PULSE AMPLITUDE ANALYSIS - A NEW DIMENSION IN SINGLE-ION TIME-OF-FLIGHT MASS-SPECTROMETRY

Using single secondary ion detection, it has been shown that the amplitude of pulses formed by a microchannel plate electron multiplier array is dependent on the number of electrons and other ionizing species that are ejected into the microchannel when the ion impacts at the entrance of the microchannel. We have measured the pulse amplitude distribution for impacts where a single electron is ejected and have observed the onset of multiple electron emission when the incident ion velocity is increased. We have shown that pulse amplitude analysis is sensitive to the velocity and number of atoms in a molecular ion. By using pulse amplitude windows, we have measured Cf-252-PD mass spectra where the 1+ or 2+ ion of insulin can be selectively chosen or rejected on the basis of pulse amplitude. A new component in the Cf-252-PD mass spectrum in the high mass region has been identified as being due to low velocity neutral species formed by metastable decay in the acceleration region. Elimination of this component should enhance the sensitivity for low intensity high molecular mass species.