Guide, quadrupole

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.

Note that in mass spectrometry/mass spectrometry (MS/MS) applications, quadrupole and magnetic sectors can be used together advantageously. It is also worth noting that the quadrapole can be operated without the DC voltages. In this RF-only mode, no mass separation occurs, and these quadrapoles are used as ion transmission guides, described in Chapter 49. 

It is perhaps worth noting here that, if a quadrupole assembly is used in this all-RF mode, there is no significant mass separation as ions of different mass move through the guide. However, if a DC potential is applied to one pair of rods, the guiding potential changes to that shown in Equation 49.5, in which F is the applied DC potential. 

The octopole configuration is similar to the quadrupole and hexapole in affording a good ion guidance system in its all-RF mode. The poles are connected in two pairs of four nonadjacent rods, and each pair is connected to an RF voltage supply, 180° out of phase with each other. The octopole is even more efficient than the hexapole as an ion guide. 

As the number of rods is increased, the rate of increase in ion efficiency falls off, and the devices become more difficult to construct. Consequently, quadrupoles and hexapoles are used most frequently in commercial instruments. More recently, the concept has been extended to a set of ring electrodes, which are the most efficient ion guides (Figure 49.8). Flowever, although the ring set (ion tunnel) uses RF fields similar to the ones outlined here, there are sufficient differences that are not discussed here. 

If no DC (static) voltage is used, the remaining all-RF field guides all ions through the quadrupole assembly. There is no separation by m/z, and the quadrupole in this mode is often used as an ion/gas collision cell. 

The most common ion guide is the quadrupole, in which there are four parallel metal rods spaced evenly about a central axis. 

The efficiency of the ion guiding effect increases from quadrupole to hexapole to octopole. 

Thomson, B.A. 1997 McBryde Medal Award Lecture Radio Frequency Quadrupole Ion Guides in Modem Mass Spectrometry. Can. J. Chem. 1998, 76, 499-505. 

Figure 4.2 The micro-Flow Tube Reaetor/Mass Spectrometer instrument. 1 — heated gas inlet/vacuum feedthrough, 2 — hot zone of flow tube, 3 — multiion source block, 4 — ion guide, 5 — quadrupole mass spectrometer, 6 — ion guides, 7 — reaction cell, 8 — quadrupole mass spectrometer, 9 — daly detector... 

Figure 12 Offset dependence of the intensity of the CT resonance in MAS NMR spectra of Na2MgEDTA-4H20, recorded with a single HS pulse placed before the spin-echo sequence. The experimental data (square points) were recorded using a 7mm probe (with 7kHz MAS rate) the solid red line is just a guide for the eyes. Also shown (blue line) is the simulated first-order quadrupole static powder pattern for the single Mg site in this compound.

Once the ions have been focussed through the orifice and into the analyser portion they are further focussed by electrostatic lenses into the first of three axially aligned quadrupole arrays. The first and third arrays can be operated as mass filters while the central quad (Q2 in Figure 1) functions only as an ion guide, incapable of resolving one mass from another. 

Cha, B., Blades, M. W., and Douglas, D. J. (2000). An interface with a linear quadrupole ion guide for an electrospray-ion trap mass spectrometer system. Anal. Chem. 72 5647-5654. 

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