Axial ejection

In a linear ion trap one of the most efficient ways to perform mass analysis is to eject ions radially. Hager [60] demonstrated that, by using fringe field effects, ions can also be mass-selectively ejected in the axial direction. There are several benefits for axial ejection (i) it does not require open slits in the quadrupole, (ii) the device can be operated either as a regular quadrupole or a LIT using one detector. A commercial hybrid mass spectrometer was developed based on a triple quadrupole platform where Q3 can be operated either in normal RF/DC mode or in the LIT ion trap mode (Fig. 1.24). 

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... 

The linear trap with axial ejection was invented by Hager, from MDS Sciex, in 2002 [20], Figure 2.31 displays a scheme of such an ion trap included in the ion path of a triple quadrupole mass spectrometer. 

This instrument can be operated as a normal triple quadrupole with all its scan modes or as a trap in various combinations with the use of the other quadrupoles. If a slow scan rate is used to expel the ions a resolution up to 6000 FWHM can be reached by scanning at 5 Th s 1 using q2 and at 100 Th s 1 using Q3, which is at a lower pressure. As fringe field effects are used, only ions close to the exit lens are expelled. In consequence, mass selective ejection in the axial direction based on this technique is characterized by low ejection efficiency. For instance, an ejection efficiency of less than 20 % is achieved at 1 Th ms-1 scan rate. Different techniques have been proposed to improve the axial ejection efficiency [21], but the most promising technique for mass selective axial ejection is the technique named axial resonant excitation (AREX) [22]. Lenses are introduced between each rod of the quadrupole... 

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 results from simulations using all three models are presented in Table 9.3. No significant differences in the simulated overall detection efficiencies were observed, suggesting that the insertion of holes in the ring electrode does not affect adversely detection efficiency. It is perhaps not surprising that the axial ejection of ions is relatively insensitive to changes in the radial geometry of the trap. 

Chapter 11 Linear Ion Trap Mass Spectrometry with Mass-Selective Axial Ejection... 

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 3.19. Schematic of an AREX linear ion trap. In this design, ions are axially ejected by inserting two sets of eight vane lenses into quadrupole electrodes. (Reproduced from ref. 53 by permission of Elsevier Science, copyright 2006 American Society for Mass Spectrometry.)...

Ion ejection The efficiency of mass-selective radial ion ejection was 44% relative to the number of ions detected under mass-independent ion ejection conditions, yielding an overall efficiency of 12.7%. Mass-selective axial ejection yielded an overall extraction efficiency of 8% at 5.0 x 10 " Torr to 18% at 1.2 X 10 Torr. The ejected ion signal intensity increased threefold as the auxiliary AC frequency was increased from 334 to 762 kHz. 

Waveforms Operation of a mini-CIT requires five waveforms drive rf (2.000 MHz) and rf amplitude modulation potentials, ion isolation waveform, and ac potentials for ion excitation and for mass-selective axial ejection. Typical ac amplitude used in the mini-CIT for CID is 2Vo p and is applied in dipolar mode to the end-cap electrodes. The frequency used for resonant ejection is an octapole resonance that causes rapid ejection of ions. In Figure 8 is shown a typical timing diagram of scan function by which an El mass spectrum may be obtained. A scan function is a diagram showing the temporal relationships of the various potentials applied during... 

Figure 6.30 Sketch of a triple quadrupole instrument adapted to demonstrate use of a linear quadrupole as a 2D trap mass spectrometer with axial ejection. Qi was operated as a conventional linear quadrupole miz filter. Either the pressurized (> 0.1 mtorr) RF-only quadrupole q2/x or the evacuated ( 10 torr) Q3/X could be used in trapping mode. The RF-only quadrupole q2/x could be operated as a conventional collision cell, and Q3/X could also be used as a conventional RF-DC quadrupole m z analyzer. The ion guide quadrupole qo is 12 cm long and is maintained at 7 mtorr. All of Qj, q2/x and Q3/X are 127 mm long with ro = 4.17 mm, and the RF drive frequency was 1 MHz. IQl - IQ3 are differential pumping apertures that can also act as ion lenses. Adapted from Hager, Rapid Commum. Mass Spectrom. 16, 512 (2002), with permission of John Wiley Sons, Ltd.

The experimental apparatus incorporating an axial ejection linear trap (Hager 2002), introduced in Section 6.4.5b, has appeared on the market (Hager 2003) as a fully engineered version. This new QqQnap instrument (Figure 6.31) is more fully described as qoQiq2/xQ3/T> where the subscript n/T with n = 0 — 3 indicates that the... 

Sugiyama, M., Hasegawa, H., Hashimoto, Y. (2009) Mass-selective axial ejection from a hnear ion trap with a direct current extraction field. Rapid Commun. Mass Spectrom., 23, 2917. 

Fig. 4.34. Operation of a linear RF multipole ion trap illustrated using typical DC voltage offsets for positive ions, (a) Ion accumulation with backside potential wall up, (b) storage of ions with both trapping potentials up, and (c) axial ejection of ions from trap using an attractive exit plate potential. The trapping RF multipole section is held at constant DC offset of + 5 V in this example.

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