The Linear Ion Trap in PTR-MS

An alternative to the 3D quadrupole ion trap (Paul trap) is the linear quadrupole ion trap. The linear ion trap is akin to a hybrid of the quadrupole mass filter and the 3D ion trap in that it consists of a four-rod assembly, like the quadrupole filter, but also it has entrance and end electrodes like the 3D ion trap. Confinement of ions along the axial direction is provided by DC potentials applied to the end electrodes. The quadrupole rods produce radial motion of the ions through application of an RF electric field, in a similar manner to that already described for the quadrupole mass filter. To record a mass spectrum axial ion ejection, initiated by RF excitation, can be used in a procedure similar to that used for the 3D ion trap. 

Three-dimensional ion traps have problems with both poor ion collection efficiencies and space-charge effects, where the latter provides an upper limit on the number of ions that can be stored within the trap. The much larger spatial volume within a linear ion trap reduces the space-charge problem and therefore a larger quantity of ions can be stored in this trap when compared to the 3D quadrupole ion trap. 

The incorporation of linear ion trap technology into PTR-MS was reported by Mielke and co-workers in 2008 [54], The quantity of ions that could be stored within the ion trap was 50 times larger than that in a 3D ion trap, which should confer a concomitant improvement in the limit of detection. Thus PTR-MS based on a linear ion trap as a mass analyser offers a detection sensitivity which has the potential to approach that achievable with systems based on quadrupole mass filters, although the best limit of detection so far achieved is near 100 pptv. Also, the linear ion trap preserves the easy use of MS/MS techniques exploited in 3D ion traps, which thus aids in compound identification. 

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