AN IMPROVED METHOD FOR CAPTURING LASER-DESORBED IONS IN AN ION-TRAP MASS-SPECTROMETER - DYNAMIC R.F. TRAPPING

We have developed an improved method, dynamic r.f. trapping, for capturing laser desorbed ions in a quadrupole ion trap mass spectrometer (ITMS). Trapping efficiency is enhanced by over an order of magnitude over previous methods. A 308 nm excimer laser pulse desorbs the sample - trimethylphenylammonium iodide (TPA-I) is used in most of the work reported - from a probe inserted through the ring electrode. The laser is fired as the r.f. trapping potential (risetime about 175 mu s) is applied to the ring electrode. Laser desorbed ions penetrate the trap while the trapping potential is low, but cannot escape because the r.f. potential rises substantially during their transit across the trap. The trapping efficiency is found to depend critically on the kinetic energy of the laser desorbed ions, and on the r.f. amplitude, phase, and rate of change of the r.f. amplitude when the laser fires. Cation and anion signals are recorded as functions of coarse and fine steps in the laser-to-r.f. timing. Coarse and fine timing steps test the effects of laser-to-r.f. delay and phase respectively. We also report effects on trapping efficiency of buffer gas pressure and composition (He neat versus He:Xe mixtures) and the amplitude of the ring electrode steady state r.f. potential. The delay and phase dependence of the experimental data is analyzed with reference to an effective potential barrier model. Differences in the phase and delay dependences for anions and cations are attributed to differences in Debye shielding early in the expansion of the laser desorbed plume. Cation and anion mass spectra are presented for laser desorption/ionization of TPA-I and pyrene. For TPA-I desorption, reactions between laser desorbed cations and neutral TPA fragments in the early, high density portion of the laser plume lead to production of high mass cations.