ion MALDI mass spectra. The negative ions were pre-formed in solution through an acid/base Brönsted-Lowry reaction promoted by the highly basic matrices (9-AA and DMAN). The DMAN afforded cleaner spectra with absence of clusters, aggregates or matrix interferences at low m/z; in addition, it provides molecular weight distributions similar to those observed with negative ion electrospray ionization (ESI). In contrast, MALDI spectra obtained with 9-AA exhibits matrix interferences at low mass, and bias towards aliphatic NAs. Finally, we observe the presence of monomeric species in the low mass region and non-covalent aggregates at higher masses in the negative mode MALDI spectra with both matrices, much like with negative mode ESI. We demonstrate that MALDI-TOF/MS is a viable tool for NAs analyses with added advantages such as high throughput and exceptional tolerance to the presence of impurities and salts in the samples, when compared with ESI.

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