Comparison of ion coupling strategies for a microengineered quadrupole mass filter

Abstract

The limitations of conventional machining and assembly techniques require that designs for quadrupole mass analyzers with rod diameters less than a millimeter are not merely scale versions of larger instruments. We show how silicon planar processing techniques and microelectromechanical systems (MEMS) design concepts can be used to incorporate complex features into the construction of a miniature quadrupole mass filter chip that could not easily be achieved using other microengineering approaches. Three designs for the entrance and exit to the filter consistent with the chosen materials and techniques have been evaluated. The differences between these seemingly similar structures have a significant effect on the performance. Although one of the designs results in severe attenuation of transmission with increasing mass, the other two can be scanned to m/z ϭ 400 without any corruption of the mass spectrum. At m/z ϭ 219, the variation in the transmission of the three designs was found to be approximately four orders of magnitude. A maximum resolution of M/⌬M ϭ 87 at 10% peak height has been achieved at m/z ϭ 219 with a filter operated at 6 MHz and constructed using rods measuring ( nterest in the miniaturization of mass analyzers has been growing rapidly in recent years, largely driven by the need for compact, lightweight systems for use in environmental, security, and space applications. While there have been numerous attempts to make small analyzers, not all can reasonably be called miniature [1]. However, quadrupole mass filters [2-6] and various types of ion traps [7][8][9] with characteristic dimensions of the order of a few millimeters or less have been demonstrated. The size and weight contribution to a complete mass spectrometer system is not the only benefit of a miniaturized quadrupole mass filter. The rf amplitude required to achieve a particular mass range increases with the square of the rod radius. Hence, the rf supplies for miniature filters can be smaller and require less power than those needed for larger filters. More importantly, smaller pumps may be used, as the short path length of ions in the filter means that a higher pressure can be tolerated. Although this paper is focused on miniaturization of the filter, reports on the development of other miniaturized components that might be incorporated into a system, such as ion sources, gauges, and pumps can be found elsewhere [10 -12].The factors that determine the size of a miniature quadrupole mass filter are the required signal level, the accuracy of the construction technique, and the number of rf cycles needed to achieve the desired resolution. Clearly, as the size of the entrance aperture decreases, there will be an inevitable loss of signal, although some of this can be recovered through the use of an array [2,4]. There is a well-known correlation [13] between the fractional geometrical error and ultimate resolution. Hence, one fundamental limitation is set by the accuracy of the fabrication technique and the minimum acceptable resolution. Si...

MEMS-Based Nanospray-Ionization Mass Spectrometer

“…The first is a simple quadrupole fabricated using bonded silicon-on-insulator material [83,84] To construct a complete filter, two dies are stacked together back-to-back so that the quadrupole geometry is formed. In the bonded silicon device, the features etched into the device layers also form sets of entrance and exit electrodes containing central pupils.…”

“…These pupils are, however, both rectangular and relatively small, and significantly limit ion transmission [84]. The assembly is then attached to a small printed circuit board, and wire-bond connections are made to the electrode contacts.…”

“…The first is a simple quadrupole fabricated using bonded silicon-on-insulator material [83,84]. The second is a quadrupole with a Brubaker prefilter (the use of which can improve ion 16 transmission, especially in small devices) fabricated using silicon-on-glass [87].…”

“…A more successful filter, with a mass range of m/z = 400 and a resolution of m/∆m = 80 at 10% peak height, was developed by Microsaic Systems [83,84] and used in complete benchtop systems [85]. The rods were supported in a silicon frame, fabricated by deep reactive ion etching (DRIE) of bonded silicon-on-insulator (BSOI) material.…”

“…The first is a simple quadrupole fabricated using bonded silicon-on-insulator material [83,84] To construct a complete filter, two dies are stacked together back-to-back so that the quadrupole geometry is formed. In the bonded silicon device, the features etched into the device layers also form sets of entrance and exit electrodes containing central pupils.These pupils are, however, both rectangular and relatively small, and significantly limit ion transmission [84]. The assembly is then attached to a small printed circuit board, and wire-bond connections are made to the electrode contacts.…”