Ion-trap system

In order for an instrument to carry out MS-MS, it must be able to do the three operations outlined above. As we have seen however, ion-trap systems capable of MS-MS and MS(n) do not use a tandem arrangement of mass analyzers at all, but rather use a single ion trap for all three operations simultaneously. As has already been stated, these ion-trap tandem mass spectrometer experiments are very sensitive and are now user friendly. The ion trap brings the capability for carrying out MS-MS experiments to the benchtop at relatively low cost. 

All major mass spectral data collections consist of El mass spectra, mostly recorded under accepted standardized conditions such as an ionization voltage of 70 eV, an emission current of 100-200 xA, and an ion source temperature of 150-200°C. Several types of GC/MS systems may be applied, for instance, magnetic sector, quadrupole, or ion trap analyzers. Ion trap systems are considered less applicable, when data comparison is required with spectra from a reference library. In particular, basic compounds related to VX or the three nitrogen mustards tend to produce protonated molecular ions by self-protonation. Magnetic sector and quadrupole mass spectrometers suffer less from interference of self-protonation, and spectra produced with these types of instruments are generally reproducible. 

Structural information about toxaphene components can be obtained by gas chromatography coupled to electron ionization mass spectrometry (GC/EI-MS), positive ion chemical ionization mass spectrometry (GC/PICI-MS) or negative ion chemical ionization MS(GC/NICI-MS). Mass separation is performed by low resolution quadrupole or high resolution magnetic field instruments, by ion trap systems (GC/IT-MS), or by tandem mass spectrometry (MS/MS) offering a broad spectrum of possibihties. 

Data-dependent acqnisition (DDA) is a mode of operation, where the MS experiment performed in a particular scan is based on the data acqnired in a previons scan. In a simple form, a DDA experiment switches the instrument from full-scan MS acquisition to full-scan product-ion MS-MS when the total-ion intensity or a selected-ion intensity exceeds a preset threshold. This avoids the need to perform two consecutive injections for the identification of unknowns in a mixture first to obtain the m/z values for the intact protonated molecules of the unknowns, and second to acquire the product-ion MS-MS spectra of these unknowns in a time-scheduled procedure, switching between various preselected precursor ions as a function of the chromatographic retention time. The DDA was promoted by Thermo Finnigan upon the introduction of the API-ion trap combinations [44-46]. Similar procedures are available for other commercial ion-trap systems, as well as for triple-quadrupoles, e.g.. Information Dependent Acquisition (IDA) from Applied Biosystems MDS Sciex, Data-directed Analysis (DDA) from Waters, and Smart Select from Bruker. 

Figure 5.14 Schematic diagram of the ESI-ion-trap system. Reprinted from [108] with permission, 1991, Ameriean Chemieal Soeiety.

Benijts et al. [19] applied negative-ion ESI on an ion-trap system for the determination of the estrogens estrone, estradiol, estriol, ethinyl estradiol, and diethylstilbestrol in enviromnental water samples. With manual off-line SPE on 50-ml samples, the deteetion hmits ranged from 3.2 to 10.6 ng/1. 

To perform the analysis using an ion trap system allows the operation of normal scan recording, and collision induced dissociation (CID) experiments by selecting [M + H]+ ion of diacetyl-mono derivatives at... 

The TCA can be determined using an ion trap system performing collision-induced dissociation (CID). Quantification is based on the daughter ion signals of the M+ species at m/z 210 and 212 used as precursor ions. Depending on the system used, CID can be performed in either resonant or non-resonant mode. In the former condition, the most intense daughter ions are at m/z 195 and 197, in non-resonant mode the principal signals are at m/z 167 and 169. The CID of a wine spiked with... 

Ajmalicine formed as the major end product of this pathway in the cultured cells diffuses through membranes as the neutral base and is accumulated in the vacuole through an ion-trap system (see later discussion) (Fig. 21). In the vacuole, ajmalicine is further converted into serpentine, which remains trapped in the vacuole as it cannot pass the tonoplast (29,60,271). 

