Collisional Activation in an Ion Trap

Operating in MS/MS with an ion trap involves the addition of an electronic card, not a physical change of the device. While the activation by collisions takes place in a cell when one uses a TQ, activation occurs between the electrodes in an ion trap mass spectrometer. The collision gas must be helium, a gas that is already present in the ion trap as a thermalization component. 

This leads to an important question. How can helium be efficient as a collision gas when it was chosen to reduce the kinetic energy of the ions in the trap withont fragmenting them In other words, how can a gas behave simultaneously to induce thermalization and activation As mentioned in Section 5.4.1, the collisions in helium are not very efficient due to its weak atomic mass. Many collisions at high speed (high kinetic energy) can nevertheless allow a sufficient increase of the internal energy of the precursor ion to allow it to dissociate.  

The following metaphor can aid understanding of this concept. Consider a car moving at a speed of 5 km/h on a circular circuit delimited by hay stacks. The car, the circuit, and the hay stacks, respectively, symbolize the ion, the ion trap, and the helium atoms. If the car slides and collides with a hay stack at 5 km/h, the car is 

FIGURE 5.19 Illustration of decreased risk of false negative in MRM. Relative abundances of ions of A are in agreement with those expected despite the coelution with B, which contains the m/z 200 ion in common with A. No false negative results. 

To operate MS/MS with an ion trap, we must accelerate the precursor ions sufficiently to allow the collisions with the helium atoms to become activating (and no longer thermalizing). The two ways to accelerate an ion in an ion trap are the resonant and non-resonant modes described below. 

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Wan, K.X. Gross, M.L. Shibue, T. Gas-Phase Stability of Double-Stranded Oligodeoxynucleotides and Their Noncovalent Complexes with DNA-Binding Drugs as Revealed by Collisional Activation in an Ion Trap, J. Am. Soc. Mass Spectrom. 11,450 57 (2000). 

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