Probing trapped ion energies via ion-molecule reaction kinetics: Quadrupole ion trap mass spectrometry

We present a detailed study of the energies of the ions stored in a quadrupole ion trap mass spectrometer (QITMS). Previous studies have shown that the rate constant, k, for the charge exchange reaction Ar(+) N(+) 2 →, N(+) 2+Ar increases with increasing ion-molecule center-of-mass kinetic energy (K.E.cm). Thus, we have determined k for this chemical "thermometer" reaction at a variety of Ar and N2 pressures and have assigned K.E.cm values as a function of the q2 of the Ar(+) ion both with and without He buffer gas present in the trap. The K.E.cm energies are found to lie within the range 0.11-0.34 eV over the variety of experimental conditions investigated. Quantitative "cooling" effects due to the presence of He buffer gas are reported, as are increases in K.E.cm due to an increase in the q2 of the Ar(+) ion. "Effective" temperatures of the Ar(+) ions in He buffer are determined based on a Maxwell-Boltzmann distribution of ion energies. The resulting temperatures are found to lie within the range ≈ 1700-3300 K. We have also examined the K.E.cm, values arising from the chemical thermometer reaction of O(+) 2 with CH4, as previous assignments of effective ion temperatures based on this reaction have been called into question.