APPLICATION OF SUSTAINED OFF-RESONANCE IRRADIATION - THE BEAT FREQUENCY-MEASUREMENT AND RADIAL SEPARATION OF MASS-SELECTED IONS

Newer applications of sustained off-resonance irradiation (SORI) in Fo urier-transform ion cyclotron resonance (ET-ICR) spectrometry are desc ribed. SORI induced circular gyrations of ions with their cyclotron ra dii varying sinusoidally at the corresponding beat frequency. The beat is observed in the temporal variation of ET-ICR peak height as a func tion of the duration of the SORI burst, and it allows the direct measu rement of the beat frequency. The effective cyclotron frequency is det ermined from the measured beat frequency and the applied burst frequen cy. The frequency uncertainty is greatly reduced by nearly 3 orders of magnitude compared with the uncertainty in FT-ICR peak frequency that fluctuates with the rf burst duration. In addition, the oscillating c yclotron radii of the irradiated ions allow the radial separation of m ass-selected ions so that the mass-selective photodissociation can be carried out with m-bromotoluene ions. (C) 1997 American Society for Ma ss Spectrometry