Analysis of complex mixtures by photoionization mass spectrometry with a vacuum-ultraviolet hydrogen-laser source

Trace organic analysis of a complex matrix presents one of the most challenging problems in analytical mass spectrometry. Inselective electron-impact ionization often requires extensive sample clean-up to isolate the analyte from the matrix. Sample preparation can be greatly reduced when a hydrogen laser is used for selective photoionization of only a small fraction of the compounds introduced into the ion source. This device produces parent ions only for all compounds with ionization potentials that lie below a threshold value limited by the photon energy of 7.8 eV. The only observed interference arises from electron-impact ionization when scattered laser radiation interacts with metal surfaces, producing electrons which are then accelerated by potential fields inside the source. These can be suppressed to levels acceptable for practical analysis through proper instrumental design. Results are presented which indicate the ability of this ion source to discriminate against interfering matrix components in simple extracts from real samples such as brewed coffee, beer, and urine.