The authors are developing a field instrument for direct air sampling which combines rapid screening with high sensitivity. These qualities are achieved by combining a pneumatically assisted two-stage membrane sampling interface with a Finnigan ion trap mass spectrometer. The instrument will be suitable for monitoring contaminant releases in outdoor air during waste site operations. Additional applications include indoor air monitoring, and stack and process monitoring.

We report a new method for trapping laser desorbed ions in a quadrupole ion trap mass spectrometer (ITMS). This method, gated RF trapping, improves the trapping efficiency by over two orders of magnitude over previous methods. If the RF trapping field is applied before the laser fires, there will be a large potential barrier for injection of ions into the trap.1 In general, ions either have insufficient energy to penetrate the entrance barrier, or they penetrate the entrance barrier and pass through the trap because the entrance and exit barrier heights are the same. Collisions reduce the kinetic energy of desorbed ions and increase the probability of trapping. Gated RF trapping takes advantage of the risetime of the RF in the ITMS: the laser is fired early in the rise of the RF and ions are presented with a small potential barrier to entry into the trap, but by the time the ions cross the trap the barrier has risen substantially, hindering their escape. Ions are more efficiently trapped than by buffer gas collisions alone and good sensitivity is obtained at very low buffer gas pressures (10-5 Torr) allowing high mass resolution operation. This simple picture is complicated by factors such as Debye shielding, plume reactions, and buffer gas collisions. In this paper, we describe the effects of laser-to-RF phase and delay, and buffer gas pressure and composition.2,3

Laser desorption in an ion trap mass spectrometer shows significant promise for both qualitative and trace analysis. In this work, we explore various combinations of time-varying DC and radiofrequency (RF) fields in order to optimize laser-generated signals. By judicious choice of timing between the laser desorption pulse and the rise in the applied RF trapping potential, we observed over an order of magnitude enhancement in the trapped ion signal. This new method for laser desorption has enabled us to observe mass spectra of many compounds (e.g., pyrene, dichlorobenzene, and ferrocene) that are barely detectable using previous laser desorption methods. Effects of laser timing and the magnitude of the steady-state RF potential are discussed.