Mapping the distribution of ion positions as a function of quadrupole ion trap mass spectrometer operating parameters to optimize infrared multiphoton dissociation.

Infrared multiphoton dissociation (IRMPD) combined with ion trajectory simulations has been used to obtain probability maps of ion position as a function of different operating parameters in a quadrupole ion trap mass spectrometer. The factors that contribute to the depth of the pseudopotential trapping well are analyzed, and their effects on the efficiency of IRMPD are demonstrated. Ion trajectory simulations are used to substantiate experimental results and demonstrate in greater detail the dynamic nature of the ion population's positional distribution. In particular, it is shown that the so-called "q(z) value" used during photodissociation can be of great consequence, as can the frequency of ac trapping voltage applied to the ring electrode. The results reveal that parameters which increase the pseudopotential well have the effect of decreasing the size of the ion cloud and maximizing overlap between the irradiating laser and the ions. Thus, while the common understanding of IRMPD dictates otherwise, IRMPD fragmentation efficiencies really depend on many ion trap operating parameters, much as collision-induced dissociation does.