Ion trap instruments

This focusing action gives an ion beam, in which the m/z values can be measured so accurately that the resolution of a magnetic/electric-sector instrument (separation of ions of different m/z values) is measured as a few parts per million, compared to the more modest few parts per thousand in, say, a quadmpole or ion-trap instrument. 

The result of the Back-to-Basics series is an accumulation of some 50 separate but interrelated expositions of mass spectrometric principles and apparatus. Some areas of mass spectrometry, such as ion cyclotron resonance and ion trap instruments, have not been covered except for passing references. This decision has not been due to any bias by the authors or Micromass but simply reflects the large amount of writing that had to be done and the needs of the greatest proportion of users. 

Tandem quadrupole and magnetic-sector mass spectrometers as well as FT-ICR and ion trap instruments have been employed in MS/MS experiments involving precursor/product/neutral relationships. Fragmentation can be the result of a metastable decomposition or collision-induced dissociation (CID). The purpose of this type of instrumentation is to identify, qualitatively or quantitatively, specific compounds contained in complex mixtures. This method provides high sensitivity and high specificity. The instrumentation commonly applied in GC/MS is discussed under the MS/MS Instrumentation heading, which appears earlier in this chapter. 

More recently, certain MS-MS scans have been made available on the ion-trap instrument. This type of system differs from those described previously in that the MS-MS capability is associated only with the way in which the ion-trap is operated, i.e. it is software controlled, and does not require the addition of a collision cell and a further analyser. This is because ion selection, decomposition and the subsequent analysis of the product ions are all carried out in the same part of the instrument, with these processes being separated solely in time, rather than time and space as is the case for the instruments described previously. 

As with GC/MS, LC/MS offers the possibility of unequivocal confirmation of analyte identity and accurate quantiation. Similarly, both quadrupole and ion-trap instruments are commercially available. However, the responses of different analytes are extremely dependent on the type of interface used to remove the mobile phase and to introduce the target analytes into the mass spectrometer. For pesticide residue analyses, the most popular interfaces are electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). Both negative and positive ionization can be used as applicable to produce characteristically abundant ions. 

Ion trap MS is particularly suited for chemical structure elucidation, as it allows for simultaneous ion storage, ion activation and fragmentation, and product ion analysis. The fragmentation pathway of selected ions and the fragmentation products provide information on the molecular structure. Compared with triple-quadrupole and especially with sector instruments, the ion trap instrument provides more efficient conversion of precursor ion into product ions. However, the CID process via resonance excitation, although quite efficient in terms of conversion yield, generally results in only one (major) product ion in the product-ion mass spectrum. MS/MS with a quadrupole ion trap offers a number of advantages ... 

These three polymeric HALS stabilisers can be detected and positively identified in extracts from polyolefins using an Agilent Ion-trap instrument with positive APCI. Figure 34 shows a chromatogram for a 5-ppm standard of Tinuvin 622 in tetrahydrofuran (THF) and the peak mass spectrum (Figure 35). Similar data for Chimassorb 944 in THF are shown in Figures 36 and 37, respectively. A Waters Xterra C8 150 x 2.00 mm 3 pm 125A column at 60°C with the mobile phase of isopropanol +700 pl/1 hexylamine was employed. 

MARCH, R.E., TODD, J.F., Practical Aspects of Ion Trap Mass Spectrometry. Vol. II, Ion Trap Instrumentation, CRC Press, Boca Raton, 1995, 320 p. 

The mass-selective instability mode of operation permits the selection and trapping of all ions created over a specified period with subsequent ejection to the detector.26 Ions with different m/z values can be confined within the ion trap and scanned singly by application of voltages that destabilize the orbits of the ions and eject them to the detector. Ion trap instruments interface readily with liquid chromatography, ESI,15 and MALDI.27 The motions of the ions and the dampening gas (e.g., helium) concentrate around the middle of the ion trap, thereby diminishing ion loss through collisions with electrodes. 

Figure 6.3. Real-life example of a tandem MS experiment in an electrospray ion trap instrument. Top panel a complex peptide mixture. Middle panel ion at 1318.9 m/z was isolated from other sample components. Note the lack of any other peaks and a very low background. Bottom panel fragmentation spectrum of the selected parent ion (1318.9 m/z), note the different scale of the m/z axis. All peaks seen in this mass spectrum are product ions that were formed due to the controlled fragmentation of the parent ion. The main peak at 1300.8 m/z corresponds to the loss of water molecule, a lower intensity parent ion at 1318.9 m/z is also seen.

The sensitivity, compactness, automation, and low prices of ion trap instruments made them very popular in biological MS P Limitations of ion traps include low resolution and mass accuracy at high m/z. In addition, in MS/MS mode, the lower end of the fragment mass range cannot be visualized. Recent developments in the linear geometry of ion traps are aimed at improving on those limitations. 

Example The ESI mass spectrum and the charge-deconvoluted molecular weights (inset) of bovine serum albumine (BSA) as obtained from a quadrupole ion trap instrument are compared below (Fig. 11.19). Ion series A belongs to the noncovalent BSA dimer, series B results from the monomer. [24]... 

Due to their comparatively low costs and easy operation, quadtupole instruments are the most common instruments used for hyphenation in CEC analyses. However, these instruments only operate at low mass resolution. Sensitivity can be enhanced by operating in selected ion monitoring mode instead of full scanning acquisitions. Unfortunately, this leads to the loss of structural information. The expansion of biological applications has been largely accommodated by the TOE, quadtupole mass filter, and ion-trap instruments. The major advantage of TOE is its potential for speed, resolution, and good mass accuracy. 

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

Triple quadmpole linear ion trap instrument. In this instrument the quadmpole Q3 is operated either in RE/DC mode or in RE mode... 

The analytical performance of four modes of LC-MS, multiple MS (MS"), and tandem MS operation (APCI and ESI with positive and negative ionizations) has been compared for two mass spectrometers, a triple-quadrupole and an ion-trap instrument.With 15 flavonoids as test compounds, the use of APCI in the negative-ion mode gave the best response, with the signal intensities and the mass-spectral characteristics not differing significantly between the two instruments. Under optimum conditions, full-scan limits of detection of... 

De Rijke, E. et al.. Liquid chromatography with atmospheric pressure chemical ionization and electrospray ionization mass spectrometry of flavonoids with triple-quadrupole and ion-trap instruments, J. Chromatogr. A, 984, 45, 2003. 

MS" A series of n steps in which m/z selection is performed. MS" can be conducted by linking a series of mass analyzers, each of which performs one selection step, or more commonly by using ion-trapping instruments such as QITs (2D or 3D) or FTICR. 

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