Radial ion ejection

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. 

Figure 6.29 A linear quadrupole ion trap for use as a mass spectrometer using radial ion ejection. Top schematic of the trap showing the ejection slot along the length of one of the x-rods. Bottom an overall view of the complete instrument showing typical potentials and pressures. The first (square) quadrupole is an ion guide to transport ions from the ESI source into the higher vacuum region and the function of the small octapole is similar but to facilitate ion transfer into the trap. Reprinted by permission of Elsevier from A Two - Dimensional Quodrupole Ion Trap. .. , by Schwartz, et al. Journal of the American Society for Mass Spectrometry, 13, p. 659-669, 2002, Eig 1, p. 660 Eig 5, p. 662, by the American Society for Mass Spectrometry.

Linear ion trap refers in general to 2D ion trap ion ejection is either axial or radial... 

In 2002, two linear ion traps were reported they had the basic structure of a quadrupole mass filter. The first linear ion trap instrument was described by J.W. Hager of MDS SCIEX and a second linear ion trap instrument was described by J.C. Schwartz, M.W. Senko, and J.E.P. Syka of Thermo Einnigan. Both instruments employ mass-selective ion ejection. Axial ion ejection is employed in the SCIEX instrument while in the Thermo Einnigan instrument ion ejection occurs radially. In addition, ion trapping in the SCIEX instrument can occur either in a pressurized collision cell region or in a low-pressure quadrupole rod array downstream of the collision cell. 

When an auxiliary ac field is applied to induce radial resonant excitation, coupling of radial and axial motions effected axial ion ejection when the ion radial secular frequency matched that of the ac field. 

Axial ejection is accomplished by resonantly exciting the ions radially near the quadrupole exit where the ions can overcome the exit DC barrier for mass analysis. Londry and Hager use the term, cone of reflection, to describe and determine what ions can be drawn past this barrier for mass analysis. The MDS Sciex team built a powerfiil hybrid triple QMF/q2/LQIT mass spectrometer, which is shown above Table 9.1, using the axial mode of ion ejection while still taking advantage of the mass filtering ability of Ql. 

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. 

An alternative to the 3D quadrupole ion trap (Paul trap) is the linear quadrupole ion trap. The linear ion trap is akin to a hybrid of the quadrupole mass filter and the 3D ion trap in that it consists of a four-rod assembly, like the quadrupole filter, but also it has entrance and end electrodes like the 3D ion trap. Confinement of ions along the axial direction is provided by DC potentials applied to the end electrodes. The quadrupole rods produce radial motion of the ions through application of an RF electric field, in a similar manner to that already described for the quadrupole mass filter. To record a mass spectrum axial ion ejection, initiated by RF excitation, can be used in a procedure similar to that used for the 3D ion trap. 

Fig. 1.23 Standalone linear ion trap. Because the ions are ejected radially two detectors are required for best sensitivity. Adapted with permission from reference [59].

Schwartz et al. [59] described a standalone linear ion trap where mass analysis is performed by ejecting the ions radially through slits of the rods using the mass instability mode. To maximize sensitivity the detection is performed by two detectors placed axially on either side of the rods (see Fig. 1.23). 

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). 

When the high energy ions collide with the source material, they cause ejection of the desired carbon radicals from the source material. The carbon radicals are ejected radially from the source material into the chamber. The carbon radicals then deposit themselves onto whatever is in their path, including the stage, the reactor itself, and the substrate (17). 

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

For low ion populations, a first estimate of achievable ejection resolution might be obtained from the cyclotron frequency spread that occurs over the range of cyclotron orbit radii through which the ion must pass to be ejected. This is based on the notion that an ejection waveform that is just adequate to eject one ion must have a frequency spectral peak that is at least as wide as the above spread of frequencies. Such a waveform would then excite, at least to some extent, all ions with frequencies falling within the width of the peak, thus limiting the ejection resolution. For ions with low z-mode amplitudes, we can use Dunbar s (46) approximate expression for the average radial field strength,... 

Here, B is the magnetic field strength, mc is the critical mass (40), q is the charge of the ion in question, rm is the maximum radius required for ejection, a is the inside length of the cubic cell, r is a radial displacement, V-p is the voltage applied to the trap plates, and a = 1.3871 (49) a geometry constant. 

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... 

LITs two modes for the mass selective ejection of ions are used either the ions are expelled axially using fringe field effects by applying AC voltages between the rods of the linear trap and the exit lens, or slots are hollowed out in two opposite rods and mass selective radial expulsion of ions is obtained by applying an appropriate AC voltage on these two rods. 

Mass selective ejection of the ions in a radial direction occurs by applying an AC voltage between the two cut rods. As for the 3D ion trap, an AC frequency corresponding to qz = 0.88 is used. Ions of successively higher masses are brought to this qz value by increasing V. An ejection efficiency of about 50 % is achieved at 5 Th s scan rate [25]. 

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 second configuration of a linear IT commercially available is schematized in Fig. 2.22. It operates, with respect to ion injection and storing, in the same way as described above, playing on the voltages applied to Gates 1 and 2, but in this case the ions are no longer ejected axially, but radially, thought two narrow slits present on two opposite quadrupole rods (Schwartz et al., 2002). 

In an ICR cell, two-dimensional spectrometry begins with application of the stored waveform inverse Fourier transform (SWIFT) excitation. This technique removes all but a single chosen ion from the trap (Marshall et al., 1985) and is performed by first determining which ions are to be ejected and their cyclotron frequency. Inverse Fourier transform then produces an excitation waveform that excites selected ions radially until they come into contact... 

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