The Hows and Whys of Ion Trapping

Abstract

48824ion trap (Ian trap} n.-a device in ion trapping has been successfulranging from fundamental physical measurements to applications in chemistry and MS-will be provided.As a benchmark, consider the ion source of a mass spectrometer. Although mass spectrometers are designed to study ionic species, conventional ion sources typically are not designed to contain ions for any period of time. Typical residence times (from ion formation to extraction) are about 10_6-10-6 s (6), In contrast, ion traps have been used to trap ions for weeks. How to trap ionsIon traps typically are classified as active or passive. The classification reflects whether the fields used to confine the ions are time-dependent (active) or time-independent (passive).We will first focus on the mechanisms of trapping for the various devices that have been reported. Although a variety of ionization methods have been used in conjunction with ion traps, by far the most common is electron impact ionization (El). Unless otherwise stated, it may be assumed that El is used to form ions in the experiments discussed. Active trapsThe quadrupole ion storage trap (quistor). The quistor or Paul trap(5), first described in 1953, falls into the same family as, and is a "closed form" of, the quadrupole mass filter (7). The quistor (7, 8) is a small "ion bottle" that has a geometry described as a hyperboloid of one sheet combined with a hyperboloid of two sheets (9)-it is composed of a ring electrode and two end caps (see Figure la). Electrons can be injected through a hole in either the end cap (9) or in the ring electrode (10).In the quistor, the end caps are electrically connected, and dc/rf potentials are applied between them and the ring electrode. The applied voltage takes the form [U + V cosfit], where U is the dc voltage; the rf voltage has amplitude V and radial frequency 12.The equations of motion for an ion in the trap with a certain mass-to-charge ratio (m/z) are derived from Newton's Law (F = ma), giving differential equa-