ABSTRACT
Radio frequency (RF) ion guides have been used for many years for transporting and guiding ions along an ion optical path. Their use has become pervasive in commercial instruments since the enormous growth in atmospheric pressure ionization mass spectrometry, driven by both the popularity of electrospray ionization for biological analysis and by the widespread use of liquid chromatography/tandem mass spectrometry (LC/MS/MS) systems for the analysis of drugs and metabolites in the eld of drug development. More recently, RF ion guides have begun to be adapted for use as ion traps in a number of applications: for temporary storage of ions while processing a previous batch at another location in the ion path, and to contain ions while they are processed by charge state separation techniques [1], by ion/photon reactions [2], by ion/molecule reactions (see Section 10.5), by ion/ion reactions [3], or even by ion/ electron reactions [4]. In addition, ion guides now nd use as local traps to store ions prior to mobility separation, and as containment devices for ion mobility drift tubes. Under software control, RF ion guides can be switched from transport mode to trapping mode and back to transport with a reversal of ion ow direction. Techniques of trapping ions inside the ion guide have expanded from the use of direct current (DC) barriers at the ends, to include the application of RF barriers at the end [5], axial elds combined with RF or DC barriers [6], and gas ow combined with DC or RF
10.1 Introduction ................................................................................................ 525 10.2 Trapping and Moving Ions in a High-Pressure RF Quadrupole ................ 528 10.3 Reversing Ion Flow for MSn ....................................................................... 531 10.4 Ion/Ion Reactions with Bi-Directional Ion Flow ........................................ 534 10.5 Ion/Molecule Reactions of Trapped Ions .................................................... 536 10.6 Branched Ion Guides for Multiple Sources ................................................ 539 10.7 Conclusion .................................................................................................. 542 References .............................................................................................................. 542
elds [7]. More techniques and applications will no doubt be developed in the future as the need for more and better methods of ion processing increases.
