Ion guide and collision cell

In this equation, 2n represents the number of rods and r is the radial distance from the centre of the rods. 

As an ion moves from the centre of the multipole towards any one of the rods, the potential increases to reach a maximum at the surface of the rod. For the quadrupole ( = 2), the potential varies as (r/r0)2, whereas the hexapole ( = 3) and the octapole ( = 4) have potentials that vary as (r/ro)4 and (r/ro)6, respectively. Consequently, the potential produced 

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This mode is executed by scanning or fixing the electric potentials of QMF I in the absence of a collision gas in the collision cell, and operating the QMF II as an ion guide. Under these conditions, the transmission efficiency of ions in the collision cell and the QMF II are optimized by selecting proper electric potentials. 

Figure 3.15 Schematic representation of a Q-TOF mass analyzer. It typically consists of a QMF, a collision cell, an ion guide, and an orthogonal TOF mass analyzer...

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. 

Another important application of a linear quadrapole is operation in the RF-only mode, i.e., where the DC voltage is zero. In this mode, all ions with m/z between a low-mass and high-mass cut-off are transmitted. RF-only quadrapoles (or hexapole or octapole) are used as an ion guide between API sources and mass analysers (Ch. 5.4.5) and as collision cells in various MS-MS instruments. 

The tandem MS includes a quadrupole mass filter, an octopole ion guide," a second quadrupole mass filter, and an ion detector. The ions from the flow tube are focused through electrostatic lenses into the first quadrupole, where a particular reactant ion is selected. These ions are then focused into the octopole, which passes through a cell that contains the collision gas. From the octopole, the dissociated and unreacted ions are focused into a second quadrupole for mass analysis. The detector is an electron multiplier operating in pulse-counting mode. 

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