Advanced System Components for the Development of a Handheld Ion Trap Mass Spectrometer

Schultze, Kevin

Advanced System Components for the Development of a Handheld Ion Trap Mass Spectrometer

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March 19, 2019

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Schultze, Kevin
- Affiliation: College of Arts and Sciences, Department of Chemistry

Abstract

This work describes the development and implementation of system components utilized in a handheld ion trap mass spectrometer (MS). Current state-of-the-art portable mass spectrometers fall in the 30 to 40 lb. range due to their large and heavy vacuum systems. The strategy used in this work to create lighter, handheld instruments is to eliminate much of the vacuum pumping requirements by operating the entire instrument under 1 Torr of background pressure, i.e. 3 orders of magnitude higher than conventional instruments. Reducing the ion trap size by a factor of 20 and operating these miniature traps at increased RF drive frequencies compensates for the ions' reduced mean free path, making mass analysis possible at these elevated pressures. The first component developed was a novel ion trap geometry. The design is a modification of the cylindrical ion trap (CIT), using three electrodes to produce an elongated two-dimensional trapping region capped by planar mesh electrode features on each end. The SLIT's increased dimensionality led to a tenfold increase in sensitivity versus a CIT with the same critical dimensions while maintaining identical resolution. This 5 mm long SLIT with critical dimensions x<sub>o</sub> = 500 μm and z<sub>o</sub> = 650 μm represents the smallest linear type ion trap reported to date and maintains higher resolution than their larger counterparts. SLIT operation in pressures exceeding 1 Torr of helium, nitrogen, and air buffer gasses also represents the highest pressure operation reported for linear ion traps. Ion detection at elevated pressures was achieved using a Faraday cup and charge sensitive amplifier. Mass spectra were collected using both a commercial amplifier and two custom made amplifiers. The smaller of the two custom Faraday cup amplifiers was incorporated into a miniature vacuum chamber along with the SLIT mass analyzer and glow discharge ion source. The operation of this MS system is the first reported miniature instrument operated without the use of turbomolecular vacuum pumps, and can in fact be operated with only a simple miniaturized roughing pump. Incorporating the miniature chamber, pump, and a custom miniature RF system, a prototype MS was developed weighing 2 lbs. and drawing 8.2 W. Finally, gas chromatography - mass spectrometry (GC MS) experiments were carried out utilizing the newly developed miniature MS instrumentation with a full size GC to demonstrate the feasibility of a future handheld GC MS device. Detection of a variety of organic analytes during 2.5 minute separations was demonstrated in both helium and nitrogen carrier gasses with limits of detection of 0.056 ng injected on-column.

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