The effects of collisional cooling on the storage and detection of ions in a quadrupole ion trap mass spectrometer are examined. Four parameters that affect the storage of ions are evaluated: the pressure of the buffer gas: the qz value which establishes the ion trapping environment; the cooling time during which ions may collapse to the center of the trap; and the supplementary a.c. voltage used to kinetically excite selected ions. The ion trap may be operated effectively at a low helium pressure (0.1 mTorr) if a sufficient delay period is added to allow the diffuse ion cloud to collapse to the center of the trap prior to detection. For a typical experiment, this modified operation at low pressures involves allowing the ions to undergo approximately 30–100 collisions (i.e. in a ≈30 ms interval at 0.1 mTorr). At low helium pressures (≈0.1 mTorr), more ions may be trapped at higher qz values. Collision-activated dissociation (CAD) at low helium pressures indicates that the average internal energy deposition is higher, and sensitivity is not lost, if a collisional cooling delay is added. This latter result potentially has great analytical relevance for CAD of biomolecules which require greater internal energy deposition to induce fragmentation.
International journal of mass spectrometry and ion processes 115, pp.67-81