Linear ion traps are finding new applications in many areas of mass spectrometry. In a linear ion trap, ions are confined radially by a two-dimensional (2D) radio frequency (RF) field, and axially by stopping potentials applied to end electrodes. This review focuses on linear ion trap instrumentation. Potentials and ion motion in linear multipole fields and methods of ion trapping, cooling, excitation, and isolation are described. This is followed by a description of various mass discrimination effects that have been reported with linear ion traps. Linear ion traps combined in various ways with three-dimensional (3D) traps, time-of-flight (TOF) mass analyzers, and Fourier transform ion cyclotron resonance mass spectrometers are then given. Linear ion traps can be used as stand alone mass analyzers, and their use for mass analysis by Fourier transforming image currents, by mass selective radial ejection, and by mass selective axial ejection are reviewed.

Linear ion traps in mass spectrometry.

The quadrupole ion trap (QIT) is constructed of three electrodes that, when held at appropriate potentials, cause the formation of a trapping pseudo-potential well so that charged particles, or gaseous ions, may be confined or stored for long periods of time. The two end-cap electrodes resemble saucers while the ring electrode resembles a napkin ring; all of the electrodes are of hyperbolic geometry. The ion trap itself functions as a mass spectrometer when the ion-confining conditions are modified such that ions are ejected mass-selectively from the trapping potential well. As ions of successive mass/charge ratios are ejected in turn from the ion trap, they impinge upon an external detector whereby ion signals are created in proportion to the ion number of each species; in this manner, a mass spectrum is generated. The QITMS (quadrupole ion trap mass spectrometer) is an extraordinary instrument in that it is physically small (the entire electrode assembly can be held in the palm of one's hand) compared with magnetic and electric sector instruments, it is relatively inexpensive, it is one of the most, if not the most, sensitive mass spectrometers and, since several mass-selective operations can be carried in succession, the ion trap can function as a tandem mass spectrometer. Tandem mass spectrometric operation is described as (MS)n. With the QITMS, (MS)n is carried out in time in the same volume of space whereas (MS)n in sector instruments is carried out in space. With sector instruments, the maximum value of n is n = 4 yet with the ion trap, (MS)n where n = 4–6 can be carried out routinely and n = 13 has been achieved. The QITMS shares several similarities with the ion cyclotron resonance mass spectrometer yet the cost of the former is about one-tenth that of the latter. One striking difference between the QITMS and all other mass spectrometers is that the QITMS operates at a pressure of 10−3 Torr compared with 10−6 to 10−9 Torr for other mass spectrometers.



The theory of ion confinement and ion trajectory manipulation in the QITMS has been explained relatively simply so far. Since the theory differs widely from those of sector instruments and ion cyclotron resonance mass spectrometry (ICR/MS), it will not be familiar to those who have not had the opportunity to examine ion motion in quadrupole fields. Optimum operation of the QITMS is effected by collisional focusing of the ion cloud to the center of the ion trap under the influence of helium buffer gas. Since the movement of ions confined in the ion trap is periodic, the trajectories of collisionally focused ions can be expanded by resonance excitation effected by the imposition of supplementary radio frequency (rf) potentials of low amplitude to the end-cap electrodes of the ion trap. This excitation operation permits isolation of selected ion species, by ejection of unwanted ion species, followed by selective ion/molecule reaction or by collision-induced dissociation (CID) with subsequent mass analysis of the product/fragment ions formed. Sample calculations are given of all of the relevant trapping parameters. Applications of the QITMS as a single stage mass spectrometer and as a tandem mass spectrometer are discussed. The operation of the QITMS for the identification of dioxins and furans co-eluting from a gas chromatograph is described. In addition, the application of chemical ionization (CI) for the identification of co-eluting polychlorinated biphenyl (PCB) congeners is discussed. The QITMS is an extraordinary instrument that is capable of great sensitivity, high mass range and high mass resolution. Since the QITMS is compatible with methods for generating ions externally, such as electrospray ionization (ESI), its continued growth in many areas of mass spectrometry (MS) is assured. The structural details of the glycoprotein and the optical spectroscopy of stored ions are applications of the QITMS combined with ESI, while the ion trap in combination with a metal-cluster aggregation source has been used for an electron diffraction study.

Quadrupole ion trap mass spectrometry

Ion isolation and sequential stages of mass spectrometry in a quadrupole ion trap mass spectrometer

The non-linear resonance ion trap. Part 2. A general theoretical analysis

The use of linear quadrupoles in mass spectrometry as mass filters and ion guides is reviewed. Following a tutorial review of the principles of mass filter operation, methods of mass analysis are reviewed. Discussed are extensions of quadrupole mass filters to higher masses, scanning with frequency sweeps of the quadrupole waveform, operation in higher stability regions, and operation with rectangular or other periodic waveforms. Two relatively new methods of mass analysis the use of "islands of stability" and "mass selective axial ejection" are then reviewed. The optimal electrode geometry for a quadrupole mass filter constructed with round rods is discussed. The use of collisional cooling in quadrupole ion guides is discussed along with ion guides that have axial fields. Finally, mass analysis with quadrupoles that have large distortions to the geometry and fields is discussed. An Appendix gives a brief tutorial review of definitions of electrical potentials and fields, as well as the units used in this article.

N.R. Whetten

Papers

Non-linear resonances in quadrupole mass spectrometers due to imperfect fields I. The quadrupole ion trap

Non-linear resonances in quadrupole mass spectrometers due to imperfect fields