Ion-trap mass spectrometers

The ion trap is sometimes considered as a variant of the quadrupole, since the appearance and operation of the two are related. However, the ion trap is potentially much more versatile and clearly has greater potential for development. At one time the ion trap had a bad reputation because the earliest versions gave inferior results compared to quadrupoles. The results were oftentimes concentration dependent relatively large sample sizes usually gave many peaks with the mass of the [ion + 1], which renders the resulting spectra useless in a search with standard El libraries. These problems have been overcome and 

In one sense, an ion trap is aptly named because, unlike the quadrupole, which merely acts as a mass filter, it can trap ions for relatively long periods of time, with important consequences. The simplest use of the trapped ions is to sequentially eject them to a detector, producing a conventional mass spectrum. Before other uses of trapped ions are briefly described, a closer look at the ion trap itself will be helpful. 

There are three basic modes in which the ion trap can be operated. First, when the ion trap is operated with a fixed RF voltage and no DC bias between the endcap and ring electrodes, all ions above a certain cutoff m/z ratio will be trapped. As the RF voltage is raised, the cutoff mlz is increased in a controlled 

The second mode of operation uses a DC potential across the endcaps the general result is that there is now both a low and high-end cutoff (m/z) of ions. The possibilities of experiments in this mode of operation are tremendous, and most operations with the ion trap use this mode. As few as one ion mass can be selected. Selective ion monitoring is an important use of this mode of operation. There is no practical limit on the number of ions masses that can be selected. 

Another way to use this kinetic energy addition mode is to selectively reject unwanted ions from the ion trap. These could be ions derived from solvent or from the matrix in FAB or LSIMS experiments. A constant frequency field at high voltage during the ionization period will selectively reject a single ion. Multiple ions can also be selected in this mode. 

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Figure Bl.7.14. Schematic cross-sectional diagram of a quadnipole ion trap mass spectrometer. The distance between the two endcap electrodes is 2zq, while the radius of the ring electrode is (reproduced with pennission of Professor R March, Trent University, Peterborough, ON, Canada).

Figure Bl.7.16. Mass spectra obtained with a Finnigan GCQ quadnipole ion trap mass spectrometer, (a)...

Brodbelt J, Liou C-C and Donovan T 1991 Selective adduct formation by dimethyl ether chemical ionization is a quadrupole ion trap mass spectrometer and a conventional ion source Ana/. Chem. 63 1205-9... 

Other types of mass spectrometer can use point, array, or both types of ion detection. Ion trap mass spectrometers can detect ions sequentially or simultaneously and in some cases, as with ion cyclotron resonance (ICR), may not use a formal electron multiplier type of ion collector at all the ions can be detected by their different electric field frequencies in flight. 

GC = gas chromatography ECD = electron capture detector EIA = enzyme-immunoassay GPC = gel permeation chromatography HPLC = high-performance liquid chromatography ITMS = ion trap mass spectrometer LSE = liquid solid extraction MS = mass spectrometry RSD = relative standard deviation SPE = solid phase extraction... 

A triple-quadrupole mass spectrometer with an electrospray interface is recommended for achieving the best sensitivity and selectivity in the quantitative determination of sulfonylurea herbicides. Ion trap mass spectrometers may also be used, but reduced sensitivity may be observed, in addition to more severe matrix suppression due to the increased need for sample concentration or to the space charge effect. Also, we have observed that two parent to daughter transitions cannot be obtained for some of the sulfonylurea compounds when ion traps are used in the MS/MS mode. Most electrospray LC/MS and LC/MS/MS analyses of sulfonylureas have been done in the positive ion mode with acidic HPLC mobile phases. The formation of (M - - H)+ ions in solution and in the gas phase under these conditions is favorable, and fragmentation or formation of undesirable adducts can easily be minimized. Owing to the acid-base nature of these molecules, negative ionization can also be used, with the formation of (M - H) ions at mobile phase pH values of approximately 5-7, but the sensitivity is often reduced as compared with the positive ion mode. 

