10.5-eV photoionization mass spectrometry of aliphatic compounds.

Vacuum ultraviolet photoionization by coherent 10.5-eV radiation is evaluated for soft ionization of organic molecules. Coherent 10.5-eV radiation is produced by frequency tripling the third harmonic of an Nd:YAG laser (355 nm) in a mixture of xenon and argon. A number of intermediate size, C6 to C8 , aliphatic compounds are studied to determine the extent and characteristics of fragmentation. Compared with 12-eV electron impact ionization, all show higher molecular ion abundances, less fragmentation, and signiftcant enhancement of low energy rearrangement ions. n-Alkanes, alkenes, ketones, carboxylic acids, and ethers all form predominant molecular ions. Aldehydes and amines show significant molecular ion abundances but also extensive fragmentation. Branched alkanes, dienes, alcohols, and esters show little or no molecular ion, but do have a single dominant fragment ion. Metastable broadening in these spectra gives qualitative information on the energetics of low energy rearrangements. (J Am Soc Mass Spectrom 1990, 1, 419-426).