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Mass Spectrometry
Vol. 1 (2012) No. 1 Special Issue: Advanced techniques of MALDI-TOF MS in ionization, instrumentation and applications p. A0005-



Original Article

We studied the ionization process of aromatic carboxylic acids, including ones with or without hydroxy groups in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), because many natural products, metabolites, and drags contain those structural units. In the actual experimental data, benzoic acid was ionized as only deprotonated molecule [M-H]. In contrast, both of negative molecular ion M and deprotonated molecule [M-H] were generated from 2-naphthoic acid and 2-anthracenecarboxylic acid, and the ratio of negative molecular ion to deprotonated molecule M/[M-H] was increased in 2-anthracenecarboxylic acid. In addition, the ratio of 2-anthracenecarboxylic acid was much higher than those of 1- and 9-anthracenecarboxylic acids among the three isomers. Therefore, 2-substitution induced the generation of the negative molecular ion M, which can made us prediction of the substituted positions from their overlapping peak isotope patterns. 2,5-Dihydroxybenzoic acid showed two deprotonated molecules, [M-H] and [M-H*-H], which was generated from a neutral hydrogen radical (H*) removal from a phenolic hydroxy group. The deprotonated molecule [M-H*-H] of 2,5-DHBA was the most abundant among six dihydroxybenzoic acids and three hydroxybenzoic acid. This observation raises the possibility that such a property of 2,5-DHBA could be a clue to explain its highest efficiency as a MALDI matrix. The order of the hydrogen radical removal from the phenolic hydroxy groups was the 3-<4-<<5-positions in the dihydroxybenzoic acids, and the 3-<4-positions in hydroxybenzoic acids. The intra-molecular hydrogen bonding between 1-carboxy and 2-hydroxy groups was an important factor in hydrogen radical removal in the hydroxylbenzoic acids and dihydroxybenzoic acids.

Copyright © 2012 The Mass Spectrometry Society of Japan

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