SELECTIVE PARENT ION AXIALIZATION FOR IMPROVED EFFICIENCY OF COLLISION-INDUCED DISSOCIATION IN LASER-DESORPTION IONIZATION FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE MASS-SPECTROMETRY

We have systematically established the excitation frequency, amplitude , duration, and buffer gas pressure for optimal axialization efficienc y and mass selectivity of quadrupolar excitation-collisional cooling f or isolation of parent ions for collision-induced dissociation in Four ier transform ion cyclotron resonance mass spectrometry. For example, at high quadrupolar excitation amplitude, ion axialization efficiency and selectivity are optimal when the applied quadrupolar excitation fr equency is lower than the unperturbed ion cyclotron frequency by up to several hundred hertz. Moreover, at high buffer gas pressure (10(-6) Torr), quadrupolar excitation duration can be quite short because of e fficient collisional cooling of the cyclotron motion produced by magne tron-to-cyclotron conversion. Efficiency, detected signal magnitude, a nd mass resolving power for collision-induced dissociation (CID) produ ct ions are siginificantly enhanced by prior parent ion axialization. With this method, we use argon CID to show that C94+ (m/z 1128) formed by Nd:YAG laser desorption-ionization behaves as a closed-cage struct ure.