GC/MS on an LC/MS instrument using atmospheric pressure photoionization

Atmospheric pressure (AP) GC/MS was first introduced by Horning et al. [E.C. Horning, M.G. Horning, D.I. Carroll, I. Dzidic, R.N. Stillwell, Anal. Chem. 45 (1973) 936] using 63Ni as a beta-emitter for ionization. Because, at the time special instrumentation was required, the technique was only applied with consistency to negative ion environmental studies where high sensitivity was required [T. Kinouchi, A.T.L. Miranda, L.G. Rushing, F.A. Beland, W.A. Korfmacher, J. High Resolut. Chromatogr., Chromatogr. Commun. 13 (1990) 281]. Currently, AP ion sources are commonly available on LC/MS instruments and recently a method was reported for converting an AP-LC/MS ion source to a combination AP-LC/MS:GC/MS source [C.N. McEwen, R.G. McKay, J. Am. Soc. Mass Spectrom. 16 (2005) 1730]. Here, we report the use of atmospheric pressure photoionization (APPI) with GC/MS and compare this to AP chemical ionization (APCI) GC/MS and electron ionization (EI) GC/MS. Using a nitrogen purge gas, we observe excellent chromatographic resolution and abundant molecular M+ and MH+ ions as well as structurally significant fragment ions. Comparison of a 9.8 eV UV lamp with a 10.6 eV lamp, as expected, shows that the higher energy lamp gives more universal ionization and more fragment ions than the lower energy lamp. While there are clear differences in the fragment ions observed by APPI-MS versus EI-MS, there are also similarities. As might be expected from the ionization mechanism, APPI ionization is similar to low energy EI. These odd electron fragment ions are useful in identifying unknown compounds by comparison to mass spectra in computer libraries.