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 Fourier-transform ion cyclotron resonance (FT-ICR) spectrometry are described. SORI induced circular gyrations of ions with their cyclotron radii varying sinusoidally at the corresponding beat frequency. The beat is observed in the temporal variation of FT-ICR peak height as a function of the duration of the SORI burst, and it allows the direct measurement of the beat frequency. The effective cyclotron frequency is determined from the measured beat frequency and the applied burst frequency. 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 cyclotron radii of the irradiated ions allow the radial separation of mass-selected ions so that the mass-selective photodissociation can be carried out with m-bromotoluene ions