Collisional cooling in CID tandem MS ion trap experiments
Résumé
Injecton 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[1]. 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 106)[2] that are extensively used to generate tandem mass spectra.
Several years ago, Ichou & al.[3] 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 (synthe�c and biologic polymers) and, different
adduct ons (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.
References
1. G.C.Stafford et al.; Int.J. Mass Spectrom.Ion Proc., 60, 85-98 (1984)
2. S.A. McLuckey; J. Am. Soc. Mass Spectrom., 3, 599-614 (1992)
3. F. Ichou, et al.; J. Mass Spectrom., 48, 179-186 (2013)