In addition to the large effort in microfabricated devices for liquid-phase analysis, the Laser Spectroscopy and Microinstrumentation Group is exploring the use of microscale mass spectrometry for gas-phase analysis. We have shown that ion trap mass spectrometers with submillimeter radius can store up to 103 ions and attain a mass resolution of less than 0.2 Da at m/z 131 Da. The micro ion traps we have been using are of cylindrical geometry, constructed of three stainless steel electrodes separated by insulating layers, as shown in the figure at right. The ring electrode is formed by drilling a 0.5-1.0-mm diameter hole in the electrode.

(The bee picture shows a ring electrode with a 0.5-mm hole that yielded useful mass spectra.) The endcaps have smaller holes than the ring electrode and have been thinned at the center. The traps are operated in the mass-selective instability mode in the same manner as the larger conventional ion traps, but the voltage and frequency of the voltage on the ring electrode are different. Typically, the ring RF voltage ranges from 50 to 200 V amplitude at a frequency of from 3 to 7.5 MHz depending on the desired mass range.

	A diagram of a micro ion trap assembly with a hot filament electron gun for electron impact ionization and a Channeltron detector is shown at right. This instrument was described in reference 2. An electron current of 3.3 µA was available for sample ionization.
	A mass spectrum of xenon obtained with the trap described above is shown. The mass peaks are less than 0.2 Da in width. This spectrum was measured with a single scan of the ring electrode voltage. From an analysis of the variation of the peak heights from scan to scan, we estimate that a lower limit of approximately 800 ions were trapped per scan.

References

(1) Oleg Kornienko, Peter T. A. Reilly, William B. Whitten, and J. M. Ramsey, "Micro ion trap mass spectrometry", Rapid Commun. Mass Spectrom. 13, 50-53 (1999).

(2) Oleg Kornienko, Peter T. A. Reilly, William B. Whitten, and J. M. Ramsey, "Electron impact ionization in a micro ion trap mass spectrometer", Rev. Sci. Instrumen. (in press).

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