Micro- and Nanotechnologies
Novel sampling strategies and highly miniaturized systems for mass spectrometric analysis
We present initial results from a high-aspect-ratio linear ion trap employing 20-micrometer-wide electrodes patterned onto ceramic substrates, with a characteristic trapping dimension of 800 micrometers. In previous efforts we showed that a variety of ion trap geometries can be made using assemblies of two ceramic plates, the facing surfaces of which are patterned with appropriately shaped electrodes. The present report shows significant miniaturization of this approach. Mass spectra of organic compounds with this device have resolution of 2-3 amu. These highly miniaturized analyzers are now being developed for portable GC-MS instrumentation. Aluminum electrodes were deposited onto one side of each ceramic substrate. Electrodes are wire-bonded to a printed circuit board, which connects with a capacitive voltage divider. Two plate-PCB assemblies are mounted in a sandwich configuration, with the trapping fields being established in the space between the plates. Prior to patterning, a tapered ejection slit, 166 micrometers wide, was laser-cut into each substrate for ion ejection. The taper is critical to prevent ions from striking the inner wall of the slit and building up space-charge, while allowing the thickness of the substrate to remain sufficiently thick for strength. Dipole resonant ejection of ions, in which the applied ejection waveform is phase-locked with the drive RF, was demonstrated by the use of special phase-tracking circuit. Alignment of the substrates was demonstrated using a set of 4 micropositioners (three linear and three angular). Low-power performance—essential for portable and hand-held mass spectrometers—was also demonstrated, with a maximum RF amplitude of 400 V at the highest point in the scan. Typical mass resolution of small organic compounds (toluene, xylenes) is 1.5 Da. Experiments using high molecular weight compounds (octofluorotoluene and perfluorotributylamine) showed typical mass resolution of 2-3 Da. The effects of higher operating pressure on mass spectra were also examined. Resolution decreased at pressures above 5 mTorr, but suitable spectra could still be obtained at pressures of up to 42 mTorr. Resolving power is decreased compared with the larger scale version of this device, possibly due to increased space charge. However, the signal to noise ratio is large due to the high aspect ratio of these traps—the ratio of the length to the characteristic trapping dimension is greater than 40, providing a large trapping volume.