Two different Paul-type quadrupole ion traps were equipped with pulsed-valve gas inlets. The duration of a gas pulse inside the trap is variable, and pulses as short as 50 ms (FWHH) have been measured, allowing the use of several gas pulses during one experiment. The benefits of pulsed valves are outlined and demonstrated for chemical ionization experiments and for the use of selective ion-molecule reactions in structure determination of ions and neutral molecules. (J Am Sot Muss Specfrom 1990, 1, 308-311) e
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unique capabilities that are characteristic of trapped-ion mass spectrometry [l] include the ability to carry out multiple-stage experiments involving consecutive dissociation and/or ion-molecule reactions [2]. Endothermic and exothermic reactions can be studied, and the dependence of reactions on pressure, time, and energy can readily be investigated [l, 31. These features allow detailed studies of structures, reactions, and thermochemistry of many ionic and neutral gas-phase species, including those that are difficult or impossible to study in solution owing to theti high reactivity.
Tandem mass spectrometry (MS/MS) experiments carried out in ion-trapping devices are pulsed in nature. In these single-region mass spectrometers, ionization, mass selection, reactions, and mass analysis occur in the same space and are separated in time, in contrast to the spatial separation employed in conventional mass spectrometers. Unwanted ions can usually be removed from the reaction region by applying appropriate voltage pulses to the trap. However, the same does not apply to neutral molecules. It follows that interfering reactions are not uncommon in trapped-ion mass spectrometry; for example, massselected ions may react faster with their neutral precursors than with the desired neutral reagents.
Pulsed-valve sample introduction was first introduced [4] for ion cyclotron resonance (ICR) traps to allow time resolution for neutral reagents. This allowed many chemical reactions requiring high pressure to be carried out while satisfying the high-vacuum requirement (<lo-' torr) for analysis of ions present in the trap. We report here the application of pulsedvalve technology for a quite different ion-trapping de-