Ion guide

Figure Bl.7.13. A schematic diagram of an ion-guide mass spectrometer. (Ervin K M and Annentrout P B 1985 Translational energy dependence of Ar + XY —> ArX + Y from thennal to 30 eV c.m. J. Chem. Phys. 83 166-89. Copyright American Institute of Physics Publishing. Reproduced with pemiission.)...

A different ion guide is the ion tunnel, which also uses only RF fields to transmit ions. It is not a rod device but consists of a series of concentric circular electrodes. It is perhaps best described as operating like a series of ion traps. This chapter gives details of some of the fundamental characteristics of rod-type transmission guides (multipoles). 

A second important need for some guidance system lies in stray electric fields. Clearly, a sufficiently large potential arranged transversely to an ion beam can serve to deflect ions away from the intended direction. Such stray fields can be produced easily by sharp edges or points on the inside of a mass spectrometer and even more so in an ion guide itself. Considerable care is needed in the construction and design of mass spectrometers to reduce these effects to a minimum. 

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. 

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. 

The ion guides are frequently used to transmit ions from an atmospheric-pressure inlet/source system (electrospray ionization, atmospheric-pressure chemical ionization) into the vacuum region of an m/z analyzer. 

The ion guides are also used as gas collision cells. When ions collide with neutral gas atoms in such a cell, it is important that ion losses due to deflection or collision should be minimized. Ion guides perform this task. 

The ion guides can also be constructed as a series of flat rings, placed parallel to each other, the centers of the rings lying along a long linear axis. 

An RF field is applied to the rings. The positive phase is applied to every other ring and the negative phase to the remaining intervening rings. Such an ion guide is called an ion tunnel. 

Ion guides are needed to transfer ions of low kinetic energy from one region to another without substantial losses, [118] e.g., from an atmospheric pressure ion source (ESI, APCI) to the entrance of a mass analyzer (Chap. 11). 

Tolmachev, A.V. Udseth, HR. Smith, R.D. Radial Stratification of Ions As a Function of Mass to Charge Ratio in Collisional Cooling Radio Frequency Multipoles Used As Ion Guides or Ion Traps. Rapid Commun. Mass Spectrom. 2000,74, 1907-1913. 

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

White, F.M. Marto, J.A. Marshall, A.G. An External Source 7 T FT-ICR Mass Spectrometer With Electrostatic Ion Guide. Rapid Common. Mass Spectrom. 1996,10, 1845-1849. 

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... 

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. 

Conventional mass spectra are obtained by scanning Q1 while operating Q3 as an ion guide. Three additional scan modes are available to the MS/MS user, they are ... 

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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