FT-ICR analyzers

The main feature of this analyzer is its extremely high resolution. However, owing to the difficulty of operation and its very high cost, the FT-ICR analyzer cannot be used for routine purposes. 

Although the above mass spectrometric tools have mass ranges and resolving powers adequate for chemical analysis, mass spectral characterization and structural analysis of biopolymers generally demand efficient detection of ions over a wide mass range, accurate mass measurements, and high mass resolution. The FT-ICR analyzer is able to combine high resolution and MS" capabilities. ... 

The next higher level of performance can be achieved by replacing the oaTOF MS2 with an FT-ICR analyzer while employing a linear ion trap (Thermo Electron LTQ-FT) or a quadmpole as MSI (Bruker Daltonik APEX-Q). 

Figure 18 Use of ion-molecule reactions to change relative ion intensities. 204Pb+ and 2WHg+ were allowed to react with benzene in the Fourier transform ion cyclotron resonance (FT-ICR) analyzer cell for progressively longer periods.

The FT-ICR analyzer is the most complex and difficult to operate, but has by far the highest resolution, mass accuracy, and sensitivity. The operating principle is that ions in a magnetic field will orbit at a frequency that is related to the ion s mass m), charge z), and the strength of the magnetic field (B). This is called the cyclotron frequency fc). The relationship can be described by the following equation ... 

Note AGC is not only relevant for QITs but also for all otha- types of ion traps. AGC is well implemented in LITs that are not only employed as standalone devices but also in conjunction with FT-ICR analyzers to serve for ion selection and dosing into the ICR cell (Chap. 4.9.1). 

As ion trapping devices, FT-ICR instruments belong to the tandem-in-time category of instruments. The stage of precursor ion selection (MSI) is accomplished by selectively storing the ions of interest, whereas all others are ejected by means of a suitably tailored excitation pulse. For this purpose, the SWIFT technique [124] or correlated sweep excitation (CHEF) [125,126] are used (Chap. 4.7.7). Both methods generate tailored waveforms that cause excitation of all but the selected ions. Like QITs and LITs, FT-ICR analyzers are also capable of MS". 

Currently, at least four types of mass analyzer are used in mass spectrometry, namely ToF, magnetic/electric sector, quadrupole ion trap (see Figure 18.6), and FT-ICR. The ToF and magnetic/electric sector mass analyzers are most commonly used in mass spectrometry imaging, while FT-ICR analyzers have been applied only rarely to mass spectrometry imaging [117]. 

In the following years, the latter category of m/z analyzers has been combined with both ionization techniques [14], as have been FT-ICR MS (Fig. If), TOF MS in orthogonal configuration, and a series of hybrid instruments that combine a quadrupole MS or electrostatic ion trap with an orthogonal TOF [15-18] (Fig. Id) or FT-ICR analyzer [19-23]. The most recent innovation has been a new electrostatic ion trap design, called or bitrap [24,27], which in combination with an upfront linear ion trap [25,26] provides excellent MS/MS and MS/MS/MS performance, impressive resolving power (> 50 000 FWHM), and a mass accuracy of 0.5-2 ppm for peptides that comes close to what, so far, has only been possible with very expensive FT-ICR MS instrumentation [20,27-29]. This means that for a 1-kDa peptide the expected experimental error is only 0.0005-0.002 Da or 1-4 times the mass of an electron. 

For the mass spectrometric analysis of peptides, the two ionization techniques are similarly complementary as they are for proteins [38]. Today, most mass spectrometers equipped with an ESI or a MALDI source provide the possibility of isolating analyte ions on the basis of their m/z ratio and, by different activation methods, transfer energy to them which results in their decomposition. Mass analysis of the resulting fragment ions can provide detailed structural information and is termed tandem-MS or MS/MS analysis (Fig. 2). Electrostatic ion traps and FT-ICR analyzers also provide multiple stages of ion isolation and fragmentation experiments (MS ), of which MS is especially useful for protein identification and modification analyses. 

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