New operating mode of a quadrupole ion trap in mass spectrometry 4. Simulation studies

Computer Modeling of an Ion Trap Mass Analyzer, Part I: Low Pressure Regime

We present the multi-particle simulation program suite Computational Ion Trap Analyzer (CITA) designed to calculate the ion trajectories within a Paul quadrupole ion trap developed by the Jet Propulsion Laboratory (JPL). CITA uses an analytical expression of the electrodynamic field, employing up to six terms in multipole expansion and a modified velocity-Verlet method to numerically calculate ion trajectories. The computer code is multithreaded and designed to run on shared-memory architectures. CITA yields near real-time simulations with full propagation of 26 particles per second per core. As a consequence, a realistic numbers of trapped ions (100+ million) can be used and their trajectories modeled, yielding a representative prediction of mass spectrometer analysis of trace gas species. When the model is compared with experimental results conducted at low pressures using the conventional quadrupole and dipole excitation modes, there is an excellent agreement with the observed peak shapes. Owing to the program's efficiency, CITA has been used to explore regions of trapping stability that are of interest to experimental research. These results are expected to facilitate a fast and reliable modeling of ion dynamics in miniature quadrupole ion trap and improve the interpretation of observed mass spectra. Graphical Abstract ᅟ.

Simulations of ion trapping in a micrometer-sized cylindrical ion trap

We have performed detailed SIMION simulations of ion behavior in micrometer-sized cylindrical ion traps (r0 = 1 microm). Simulations examined the effects of ion and neutral temperature, the pressure and nature of cooling gas, ion mass, trap voltage and frequency, space-charge, fabrication defects, and other parameters on the ability of micrometer-sized traps to store ions. At this size scale voltage and power limitations constrain trap operation to frequencies about 1 GHz and rf amplitudes of tens of volts. Correspondingly, the pseudopotential well depth of traps is shallow, and thermal energies contribute significantly to ion losses. Trapping efficiency falls off gradually as qz approaches 0.908, possibly complicating mass-selective trapping, ejection, or quantitation. Coulombic repulsion caused by multiple ions in a small-volume results in a trapping limit of a single ion per trap. If multiple ions are produced in a trap, all but one ion are ejected within a few microseconds. The remaining ion tends to have favorable trapping parameters and a lifetime about hundreds of microseconds; however, this lifetime is significantly shorter than it would have been in the absence of space-charge. Typical microfabrication defects affect ion trapping only minimally. We recently reported (IJMS 2004, 236, 91-104) on the construction of a massively parallel array of ion traps with dimensions of r0 = 1 microm. The relationship of the simulations to the expected performance of the microfabricated array is discussed.