Linear Quadrupole Ion Traps

Principle. The cylindrical quadrupole ion trap is based on the same principle as the quadrupole mass filter, but the geometry is different (Fig. 2.16). The cylindrical QIT, or Paul trap, was developed almost simultaneously with the quadrupole mass filter [232, 233]. Recently, a variant of the theme has emerged, the linear quadrupole ion trap [236], which is a device built like a quadrupole mass filter with extra trapping end electrodes for the axial direction. Under stable conditions, ions moving around inside such traps will ideally continue to do that forever. 

Figure 2.17. Schematic of a linear quadrupole ion trap (QIT). This type of analyzer consists in principle of a quadrupole analyzer with electrodes at the ends to block ion passage in the z-direction.

Macek, B., Waanders, L. F., Olsen, J. V., and Mann, M. (2006). Top-down protein sequencing and MS3 on a hybrid linear quadrupole ion trap-orbitrap mass spectrometer. Mol. Cell. Proteomics 5 949-958. 

Scherperel, G., Yan, H.G., Wang, Y. and Reid, G.E. (2006) Top-down characterization of site-directed mutagenesis products of Staphylococcus aureus dihydroneopterin aldolase by multistage tandem mass spectrometry in a linear quadrupole ion trap. Analyst, 131, 291-302. 

J. E. P. Syka, J. A. Marto, D. L. Bai, S. Homing, and M. W. Senko, Novel linear quadrupole ion trap/FT mass spectrometer Performance characterization and use in the comparative analysis of histone H3 post-translational modifications. J. Proteome Res. 3 (2004), 621-626. 

The sample inlet is constituted of a heated fused silica capillary, which is maintained at approximately 200 "C and is encased in a flexible tube. The ion source, in the case of electronic ionization, is composed of electrically heated metallic filaments. Mass analyzers, separating the analytes, include time-of-flight (TOF), linear quadmpole (Q), linear quadrupole ion trap (LIT), quadmpole ion trap (QIT), Fourier transform ion cyclotron resonance (FT-ICR), etc. These detectors differ in their capacity to treat ion beams in a continuous or pulsed (TOF). Quadmpole mass analyzers stabilize and destabilize the ion paths with an oscillating electrical field. A triple quad is more recent technology and consists of three quadmpole stages. Quadmpole ion traps will sequentially eject ions that have been trapped in a ring electrode between two endcap electrodes. 

Mao, D. Ding, C. Douglas, D.J. Hydrogen/deuterium exchange of myoglobin ions in a linear quadrupole ion trap. Rapid Commun. Mass Spectrum. 2002,16, 1941-1945. 

FIGURE 4.5 (a) Linear quadrupole ion trap Pulsed Q CoUision-Induced Dissociation... 

An alternative approach was introduced by Hunt and co-workers [87], Those researchers coupled a linear quadrupole ion trap, consisting of four rods of hyberbolic cross-section, with an FT-ICR mass spectrometer. The linear ion trap allows accumulation of larger populations of ions than does a standard three-dimensional (3D) ion trap. The hybrid linear ion trap-FT-ICR instrument enables simultaneous detection in both mass analyzers. This aspect is particularly advantageous for data-dependent MS/ MS methods used in proteomics, and is discussed further below. The commercial version of this instrument features automated gain control that accumulates a fixed number of charges before delivery to the ICR cell. Because the ideal ion density is attained in the cell, space-charge effects resulting in loss of mass resolution and mass accuracy, are eliminated. 

Figure 6.29 A linear quadrupole ion trap for use as a mass spectrometer using radial ion ejection. Top schematic of the trap showing the ejection slot along the length of one of the x-rods. Bottom an overall view of the complete instrument showing typical potentials and pressures. The first (square) quadrupole is an ion guide to transport ions from the ESI source into the higher vacuum region and the function of the small octapole is similar but to facilitate ion transfer into the trap. Reprinted by permission of Elsevier from A Two - Dimensional Quodrupole Ion Trap. .. , by Schwartz, et al. Journal of the American Society for Mass Spectrometry, 13, p. 659-669, 2002, Eig 1, p. 660 Eig 5, p. 662, by the American Society for Mass Spectrometry.

D quadrupole ion traps and 2D linear quadrupole ion traps are closely related in operating principles to quadrupoles and as such share many of the same performance features, such as unit mass resolution and nearground potentials that allow facile coupling to both GC... 

Jarrell, T, Riedeman, J., Carlsen, M., Replogle, R., Selby, T, Kenttamaa, H. (2014) Multiported Pulsed Valve Interface for a Linear Quadrupole Ion Trap Mass Spectrometer to Enable Rapid Screening of Multiple Functional-group Selective Ion-Molecule Reactions. Anal. Chem. 86 6533-6539. 

Fig. 2.4 Triple quadrupole MS, QQQ, where Q1 and Q3 indicate scanning quadrupoles and Q2 indicates the collision cell consisting of either an RF-only quadrupole, hexapole, or octapole. In some hybrid instruments Q3 is replaced by either a TOF or a linear quadrupole ion trap. (See text for more details and references for further reading)...

Londry FA, Hager JW. Mass selective axial ion ejection from a linear quadrupole ion trap. J Am Soc Mass Spectrom. 2003 14(10) 1130 7. 

Fig. 2.11. Quadruple Ion traps provide a versatile approach for storage, fragmentation, and selection of ions, (a) Schematic of 3D quadmpole ion trap consisting of a ring electrode and two end-cap electrodes. Only one ion ejection pathway is shown. A combination of DC and RF potentials is applied to the electrodes. 1 mtorr of helium is typically added to the mass analyzer, (b) Schematic of a linear quadrupole ion trap. During ion trapping, DC and RF Ion-guiding potentials are maintained on the central electrodes while the end section electrodes maintain stop potential, SP.

A three-dimensional ion trap (e.g., Q ion trap) bears the same physical principles as the quadrupole mass analyzer, but the ions are trapped and sequentially ejected mainly by using an RF field, within a space defined by a ring electrode between two end-cap electrodes (Figure 2.6b). A linear quadrupole ion trap is similar to a three-dimensional ion trap, but it traps ions in a two-dimensional, instead of a three-dimensional, quadrupole field. Some of the figures of merit of the ion-trap instrument are as follows ... 

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