Hexapoles

The three moments higher than the quadrupole are the octopole, hexapole and decapoli. Methane is an example of a molecule whose lowest non-zero multipole moment is the octopole. The entire set of electric moments is required to completely and exactly describe the distribution of charge in a molecule. However, the series expansion is often truncated after the dipole or quadrupole as these are often the most significant. 

The cold plasmas tend to be unstable, are sometimes difficult to maintain, and provide ion yields that are less than those of the hot plasmas. To obviate the difficulties of the interfering isobaric molecular ions from hot plasmas, it has been found highly beneficial to include a collision cell (hexapole see Chapter 22) before the mass analyzer itself. This collision cell contains a low pressure of hydrogen gas. lon/molecule collisions between the hydrogen and, for example, ArO+... 

A liquid chromatograph (LC) is combined with a TOF instrument through a Z-SPRAY ion source. Two hexapoles are used to focus the ion beam before it is examined by a TOF analyzer, as described in Figure 20.3. 

The hexapole cannot act as a mass filter by applying a DC field and is used only in its all-RF mode, in which it allows all ions in a beam to pass through, whatever their m/z values. In doing so, the ion beam is constrained, so it leaves the hexapole as a narrow beam. This constraint is important because the ion beam from the inlet system tends to spread due to mutual ion repulsion and collision with residual air and solvent molecules. By injecting this divergent beam into a hexapole unit, it can be refocused. At the same time, vacuum pumps reduce the background pressure to about 10 mbar (Figure 22.1). The pressure needed in the TOF analyzer is about 10 ... 

Hybrid Hexapole Time-of-Flight (Hexapole/TOF) Instruments... 

The term Q/TOF is used to describe a type of hybrid mass spectrometer system in which a quadrupole analyzer (Q) is used in conjunction with a time-of-flight analyzer (TOP). The use of two analyzers together (hybridized) provides distinct advantages that cannot be achieved by either analyzer individually. In the Q/TOF, the quadrupole is used in one of two modes to select the ions to be examined, and the TOF analyzer measures the actual mass spectrum. Hexapole assemblies are also used to help collimate the ion beams. The hybrid orthogonal Q/TOF instrument is illustrated in Figure 23.1. 

Schematic diagram of an orthogonal Q/TOF instrument. In this example, an ion beam is produced by electrospray ionization. The solution can be an effluent from a liquid chromatography column or simply a solution of an analyte. The sampling cone and the skimmer help to separate analyte ions from solvent, The RF hexapoles cannot separate ions according to m/z values and are instead used to help confine the ions into a narrow beam. The quadrupole can be made to operate in two modes. In one (wide band-pass mode), all of the ion beam passes through. In the other (narrow band-pass mode), only ions selected according to m/z value are allowed through. In narrow band-pass mode, the gas pressure in the middle hexapole is increased so that ions selected in the quadrupole are caused to fragment following collisions with gas molecules. In both modes, the TOF analyzer is used to produce the final mass spectrum.

Quadmpoles or hexapoles are used as transmission guides for both slow and fast ions. In both cases, the objective is to ensure that as many ions as possible are guided from the entrance of the device to its exit. The ions are usually in transit in a straight line between an ion source and a mass analyzer. Any ions within the transmission guides that are deflected from the desired trajectory are pushed or pulled back on course by the action of the inhomogeneous RF fields applied to the poles of the guides. 

From Figure 49.2, it can be seen that the quadrupole assembly provides a potential well to contain the ions in their journey along the main quadrupole axis. The potential well of the quadinpole has not very steep sides and, compared with steep-sided hexapoles or higher -poles or ion tunnels, the quadrupole is not as efficient as the others in containing ions inside the rod assembly. 

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. 

In the hybrid hexapole/TOF instrument, these ions are collimated or guided through two consecutive hexapole sections, each operating at different pressures. 

A hexapole assembly is incapable of separating ions according to their m/z values. However, it is capable of accepting an ion beam and ensuring that the beam is kept as narrow as possible and remains on a straight-line track. 

The gas pressure in the first hexapole section is influenced by gas leaking in from the inlet system and, at about 10" mbar, is higher than desirable if excessive ion/neutral collisions are to be prevented. Therefore, the first hexapole is separated from the second hexapole section by a small orifice, which allows ions to pass through. 

Differential pumping of the two hexapole sections keeps the second at a pressure of about ICk mbar. The two consecutive hexapole sections are sometimes described as bridges between the pressure in the inlet and the following vacuum in the TOP mass analyzer. 

After passing through the hexapoles, the ion beam emerges in front of a pusher electrode built into the end of the TOP analyzer. 

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