Quadmpole Ion Trap

In the quadmpole ion trap (QIT), ions are trapped and stored in a potential well. A mass spectrum is acquired by ejecting the ions from the potential well in order of ascending m/z and detecting them. The trap can also be used to selectively store ions with a 

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. 

The helium gas in the trap not only helps in trapping the ions but also cools them (i.e., the kinetic energy of a trapped ion is dissipated through repeated collisions with the He gas), thus forcing the ions to the center of the trap where the quadrupole field is best defined. Both sensitivity and mass resolution are significantly enhanced by the presence of the He gas. Moreover, the same He can also be used to induce fragmentation when working in the MS mode (see below). 

In most commercial cylindrical ion trap instalments the end-cap electrodes are held at ground potential and usually only a RF potential is applied to the ring electrode. When the RF amplitude is set to a low, so-called storage voltage, all ions above a certain m/z are trapped. This voltage is usually chosen so the lowest trapped m/z is greater than those of water, air, and solvent ions (i.e., above 100 to 150 Th), depending on the nature of the measured species. 

One method of acquiring a mass spectrum is the mass selective instability scan. As the RF voltage increases, the ions with lowest m/z become unstable and are ejected through small holes in the end cap to hit a detector. As the RF voltage is further increased, heavier ions become successively unstable and are ejected, thus yielding a mass spectrum. 

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In many respects, the applications of FT-ICR are similar to those of the quadmpole ion trap, as they are both trapping instmments. The major difference is in the ion motion inside the trapping cell and the wavefomi detection. In recent... 

As with the quadmpole ion trap, ions with a particular m/z ratio can be selected and stored in tlie FT-ICR cell by the resonant ejection of all other ions. Once isolated, the ions can be stored for variable periods of time (even hours) and allowed to react with neutral reagents that are introduced into the trapping cell. In this maimer, the products of bi-molecular reactions can be monitored and, if done as a fiinction of trapping time, it is possible to derive rate constants for the reactions [47]. Collision-induced dissociation can also be perfomied in the FT-ICR cell by tlie isolation and subsequent excitation of the cyclotron frequency of the ions. The extra translational kinetic energy of the ion packet results in energetic collisions between the ions and background... 

Electrospray ionization mass spectrometry (ESI-MS) is an analytical method for mass determination of ionized molecules. It is a commonly used method for soft ionization of peptides and proteins in quadmpole, ion-trap, or time-of-flight mass spectrometers. The ionization is performed by application of a high voltage to a stream of liquid emitted from a capillaty. The highly charged droplets are shrunk and the resulting peptide or protein ions are sampled and separated by the mass spectrometer. 

Quadmpole ion traps were originally coupled to continuous ion sources but also work well with pulsed ion sources. The cylindrical QIT is a compact device with a diameter and length of 5 cm and the linear QIT is the size of a quadmpole mass filter, that is, 20 cm long. As stand-alone instruments they are nowadays of benchtop size and, together with quadmpole mass filters, are considered to be standard low-cost devices, which are commonly coupled to LC systems. 

S. C. Moyer, L. A. Marzilh, A. S. Woods, V. V. Laiko, V. M. Doroshenko, and R. J. Cotter. Atmospheric Pressure Matrix-Assisted Laser Desorption/Ionization (AP MALDI) on a Quadmpole Ion Trap Mass Spectrometer. Int. J. Mass Spectrom., 226(2003) 133-150. 

Low Mass Region. All spectra shown in the examples were acquired using the quadmpole ion trap mass spectrometer. As noted previously, this widely used and relatively cheap mass analyzer suffers the low-mass cut-off phenomena. In addition to techniques used in the examples shown above, other mass analyzers applied for tandem mass spectrometers may cover the low mass region of the fragmentation spectmm that can be information rich. 

Linear quadmpole ion trap LIT Continuous ion beam and trapped ions storage and eventually separation in linear radio frequency quad-rapole field due to stability of trajectories... 

The quadrupole ion trap (QIT) creates a three-dimensional RF quadrupole field to store ions within defined boundaries. Its invention goes back to 1953, [103-105] however, it took until the mid-1980s to access the full analytical potential of quad-mpole ion traps. [137-140] The first commercial quadmpole ion traps were incorporated in GC-MS benchtop instruments (Finnigan MAT ITD and ITMS). Electron ionization was effected inside the trap by admitting the GC effluent and a beam of electrons directly into the storage volume of the trap. Later, external ion sources became available, and soon a large number of ionization methods could be... 

Note Paul himself preferred to call the device lonenkdfig (ion cage) rather than the nowadays accepted term quadmpole ion trap because it does not actively act to catch ions from outside. The acronym QUISTOR derived from quadrupole ion store was also widespread in use. 

Fig. 4.40. Schematic of a quadmpole ion trap, (a) QIT with external ion source (illustration stretched in z-direction) and (b) section in the rz-plane (in scale), (a) Reproduced from Ref. [144] by permission. lohn Whey Sons, 2000.

Fig. 4.41. Electrodes of the Finnigan MAT ITS40 quadmpole ion trap. By courtesy of Thermo Electron (Bremen) GmbH.

Example Tandem mass spectrometric experiments in quadmpole ion traps are performed by combining the techniques of resonant ejection, and forward and reverse scanning to achieve an optimum in precursor ion selection, ion activation, and fragment ion scanning (Fig. 4.45). [156]... 

Quadmpole ion traps are very useful instruments that can be interfaced between various ionization sources and the mass detector. These instruments store ions of a selected mass-to-charge (m/z) ratio within their... 

Quadmpole ion trap refers in general to a 3D ion trap instrument... 

Syka, J. E. P. Marto, J. A. Bai, D. L. Homing, S. Senko, M. W Schwartz, J. C. Ueberheide, B. Garcia, B. Busby, S. Muratore, T Shabanowitz, J. Hunt, D. F. Novel linear quadmpole ion trap/FT mass spectrometer performance characterization and use in the comparative analysis of histone H3 post-translational modifications. 

March, R. E. (1997). An introduction to quadmpole ion trap mass spectrometery. J. Mass Spectrom. 32 351-369. 

Wieboldt, R., Campbell, D. A., and Henion, J. (1998). Quantitative liquid chromatographic-tandem mass spectrometric determination of orlistat in plasma with a quadmpole ion trap. J. Chromatogr. B Biomed. Sci. Appl. 708 121-129. 

Modes of MS ionization are El, Cl, FAB, PDI, MALDI and ESI and major MS analysers include time-of-llight (TOE) quadmpole ion-trap (QIT) Fourier transform ion cyclotron resonance (FTICR) and orbitrap. 

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