Tandem TOF and Hybrid Instruments

Since the ESI ion source is compatible with nearly all of mass analyzers in which tandem MS can be performed, a variety of types of mass spectrometers are employed for characterization of lipids. Some examples of the instruments that are used to perform tandem MS include tandem sectors, QqQ, ion-trap, ion-cyclotron resonance, TOF/TOF, and hybrid instruments such as Q-TOF. The majority of tandem MS analyses presented in this part are conducted by using the QqQ-type instrument since majority of the early studies on characterization of lipid species were conducted with this type of instrument, and the characterized fragmentation pattern from this type of instrument can well represent those obtained from other types of instruments even including MALDI-MS [4, 5]. 

TOF analyzers are especially compatible with MALDI ion sources and hence are frequently coupled in aMALDI-TOF configuration. Nevertheless, many commercial mass spectrometers combine ESI with TOF with great success. For proteomics applications, the quadrupole TOF (QqTOF) hybrid instruments with their superior mass accuracy, mass range, and mass resolution are of much greater utility than simple TOF instruments.21,22 Moreover, TOF instruments feature high sensitivity because they can generate full scan data without the necessity for scanning that causes ion loss and decreased sensitivity. Linear mode TOF instruments cannot perform tandem mass spectrometry. This problem is addressed by hybrid instruments that incorporate analyzers with mass selective capability (e.g., QqTOF) in front of a TOF instrument. 

It should be pointed out that FAB, MALDI, and ESI can be used to provide ions for peptide mass maps or for microsequencing and that any kind of ion analyzer can support searches based only on molecular masses. Fragment or sequence ions are provided by instruments that can both select precursor ions and record their fragmentation. Such mass spectrometers include ion traps, Fourier transform ion cyclotron resonance, tandem quadrupole, tandem magnetic sector, several configurations of time-of-flight (TOF) analyzers, and hybrid systems such as quadrupole-TOF and ion trap-TOF analyzers. 

Finally, it is important to note there are many other instruments and configurations that are often referred to as tandem mass spectrometers. There are hybrid instruments that use another form of mass separation, time-of-flight (TOF) mass spectrometry. TOF mass spectrometry separates ions based on the time it takes to... 

The most common types of MS/MS instruments available to researchers in food chemistry include triple quadrupole mass spectrometers and ion traps. Less common but commercially produced tandem mass spectrometers include magnetic sector instruments, Fourier transform ion cyclotron resonance (FTICR) mass spectrometers, and quadrupole time-of-flight (QTOF) hybrid instruments (Table A.3A.1). Beginning in 2001, TOF-TOF tandem mass spectrometers became available from instrument manufacturers. These instruments have the potential to deliver high-resolution tandem mass spectra with high speed and should be compatible with the chip-based chromatography systems now under development. 

The ESI and APCI are both soft ionization techniques, hence the spectra obtained using these methods are usually very simple and consist predominantly of the pseudomolecular ion. When more information is required, tandem mass spectrometry is a popular option. Several different types of tandem LC/MS systems can be found, with triple quadrupole and ion trap being the most popular choices. Hybrid systems including magnetic sector-quadmpole, magnetic sector-TOF, quadrupole-TOF (Q-TOF) and ion trap-TOF have been described. The most successful among these hybrids is the Q-TOF instrument. 

Various hybrid tandem mass spectrometers, which combine two or more distinct types of mass analyzers, have been developed to maximize analytical performance and functionality. From the standpoint of ion/ion reactions, the incorporation of an electrodynamic ion trap into a hybrid instrument allows for the physical separation of the three basic steps involved in an ion/ion reaction experiment, that is, ionization, ion/ion reaction, and mass analysis of reaction products. The separation of these processes provides for the highest degree of flexibility and minimal compromises in the optimization of each step. To date, three major types of hybrid instrnments have been described for ion/ion reaction studies using an electrodynamic ion trap as the reaction vessel. The three major types of hybrid instruments are (i) quadrupole/TOF tandem mass spectrometer (ii) Orbitrap and (iii) LIT /FT-ICR. 

All four scan modes can be implemented with magnetic sector- and quadrupole-based true or hybrid tandem instruments. Time-of-flight (TOF) and tandem-in-time devices are also suitable for product scan experiments, but they are unable to perform the other three scans. 

The MS-MS instrument comprises a combination of two mass analyzers. The first and second stage of mass analysis may be performed by the same type of mass analyzer, like in a triple quadrapole or an ion trap instrument. In TQ instruments, the three steps of the MS-MS process (precursor ion selection, CID, and mass analysis of product ions) are performed in spatially separated devices ( tandem-in-space ), whereas in an ion trap instalment, the three steps are performed one-after-another in the same device ( tandem-in-time ) [35]. In hybrid instruments, the first and second stage of mass analysis is performed in two different types of mass analyzers, e.g., in a first-stage quadrapole and second-stage TOP in a Q-TOF instrument, or a first-stage quadrapole and second-stage hnear ion trap in a Q-LfT instrument. 

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