Ion trap-FT-ICR

A third type of MS/MS instruments is a hybrid of tandem-in-space and tandem-in-time devices, including the Q-trap (QQ-2D-linear trap) [45] and the ion trap-FT-ICR (2D-linear ion trap-FT-ICR) [46]. The Q-trap takes the configuration of triple quadrupole, with the third quadrupole replaced by a 2D-linear ion trap. The uniqueness of this design is that the 2D-linear ion trap component can be used to perform either (a) a normal quadrupole scan function in the RF/DC mode or (b) a trap scan function by applying the RF potential to the quadrupole. It is well-suited for both qualitative and quantitative studies. In the case of ion Trap-FT-ICR, it combines ion accumulation and MS" features of a 2D-linear ion trap with excellent mass analysis capability (mass resolution, mass accuracy, and sensitivity) of FT-ICR. 

Since 2000, the field has moved increasingly toward hybrid FT-ICR instruments in which the FT-ICR is interfaced with a front-end mass analyzer. The groups of Marshall [46,47] and Smith [48,49] introduced the quadrupole-FT-ICR. That configuration is available commercially. The hybrid linear ion trap FT-ICR [87] was introduced commercially in 2003. Hybrid instruments offer greater versatility in terms of mass-selective external accumulation with the associated increase in sensitivity and dynamic range. 

An alternative approach was introduced by Hunt and co-workers [87], Those researchers coupled a linear quadrupole ion trap, consisting of four rods of hyberbolic cross-section, with an FT-ICR mass spectrometer. The linear ion trap allows accumulation of larger populations of ions than does a standard three-dimensional (3D) ion trap. The hybrid linear ion trap-FT-ICR instrument enables simultaneous detection in both mass analyzers. This aspect is particularly advantageous for data-dependent MS/ MS methods used in proteomics, and is discussed further below. The commercial version of this instrument features automated gain control that accumulates a fixed number of charges before delivery to the ICR cell. Because the ideal ion density is attained in the cell, space-charge effects resulting in loss of mass resolution and mass accuracy, are eliminated. 

Quadrupole and magnetic-sector instruments are the conunon type of mass analyzers for the detection of the GD-formed ions. The use of quadrupole ion traps, FT-ICR, and TOF instruments has also been explored [11-13], Specifically, quadrupole ion traps and FT-ICR mass spectrometers are of particular interest because they can accumulate and store ions for a desired length of time for subsequent CID or ion-molecule reactions that will eliminate isobaric interferences. The fast-scan facility of a TOF mass analyzer has the advantage that it can be used to monitor even short-lived transient signals. 

Lioe, H. O Hair, R. Comparison of collision-induced dissociation and electron-induced dissociation of singly protonated aromatic amino acids, cystine and related simple peptides using a hybrid linear ion trap-FT-ICR mass spectrometer. Analytical and Bioanalytical Chemistry 2007, 389, 1429-1437. 

ICR instrument (Thermo Scientific LTQ-FT series). In the LTQ-FT instruments, the LIT shields the ultrahigh vacuum of the FT-ICR from collision gas and decomposition products in order to operate under optimum conditions. More importantly from the analytical point of view, the LIT can accumulate, mass-select and fragment selected ions prior to the next FT-ICR cycle while the ICR cell is still busy with the previous ion package (FT-ICR cf. Chap. 4.7). By using an orbi-trap analyzer (Chap. 4.8) in place of the FT-ICR cell, the same company offers the Thermo Scientific LTQ-Orbitrap series as an economical nonetheless powerful alternative to their LTQ-FT. 

Stored waveform inverse Fourier transformation, technique to create excitation waveforms for ions in FT-ICR mass spectrometer or Paul ion traps. An excitation waveform in the time-domain is generated by taking the inverse Fourier transform of an appropriate frequency domain programmed excitation spectrum, in which the resonance frequencies of ions to be excited are included. This procedure may be used for selection of precursor ions in MS/MS experiments. 

In the other types of mass spectrometer discussed in this chapter, ions are detected by having them hit a detector such as an electron multiplier. In early ICR instruments, the same approach was taken, but FT-ICR uses a very different teclmique. If an RF potential is applied to the excitation plates of the trapping cell (figure B 1.7.18(b)) equal to the cyclotron frequency of a particular ion m/z ratio, resonant excitation of the ion trajectories takes place (without changing the cyclotron frequency). The result is ion trajectories of higher... 

In many respects, the applications of FT-ICR are similar to those of the quadmpole ion trap, as they are both trapping instmments. The major difference is in the ion motion inside the trapping cell and the wavefomi detection. In recent... 

As with the quadmpole ion trap, ions with a particular m/z ratio can be selected and stored in tlie FT-ICR cell by the resonant ejection of all other ions. Once isolated, the ions can be stored for variable periods of time (even hours) and allowed to react with neutral reagents that are introduced into the trapping cell. In this maimer, the products of bi-molecular reactions can be monitored and, if done as a fiinction of trapping time, it is possible to derive rate constants for the reactions [47]. Collision-induced dissociation can also be perfomied in the FT-ICR cell by tlie isolation and subsequent excitation of the cyclotron frequency of the ions. The extra translational kinetic energy of the ion packet results in energetic collisions between the ions and background... 

Instruments are available that can perform MS/MS type experiments using a single analyzer. These instruments trap and manipulate ions in a trapping cell, which also serves as the mass analyzer. The ion trap and fourier transform ion cyclotron resonance (FT-ICR) mass spectrometers are examples. 

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. 

Various analyzers have been used to analyze phenolic compounds. The choice of the MS analyzer is influenced by the main objective of the study. The triple quadrupole (QqQ) has been used to quantify, applying multiple reaction monitoring experiments, whereas the ion trap has been used for both identification and structure elucidation of phenolic compounds. Moreover, time-of-flight (TOF) and Fourier-transform ion cyclotron resonance (FT-ICR) are mainly recommended for studies focused on obtaining accurate mass measurements with errors below 5 ppm and sub-ppm errors, respectively (Werner and others 2008). Nowadays, hybrid equipment also exists, including different ionization sources with different analyzers, for instance electrospray or atmospheric pressure chemical ionization with triple quadrupole and time-of-flight (Waridel and others 2001). 

The FT-ICR/MS is an ideal instrument for studying ion-molecule reactions over an extended time scale due to the excellent trapping of ions in the cell and the unmatched mass resolution and mass accuracy. Mass resolution is defined as the mass divided by the peak width at half height... 

The ion trapping capability of the FT-ICR is much greater than the other types of mass spectrometer and ions may be trapped for several minutes under ideal conditions. 

FT-ICR detection is accomplished by monitoring the image current induced by the orbiting ion packet as it cycles between the two receiver plates of the ceU. After formation by an ionization event, all trapped ions of a given mIz have the same cyclotron frequency but have random positions in the FT-ICR cell. The net motion of the ions under these conditions does not generate a signal on the receiver plates of the FT-ICR cell because of the random locations of ions. To detect cyclotron motion, an excitation pulse must be applied to the FT-ICR cell so that the ions bunch... 

As a result of this excitation step, the net coherent ion motion produces a time-dependent signal on the receiver plates, termed the image current , which represents aU ions in the FT-ICR cell. The image current is converted to a voltage, ampMed, digitized, and Fourier transformed to yield a frequency spectrum that contains complete information about frequencies and abundances of all ions trapped in the cell. A mass spectrum can then be determined by converting frequency into mass because frequency can be measured precisely, the mass of an ion can be determined to one part in 10 or better. 

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