TRAPPING AND DETECTION OF IONS GENERATED IN A HIGH MAGNETIC-FIELD ELECTROSPRAY IONIZATION FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE MASS-SPECTROMETER

The trapping and detection parameters employed with a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer that is interfaced to a high magnetic field electrospray ionization (ESI) source are presented. ESI occurs at atmospheric pressure in a 1.5-T field, and FTICR detection occurs 25 cm away at 3.0 T in either one of two cells separated by a conductance limit and maintained ai pressure differentials of 5 x 10(-5) and 2 X 10(-7) torr, respectively. The continuous electrospray ion current traversing the high- and low-pressure cells is 350 100 pA, respectively. Retarding grid studies at the high-pressure cell indicate electrospray ion kinetic energies are controllable from less than an electronvolt to more than 10 eV. These kinetic energies are a function of desolvating capillary-skimmer assembly distance and the skimmer potential. Efficient accumulation of injected ions is accomplished only when the trap-plate potential matches the ion kinetic energy. If this condition is satisfied, the trapped ion cell fills to the ion space charge limit within a few hundred milliseconds. It is concluded that even at high pressures used, the primary trapping mechanism cannot be solely collision dependent because the rate of ion accumulation is independent of background pressure. However, optimized FTICR excitation conditions for peptides and proteins in the mass range from 10(3) to more than 10(6) kDa are found to vary strongly with pressure; this is attributed to large mass- and charge-dependent differences in ion-molecule collision frequency.