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Secondary-ion mass spectrometry, using Cs' ion bombardment of solid and liquid matrices, has been implemented on a quadrupole ion trap. Tetraalkylammonium cations and protonated peptides, generated in an external source using a pulsed primary-ion beam, are injected into the ion trap where they are decelerated and trapped in the course of collisions with the helium bath gas. The contents of the trap are then examined by recording a mass spectrum using the mass-selective instability scan. To improve performance in the examination of these trapped ions, the technique of axial modulation is used to cause resonance ejection of ions from the trap during the mass-analysis step. The supplementary AC voltage applied to the end-caps causes resonant ejection of ions at qr values below those which correspond to the normal stability limit. This is demonstrated to increase the mass limit of a commercial ion trap from 650Da to approximately 1300Da with no loss in resolution or sensitivity, and to more than 20 OOO Da at reduced sensitivity.As both a reactor for gas-phase ion/molecule reactions and as a sensitive mass analyzer, the quadrupole iontrap mass spectrometer has proved to be a versatile tool for the qualitative and quantitative analysis of volatile organic compounds.' The sensitivity of ion traps' and the ability to dissociate ions efficiently in the course of tandem experiment^,^-^ has increased the value of these instruments. Ions are typically generated within the cavity of the cell by electron ionization, chemical i~n i z a t i o n~.~ or photoionization.' However, internal ionization is not the only means of ion-trap operation; ionization by laser desorption with injection of the desorbed ions has also been described. In particular, laser desorption and subsequent injection of both metal ions and organic cations has been reported.'-" Growing interest in the study of biomolecules makes the desorption ionization (DI) methods such as secondary-ion mass spectrometry (SIMS), fast-atom bombardment (FAB) and laser desorption (LD), which are applicable to non-volatile compounds, of increased interest. This paper describes the implementation of desorptionionization techniques with ion traps.To exploit the DI capabilities described here, the mass range of the ion trap must be extended. This can be achieved by using reverse scans,'' by reducing the RF freq~ency'~ or by employing devices of reduced size, as recently r e p~r t e d . '~ An alternative procedure, used here, employs the simple expedient of applying between the end-cap electrodes an AC voltage of a frequency less than one half of the fundamental trapping frequency so permitting a method of mass analysis with an increased mass range. This method called "axial modulation" was first described in a patent by Syka and coworkers3 and was recently reported to provide enhanced resolution for the special case of an applied AC voltage of nearly one half the fundamental trapping frequency (so that the mass range was not extended significantly). l5

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