Modification of a double focusing mass spectrometer for the observation of metastable ions decaying in two field-free regions

The occurrence of ‘metastable’ ions formed in the ion source as one species decays to a lower mass ion prior to mass analysis provides direct evidence for dissociation mechanisms in organic mass spectra. Both unimolecular and collision-induced decay processes provide useful information on available dissociation pathways, ion structures, and energetics. In double-focusing mass spectrom-eters the existence of two field-free regions presents a convenient vehicle for studying these processes and for obtaining kinetic information on consecutive metastables through simple modification in circuitry and operating conditions. These modifications are described and the method is used to demonstrate consecutive metastable decay processes in acetyl acetone, 1-butene, toluene and alumin-um hexafluoroacetylacetonate. Drift tube pressure studies showed that, within experimental error, consecutive metastables observed in acetyl acetone and 1-butene were collision-induced, while toluene and the aluminum chelate both exhibit two-step unimolecular decays.