Melchert, H.-U. Pabel, E (2004). Reliable identification and quantification of trichothecenes and other mycotoxins by electron impact and chemical ionization-gas chromatography-mass spectrometry, using an ion-trap system in the multiple mass spectrometry mode. Candidate reference method for complex matrices. Journal of Chromatography A, Vol. 1056, No. 1-2, (November 2004), 195-199, ISSN 0021-9673. 

Ford, M.J. and Van Berkel, G.J. 2004. An improved thin-layer chromatography-mass spectrometry coupling using a surface sampling probe electrospray ion trap system. Rapid Commun. Mass Spectrom., 18 1303-1309. 

A Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR-MS) can be considered as an ion trap system, where the ions are trapped in the magnetic field rather than in a quadrapole electric field. The ICR cell is a cubic or cylindrical cell positioned in a strong magnetic field B (up to 15 T). The cell consists of two opposite trapping plates, two opposite excitation plates, and two opposite receiver plates (Fig. 4.6). Extreme high vacuum should be achieved in the cell, e.g., 10 mbar. 

Types of mass analyzers There are quite a few different types of mass analyzers. Conventional high-resolution MS exploits magnetic-sector instruments. Quadru-pole instruments have become the affordable standard for the analysis of fairly small components (up to about 1000 Da), although ion-trap systems have some specific advantages, such as the possibility of performing high-sensitivity tan-... 

The importance of El spectra for identification and structure confirmation is due to the fragmentation pattern. Searches through libraries of spectra are typically based on El spectra. With the introduction of the Cl capabilities for internal ionization ion trap systems, a commercial Cl library of spectra with more than 300 pesticides was introduced only at that time by Finnigan. 

Ion trap systems already give very high sensitivity in the full scan mode. Samples with high concentrations of matrix and detection limits below the pg level require the SIM technique (MS/MS is recommended). 

In neutral loss scan, all precursor ions, which lose a particular neutral particle (that otherwise cannot be detected in MS), are detected. Both mass analysers scan, but with a constant selected mass difference, which corresponds to the mass of the neutral particle lost. This analysis technique is particularly meaningful if molecules contain the same functional groups (e.g., metabolites as acids, glucuronides or sulfates). In this way, it is possible to identify the starting ions which are characterized by the loss of a common structural element. Both MS/MS scan techniques can be used for substance-class-specific detection in triple quadrupole systems. Ion trap systems allow the mapping of these processes by linking the scans between separate stages of MS in time. 

MS-MS in an ion-trap instrument is fundamentally different from MS-MS in sector and triple-quadru-pole instruments. While in the latter the various stages of the process, i.e. precursor ion selection, CID and product-ion mass analysis are performed in different spatial regions of the instrument, in ion-trap instruments these stages are performed consecutively within the ion trap itself. It is tandem-in-time rather than tandem-in-space mass spectrometry. A simplified diagram of an ion-trap system is shown in Figure ID. 

With the current commercial availability of various ion-trap MS-MS systems, equipped with an external ion source and for use in both GC-MS and LC-MS, MS-MS in ion-trap systems can be expected to find more elaborate analytical applications. 

Two recent instrumental innovations can be applied to further assist in the interpretation of product-ion mass spectra. As discussed above, the MS-MS capabilities of an ion-trap system allow the step-wise fragmentation of an analyte, facilitating the interpretation of the product-ion information and the fragmentation reactions involved. The use of a Q-TOF hybrid instrument allows accurate mass determination (at an accuracy of 5 ppm) of the product ions observed, which also facilitates the interpretation of the product-ion mass spectra. 

A further very effective means for the reduction of interference signals involves the use of ion-trap systems. Here, Eiden et al. [637] showed that quadmpole ion traps employing He were very effective, while Jackson et al. [638] reported on the effectiveness of Ne-filled quadrupole ion traps for the removal of biatomic interferents such as GdO. 

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