Pulsed Gas Introduction w. Bart Emary," Raymond E. Kaiser, Hilkka 1. Kenttamaa, and R. Graham Cooks Department of Chemistry, Purdue University, \-Vest Lafayette, Indiana, USA Two different Paul-type quadrupol e ion traps were equipped with pulsed-valve gas i... ilets . The duration of a gas pulse inside the trap is variable, and pulses as short as SO 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. (] Am Soc Mass Specirom 1990, 1, 308-311) r""T"1he unique capabilities that are characteristic of I trapped-ion mass spectrometry (1) include the .L ability to carry out multiple-stage experiments in- volving consecutive dissociation andlor ion-molecule reactions [2J. Endothermic and exothermic reactions can be studied, and the dependence of reactions on pressure, time, and energy can readily be investigated [1/ 3J. These features allow detailed studies of struc- tures, reactions, and thermochemistry of many ionic and neutral gas-phase species, including those that are difficult or impossible to study in solution owing to their high reactivity. Tandem mass spectrometry (MSIMS) experiments carried out in ion-trapping devices are pulsed in na- ture. In these single-region mass spectrometers , ion- ization, mass selection, reactions, and mass analysis occur in th e same and are separated in time, in contrast to the spatial separation employed in conven- tional mass spectrometers . Unwanted ions can usu- ally be removed from the reaction region by apply- ing appropriate voltage pulses to the trap. However, the same does not apply to neutral molecules . It fol- lows that interfering reactions are not uncommon in trapped-ion mass spectrometry; for example, mass- selected ions may react faster with their neutral pre- cursors than with the desired neutral reagents. Pulsed-valve sample introduction was first intro- duced [4] for ion cyclotron resonance (ICR) traps to allow time res olution for neutral r eagents. This allowed many chemical reactions requiring high pressure to be carried out while satisfying the high-vacuum re- qu irement ( :::;10- 9 torr) for anal ysis of ionsp resent in the trap . We report here the application of pulsed- valve technology for a quite different ion-trapping de- • Pres ent address: Department of Pharmacology, The Johns Hopkins Uni- versity, 72S N. Wolfe Street, Baltimore, MD 21205. Address reprint requests to Hilkka I. Kenttamaa, Department of Chem- istry, Purdue University, West Lafayette, IN 47907. © 1990 American Society for Mass Spectrometry 1044-o305I90IS3.50 vice, the Paul-type quadrupole ion trap. In this device, ions are stored in a three-dimensional quadrupole field formed between three electrodes with hyperbolic inner surfaces [5J. In contrast to the ICR technique, which uses a stationary electric field and a strong magnetic field to trap the ions, quadrupole ion traps use purely electrostatic confinement of ions . Quadrupole ion traps can operate [6J under significantly higher pressures (up to 10- 3 torr) than ICR instruments, and the use of relatively high pressures of reagent gases is therefore not a problem. However, we will demonstrate here that pulsed-valve reagent introduction provides a sim- ple means to enhance control over the reactions occur- ring in the trap . This is especially important for those quadrupole ion traps that do not have the capabilities necessary for single-ion isolation. Experimental Two different quadrupole ion traps were employed 1.'1 this study, a Finnigan ion trap detector with modified [7) sample inlet system, and a prototype Finnigan ion irap mass spectrometer (ITMS) [8j. These instruments are pumped with a 50 Lis and a 170 Lis turbomolecular pump, respectively, and the base pressure in both de- vices is in the low 10- 7 torr range. A modified version of the commercial software was used in both instru- ments [7, 8]. Ions were generated by electron ioniza- tion (EI) (typically for 1 ms). The sequence of radio- frequency (rf) and direct current (de) voltages used in the experiments described here are shown in Figure 1 for the ITMS; the same pulse sequence applies to the ion trap detector except that no dc voltages are used in this device. Typically, the ion isolation time was 4-7 rns, and the reaction time was varied from 0 to 1600 InS . The amplitude of the rf voltage applied at 1.1 MHz to the ring electrode determines the mass range of the ions trapped. Single-ion isolation and collision- activated dissociation (CAD) experiments can be car- Received December 1, 1989 Accepted March 12. 1990