A fast atom bombardment and field ionization/field desorption study of some isomeric unsaturated dicarboxylic acids

Abstract

AbstractGeometrically isomeric dicarboxylic acids, such as maleic and fumaric acid and their methyl homologues, and the isomeric phthalic acids, have been investigated using fast atom bombardment, field ionization and field desorption mass spectrometry. The most intense peak in the positive ion fast atom bombardment spectra corresponds with the [M + H]+ ion. This ion, when derived from the E‐acids, tragments either by successive loss of water and carbon monoxide or by elimination of carbon dioxide. In the case of the Z‐acids only elimination of water from the [M + H]+ ions is observed to occur to a significant extent. The same is true for the [M + H]+ ions of the isomeric phthalic acids, that is the [M + H] ions derived from iso‐ and terephthalic acid exhibit more fragmentation than those of phthalic acid. All these acids undergo much less fragmentation upon field ionization, where not only abundant [M + H]+ ions, but also abundant [M]+· ions, are observed. Upon field desorption only the [M + H]+ and [M + Na]+ ions are observed under the measuring conditions. Negative ion fast atom bombardment spectra of the acids mentioned have also been recorded. In addition to the most abundant [MH]− ions relatively intense peaks are observed, which correspond with the [M]−˙ ions. The fragmentations observed for these ions appear to be quite different from those reported in an earlier electron impact study and in a recent atmospheric pressure ionization investigation.