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

Abstract

We have performed detailed SIMION simulations of ion behavior in micrometer-sized cylindrical ion traps (r 0 ϭ 1 m). 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 q z 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) n parallel with advances in micromachining technology, numerous efforts are underway to produce miniaturized and microfabricated analytical instrumentation. Although portable instruments rarely match the performance of larger laboratory instruments, the benefits of low mass and power make small instruments well-suited for real-time in-field analytical applications. Mass spectrometers are attractive targets for miniaturization because they generally exhibit high sensitivity and chemical specificity. Many groups have focused on miniaturizing mass spectrometers, and small mass analyzers based on time-of-flight [1][2][3][4][5][6], magnetic sector [7], linear quadrupole [8], and hyperbolic and cylindrical ion traps [9 -17] have recently been described. The dimensions of some of these mass analyzers are about a millimeter or smaller. As mass analyzer dimensions decrease, vacuum requirements also typically decrease due to the shorter ion path and correspondingly shorter tolerable mean free path [11,18]. Similarly, power requirements typically decrease as the sizes of capacitive and inductive components are reduced, or as the required voltage needed for a given field strength decreases. Further reductions in size, for example, down to the micrometer range, may allow sufficiently small pumping, power, and electronics packages as to make a truly hand-held instrument.We recently reported [19] on the design considerations and development of microfabricated arrays of cylindrical ion traps for use as mass analyzers. These arrays contain up to 10 6 traps/cm 2 . The geometry of the cylindrical ion trap is amenable to multi-level microfabrication techniques, and the trap size and electrical requirements make it suitable for layout a...

Novel ion traps using planar resistive electrodes: Implications for miniaturized mass analyzers

“…In addition, smaller traps have reduced access to the trapping region, making it harder to inject ions or to introduce electrons or photons into the trap. In the extreme case, simulations of ion trapping in micrometersized cylindrical ion traps show that only a single ion can remain trapped [36]. Trap arrays have been suggested as a solution to the reduced ion counts in smaller traps.…”

Design, microfabrication, and analysis of micrometer-sized cylindrical ion trap arrays

“…Detailed simulations 19 of ion behavior in micrometersized cylindrical ion traps were performed using SIMION. 20 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 were analyzed.…”

Accelerated Simulation Study of Space Charge Effects in Quadrupole Ion Traps Using GPU Techniques

“…Coulombic force has a nonlinear contribution to the ion motion equation (Mathieu's Equation), and it is difficult to use an analytical solution to comprehensively model space charge effects in an ion trap. With approximations at different levels, several theoretical methods have been proposed [18,[36][37][38][39][40][41][42], while frequently they need to be used with assistance from ion trajectory simulations [43][44][45][46][47][48][49] to provide meaningful results. The calculation of coulombic force requires excessive amount of computation resources, and it is usually not practical to perform large-scale simulations on personal computers.…”