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 efficiency and mass selectivity of quadrupolar excitation-collisional cooling for isolation of parent ions for collision-induced dissociation in Fourier 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 frequency 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 efficient collisional cooling of the cyclotron motion produced by magnetron-to-cyclotron conversion. Efficiency, detected signal magnitude, and mass resolving power for collision-induced dissociation (CID) product 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 structure.