SELECTIVE GENERATION OF CHARGE-DEPENDENT - INDEPENDENT ION ENERGY-DISTRIBUTIONS FROM A HEATED CAPILLARY ELECTROSPRAY SOURCE

Retarding grid and Fourier transform ion cyclotron resonance (FTICR) m ass spectrometry variable trap potential measurements are performed to determine factors that contribute to the kinetic energy distribution of ions formed in an electrospray source that uses a heated capillary for desolvation. The control of ion kinetic energies is achieved by ma nipulating the skimmer position in the postcapillary expansion and by varying the potential applied to the skimmer. The selective generation of either charge-dependent or charge-independent ion energy distribut ions is demonstrated. Charge-dependent energy distributions of electro sprayed ions are created by sampling ions near the Mach disk of the su personic expansion and by using a larger diameter skimmer orifice; the FTICR spectra acquired under these conditions exhibit mass-to-charge ratio-dependent mass discrimination determined by the potential used t o trap the ions. Charge-independent energies of electrosprayed ions ar e created by positioning the capillary adjacent to the skimmer to samp le thermal ions and by using a smaller skimmer orifice to reduce expan sion cooling; under these conditions ion kinetic energy is determined primarily by the skimmer potential and no mass-to-charge ratio-depende nce is observed in the selection of optimum FTICR trapping conditions. The ability to select between proteins of different conformation on t he basis of kinetic energy differences is demonstrated. For example, a 0.4 V difference in trap potential is observed in the selective trapp ing of open and closed forms of the + 10 charge state of lysozyme. Fin ally, it is demonstrated that by operating the source under conditions which deliver a beam of ions with charge-independent energies to the cell, it is possible to obtain precursor and product ion signal magnit udes in FTICR spectra without charge-dependent mass discrimination.