Metastable Ions and Isotope Effects in the Mass Spectra of Methane and the Deuteromethanes

Abstract

A differentially pumped double‐focusing mass spectrometer was suitably modified for the study of metastable ions and collision‐induced metastables. At high resolution the same instrument easily resolved protonated and deuterated ions of the same nominal mass. Accordingly, metastable ions and isotope effects in the mass spectra of CH₄, CH₃D, CH₂D₂, and CD₄ were reinvestigated with this instrument in order to resolve certain ambiguities in previous studies. All four protonated molecular ions exhibited hydrogen‐loss metastables, while CD₄⁺ exhibited a corresponding unimolecular elimination of a deuterium atom. In addition to these true metastables, essentially every conceivable collision‐induced metastable decay process was also observed. The observed relative intensities and isotope effects in the normal spectra are rationalized with general statistical arguments and with more complete calculations using the quasiequilibrium theory of mass spectra. The behavior of the rates at threshold and their effect on methane metastable intensities are also discussed.