Quadrupole ion trap mass spectrometry

Abstract

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.