Collisional cooling in CID tandem MS ion trap experiments

Injection of a gas inside a quadrupole ion trap mass spectrometer at a sufficiently high pressure (10-5 - 10-3 mbar) increases the efficiency in trapping the ions to be mass-analyzed. This higher trapping efficiency is achieved through the collapsing of the ions orbits to the ion trap centre by collisional damping of the ionic motion. The storage of ions over long period of time (tens to hundreds of milliseconds) together with the high trapping gas pressure typically used, eventually leads to a very large collision number (up to 10+6) that are extensively used to generate tandem mass spectra. Several years ago, Ichou & al. reported Energy-Resolved tandem MS analysis on protonated leucine enkephalin peptide using a triple quadrupole instrument. They compared the Survival Yields (portion of the precursor ions surviving the CID process at constant experimental conditions) obtained at different acceleration voltages by drawing SY curves at different Ar collision gas pressures. When the Ar pressure inside the collision cell was increased, they observed a shift of the SY curve to lower acceleration voltages. This shift was atributed to an increase in the number of collisions experienced by the precursor ions and in their residence time in their journey to the detector. We conducted similar ER-MS experiments to draw SY curves at different He pressures however on two different ion trap instruments. We observed a completely opposite behavior with SY curves shifted to larger excitation voltages when collision gas pressure is increased. This observation was confirmed on a large m/z range with: different types of compounds (synthetic and biologic polymers) and, different adductions (protonated, sodiated and lithiated). These experimental results are discussed in light of collisional cooling, which may predominate inside the ion trap on contrary to CID MS/MS experiments in triple quadrupole instruments.