Hexapole assembly

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. 

A hexapole assembly of rods (poles) is built similarly to the quadrupole, but now there are three sets of opposed rods evenly spaced around a central axis. The hexapole cannot act as a mass filter by applying a DC field and is used only in its all-RF mode. It is therefore a wide band-pass filter and is used to collimate an ion beam. (Like-charged particles repel each other, and an electrically charged beam will tend to spread apart because of mutual repulsion of ions unless steps are taken to reduce the effect.)... 

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. 

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

ICP-MS was slow to adopt multipole ion optics, despite their routine use in organic mass spectrometry for some considerable time. Commercial ICP-MS models have to date employed the quadrupole and higher order multipoles such as the hexapole and octopole as gas pressurised collision cells/unpressurised ion guides. A photograph of an exposed hexapole assembly is shown in Figure 1.9. The unit comprises six highly polished rods with a circular profile, held parallel in an accurately machined ceramic former, effectively forming a tube. 

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. 

In commerdally available single-collector ICP-MS instrumentation, either a quadmpole (n = 1), a hexapole (n = 2), or an octopole n = 3) setup is used. An important differentiation has to be made, however, between a quadmpole assembly on the one hand and hexapole and octopole units on the other. Whereas the latter can only guide an ion from one point to another, a quadmpole assembly shows mass filtering capabilities [83]. 

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