MASS-SELECTIVE ISOLATION OF IONS STORED IN A QUADRUPOLE ION-TRAP .2. A SIMULATION STUDY OF CONSECUTIVE ISOLATION

A novel process of ion isolation, or mass-selective storage, in a pure quadrupole ion trap has been simulated by calculation of ion trajecto ries. This isolation process for a single ion species entails the ejec tion of ions of lower and higher mass-to-charge ratio in two ejection events. A simulation program has been applied to the calculation of io n trajectories during consecutive ion isolation; both a collision-free system and one in which collisions with helium atoms occurred were co nsidered. The simulation program used here was similar to that used pr eviously for the investigation of resonance excitation of stored ions and for concurrent ion isolation. The working points of three ion spec ies (m/z 206, 207 and 208), located initially on the q(z) axis with q( z) almost-equal-to 0.2, were moved off-axis first to the vicinity of t he beta(z) = 1 boundary at which ions of m/z 206 were ejected, then of f-axis to the beta(z) = 0 boundary at which ions of m/z 208 were eject ed. Stable trajectories were maintained only for the ion species of m/ z 207; other ion species were ejected axially. The simulation process extended over a real time of 2.84 ms. Trajectories are displayed as te mporal variations of axial and radial excursions from the centre of th e ion trap. The variations of ion total kinetic energy, together with the radial, azimuthal, and axial kinetic energy components which were calculated throughout each trajectory, were found to be substantially modified by collisional cooling prior to isolation.