Fringing fields quadrupole mass

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). 

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... 

This article begins with a brief history. It then explains the principles of operation of this mass spectrometer. The idealized view of its operation has to be tempered by consideration of some real-world situations that influence performance, such as the finite length of the field, the inevitability of fringing fields and field imperfections. Observations of performance and its limitations are illustrated. The applications section deals with residual gas analysis, gas chromatography and liquid chromatography mass spectrometry (GC-MS and LC-MS), collision induced dissociation using triple quadrupoles (MS/MS) and inductively coupled plasma mass spectrometry (ICP-MS) used for elemental analysis. 

Ions are lost in the region of the mass filter near the entrance, and to a lesser extent in the region near the exit. In these fringing field regions, fhe ion motion is destabilised because the local electric field is different from the value in the main part of the filter. For ion entry it is usual to minimise these losses by controlling carefully the ion entry conditions. This is often achieved using a short RF-only pre-filter, driven from the same RF supply as the main quadrupole, but with no DC component. An alternative approach is to use an entrance lens, driven with a voltage that scans proportionately to the RF amplitude. 

Note that although an ion may speed up or slow down as it passes through intermediate lenses before it reaches the QMF, such lenses do not themselves influence the axial velocity within the QMF. In practice, fringing flelds always exist near the entrance and exit of the QMF. These give rise to non-zero field components in the z direction, and will therefore alter the axial velocity. The influence of these flelds extends into the mass Alter to a distance of about 3ro from the entrance and exit. For simplicity this influence is largely ignored in this treatment, which applies specifically to ion motion in a perfect quadrupole field (but see Section 2.1.1.9). 

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