Chemical lonization in fourier transform mass spectrometry

An investigation of low-presure chemical lonization using the standard reagent gases methane, isobutane, water, and ammonia at pressures in the low 10^-8 torr range with a variety of analytes at partial pressures in the 10^-7-10^-8 torr range is reported. The measurements were made by using a Fourier transform mass spectrometer (FT-MS) constructed in our laboratory. For each of the reagent gases, the first experiments were designed to determine appropriate conditions for obtaining routine and reproducible chemical ionization spectra. Additional experiments demonstrate the capabilities of FT-MS instruments and allow comparison of results obtained with FT-MS and with conventional mass spectrometers. Samples have been ionized with proton transfer reactions involving both positive and negative ions. An alternative proton transfer method called "self-chemical ionization" makes use of fragment ions and no reagent gas. Both types of experiments can be conducted at prolonged ion residence times in a time-resolved manner or more rapidly using a "quench-off" approach. Other types of chemical ionization have also been investigated including isotopic exchange (for counting active hydrogens) and specialized reagents such as radical cations (for distinguishing isomeric olefins).