Radial ejection

Radial ejection of ions to detectors Axial ejection of ions to the detector... 

Ions trapped within an LIT can be mass selectively ejected either along the axis of the trap (axial ejection) or perpendicular to its axis (radial ejection). Therefore, in commercial... 

Radial ejection between the rods has been described [19] but never applied to commercial instruments. Ejection through slots cut in two opposite rods was described first by Senko and Schwartz from Thermo Finnigan in 2002 [23,24],... 

Figure 2.37 shows the scheme of the first commercially available orbitrap instrument. It is somewhat different from the one previously described. First, there is an linear ion trap (LIT) that can be used for ion storage and ejection to the orbitrap by axial ejection, or independently used as a linear trap by radial ejection. It is possible to inject all the ions from the LIT, or selected ones, or product ions from the MS" operations of the LIT. The normal acquisition cycle time in the orbitrap is 1 second. 

The first obstacle for FT-ICR-based CID, that the ions must be excited transla-tionally, may be surmounted by resonant excitation of the ions (as in pre-detection excitation, or radial ejection, described above). However, the extent of excitation is limited by the magnetic field and the size of the trap. Excess excitation wonld result in ejection of the ions from the ICR cell. A second problem is that the products are formed off-axis because the precursor ions are increasing their cyclotron radius this effect results in reduced resolving power and prevents any further fragmentation (MS ). 

Selective use of the rf and dc voltages on the rods can be used to axially eject ions in miz sequence to obtain a spectrum, to pass ions onto another analyzer, or to isolate a specific ion by radially ejecting all other ions with different mIz values. 

Figure 6.29 shows the hnear quadrupole mass spectrometer designed (Schwartz 2002) to exploit radial ejection by appropriate resonance activation techniques, which are entirely analogous to those employed in Paul traps. A conventional linear quadrupole rod set (tg = 4mm) was cut into three sections 12, 37 and 12mm long. The central section has a 30 x 0.25 mm ejection slot cut in... 

Mass-Analyzing Linear Ion Trap with Radial Ejection... 

A scanning LIT [109] can alternatively be realized using a mass-selective radial ejection noode of operation. Such a LIT can either serve as a stand-alone mass analyzer (Thermo Sdentific LTQ series) or can be combined into a hybrid LIT-FT-... 

Fig. 4.39. A segmented linear quadmpole ion trap with mass-selective radial ejection capability. Applying higher potential to the front and back sections creates an axial trapping potential for ions in the center section. Radial trapping is again provided by the RF quadmpolar field. Ions are exiting through the slot in one of the four rods. The commercial design offers doubled sensitivity by using two detectors at opposite rods. Adapted from Ref. [109] by permission. Elsevier Science, 2002.

Fig. 4.41. Radial ejection LIT mass spectrometer with ESI source. Typical operating voltages and pressures are given. Reprinted with permission from Ref. [109]. Elsevier Science Publishers, 2002.

Mass spectrometers incorporating a linear quadrupole ion trap are highly flexible instruments. As discussed in Chap. 4.5, there are two competing concepts available on the market, one employing a LU with axial ejection as part of a hybrid instrument [115], and another, featuring a pure LIT design with radial ejection [116,117],... 

Thus, ETD lends the capabilities of ECD to linear ion trap mass spectrometers. The individual steps involved in the operation of an LTQ instrument in ETD mode (Fig. 9.39) are injection of multiply protonated peptides as delivered by an ESI source application of a DC offset to store these ions in the front section of the LIT followed by injection of reagent anions from the Cl source into the center of the LIT. Then all but the peptide precursor ions and the electron-donor reagent ions are ejecteeL Next the DC potential well is switched off and a secondary RF voltage is applied to the end lens plates of the LIT causing positive and negative ion populations to mix and react. The reaction period is ended by axial ejection of reagent anions while positive product ions are retained in the center section of the LIT. Finally, mass-selective radial ejection as usual yields the ETD spectrum [160]. The attractive ETD technique has also been implemented on LITs with axial ejection [144,166] and on LIT-orbitrap hybrids [167-169]. 

As discussed earlier, in the conventional way of quadrupole operation the ions are not trapped in between the rods but fly alongside them. However, it is also possible to trap ions in between the quadrupole rods for a certain amount of time and detect them by radial ejection (Fig. 23) [44], The relatively large volume of ion... 

Major ion trap developments have occurred over the last 15 years. In 1995, Bier and Syka patented the use of the mass selective instability scan from high-charge-capacity ion trap geometries such as the linear quadmpole ion trap (LQIT), toroidal trap (TQIT), curved or banana traps (CQIT), and elliptical traps (EQIT). The LQIT with radial ejection was eventually commercialized in 2002, and it is used as a stand-alone mass analyzer and has been combined with QMFs, Fourier transform ion cyclotron resonance (FT ICR), and the orbitrap to form hybrid instmments. The LQIT analyzer has eclipsed the conventional ... 

Multidetector capability (radial ejection). Those familiar with the classical QIT operation have realized that half of the ions are lost during ejection because only one end cap is followed by a detector. (Note that some manufacturers have solved this problem.) The LQIT, however, ejects ions radially and thus allows for the use of one or more radial ion detectors. 

Dual ion source capability (radial ejection). The LQIT can be operated so that ions may be trapped efficiently from two ion sources by transferring ions into both ends of the device. This allows for convenient ion/ion (—/ +) reaction experiments. 

Mass selection of ions can be achieved within the analyzer cell by application of an excitation waveform that causes radial ejection of all unwanted ions, except the ions of... 

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