Laser Desorption Mass Spectrometry of Some Organic Acids

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Abstract
Positive and negative ion laser desorption (LD) mass spectra of organic acids are characterized by the emission of the quasimolecular ions (M+H)+ and (M-H)−. Generation of (M+H)+ ions is interpreted as evidence for the occurence of high pressure proton transfer reactions in LD. Fragment ions can be rationalized by loss of stable neutral molecules from quasimolecular ions although decomposition may also be occuring prior to ionization. Features unique to LD, including the detection of pyrolysis products along with ions characteristic of the sample, are discussed in terms of the internal energy distribution in the irradiated microvolume. Negative ion mass spectra of acids are dominated by (M-H)−, while positive ion spectra contain abundant fragment ions, underlining the utility of detecting negative ions for acidic compounds.