Californium-252 fission fragment ionization mass spectrometry of chlorophyll a

Abstract

The title study has been made by using a time-of-flight mass spectrometer, placing primary emphasis on fragmentation reactions and rates. The mass spectrometer was modified so that segmented fields with segments of different magnitudes could be applied in the ion acceleration region and retarding potentials in the region between the flight tube and the ion detector. Methods using these retarding potentials were developed for determining the identities of fragmentations in the flight tube and for measuring the rate constants of fragmentations occurring in the ion acceleration region. Positive and negative spectra were determined with and without retarding potentials at the end of the flight tube. Extensive amounts of fragmentation occur both close to the sample foil (prompt fragmentation) and in the flight tube. For example, 98.9% of the positive molecular ions from chlorophyll a entering the flight tube fragment before reaching the detector. In the spectra with retarding potential applied, M+. and (M + H)' ions are the ions in the positive molecule-ion region, and (M - H)-, M-., and (M + H)- are the ions in the negative molecule-ion region. Networks of fragmentation paths were deduced from retarding potential measurements. Rate constanis for certain fragmentations are deduced from tails on peaks and from measurements utilizing segmented fields in the ion acceleiation region. It is deduced that fragmentations occur with rate constants ranging from >lo9 s-l to lo4 s-'. It is suggested that the fission fragment induced fragmentation processes observed in Chl a involve many of the concepts embodied in the quasi-equilibrium theory of mass spectra, e.g., formation of reactant ions with a wide range of energies, which results in a network of sequential and competing unimolecular reactions with variable and wide-ranging rate constants.