Finnigan LCQ ion-trap mass spectrometer using ESI interface and Navigator 1.2 or XCalibur 1.0 SRI software... 

C. MS operating conditions for quantitative analysis on the LCQ ion-trap mass spectrometer... 

Magnetic sector mass spectrometers accelerate ions to more than 100 times the kinetic energy of ions analysed in quadrupole and ion trap mass spectrometers. The higher accelerating voltage contributes to the fact that ion source contamination is less likely to result in degraded sensitivity. This is particularly important for analysis that requires stable quantitative accuracy. 

The Mass Spectrometer Module houses the vacuum system, capillary interface assembly, and ion-trap mass spectrometer in approximately half of the module. Also included are the reagent gas and calibration gas subassembly (a temperature-controlled housing that ensures consistent gas pressures). The other half contains the electronic printed circuit boards, power supplies, and instrument control computer. 

Moyor, S. C. Marzilli, L. A. Woods, A. S. Laiko, V. V. Doroshenko, V. M. Cotter, R. J. Atmospheric pressure matrix-assisted laser desorption/ionization (AP MALDI) on a quadrupole ion trap mass spectrometer. Int. I. Mass Spectrom. 2003, 226,133-150. 

Figeys, D. Aebersold, R. High sensitivity identification of proteins by electrospray ionization tandem mass spectrometry inital comparison between an ion trap mass spectrometer and a triple quadrupole mass spectrometer. Electrophoresis 1997,18, 360-368. 

Multiply charged proteins can also be partially sequenced, and microsequences of proteins isolated from several microorganisms have been reported, accomplished with electrospray ionization and FTMS.23,90 Nonadjacent fragment ions may be used to identify bacterial proteins in these top-down strategies.91 In all cases these sequences could be related by bioinformatics to the parent species. An obvious extension would be to characterize proteins from intact microorganisms in this way. In at least one instance a microsequence has been obtained from a protein released from a contaminated intact bacteriophage sample (MS2) to provide a chemotaxonomic identification.77 This work was carried out in an ion trap mass spectrometer. 

Ion trap mass spectrometers are widely used and can be easily interfaced with all the ionization techniques. 

One of the most interesting aspects of ion trap mass spectrometers is represented by the possibility of carrying out multi mass separation steps with only one analyzer (see Section 2.9). 

Figure 3.8 Comparative saponin profiles for two cultivars of alfalfa and one cultivar of M. truncatula obtained by reverse-phase HPLC/PDA/MS using electrospray ionization and an ion trap mass spectrometer. The profiles illustrate the increased complexity of saponins in M. truncatula and offer a richer source for bio-prospecting of natural products.

One of the best tools for metabolite profiling is the hybrid QTRAP MS/MS system (Applied Biosystems).119-121 While the hybrid QTRAP MS/MS was initially considered a premier tool for metabolite identification, it has more recently been seen as a tool for quantitation and metabolite profiling. Li et al.122 described the use of a hybrid QTRAP MS/MS system for discovery PK assays plus metabolite profiling in the same analytical procedure. Because QTRAP MS/MS may be used as a triple quadrupole MS system, it can be used as part of a quantitative HPLC/MS/MS system. Because QTRAP MS/MS also has linear ion trap capabilities, it can be used for metabolite screening and characterization—essentially it combines the capabilities of a triple quadrupole mass spectrometer and a linear ion trap mass spectrometer. 

March RE (1998) Quadrupole ion trap mass spectrometer. In Meyers RA (ed) Encyclopedia of analytical chemistry. Wiley, Chichester, pp 1-25... 

Figure 2.16. A cross-section schematic of a quadrupole ion trap mass spectrometer (a), with a three-dimensional perspective view of the quadrupole ion trap (b). Reprinted from A. Westman-Brinkmalm and G. Brinkmalm (2002). In Mass Spectrometry and Hyphenated Techniques in Neuropeptide Research, J. Silberring and R. Ekman (eds.) New York John Wiley Sons, 47-105. With permission of John Wiley Sons, Inc.

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