Improving the Selectivity of High Pressure Mass Spectrometry

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  • March 19, 2019
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  • Hampton, Andrew
    • Affiliation: College of Arts and Sciences, Department of Chemistry
Abstract
  • This work describes several strategies for improving the selectivity of high pressure mass spectrometry (HPMS). HPMS is the central strategy for developing hand portable MS instrumentation, which is useful in many field applications including rapid threat detection. A drawback to operating at high pressures is a degradation of resolution, adversely affecting selectivity. Improving resolution and selectivity at high pressures would further advance the field of HPMS. The focus of this work was incorporating tandem mass spectrometry (MS/MS) in a HPMS system and also demonstrating the feasibility of coupling HPMS with gas chromatography (GC). Tandem MS requires ion isolation and fragmentation, followed by mass analysis and detection. Initial work studied the behavior of trapped ions at high pressures (~1 Torr). Stability regions were measured at high and low pressures. Mass windows were found to be smaller at 1 mTorr, but the overall regions were generally steady from 500–1500 mTorr. Higher pressures dampened resonant behavior, thus requiring higher voltages to effect resonant ejection. Also, to improve the sensitivity of a high pressure system, two strategies were tested for creating differentiated sites for ion injection and ejection. The tapered SLIT geometry was shown to affect ion location, but adding supplemental electrodes between the ring and endcaps was demonstrated as a much more effective strategy for controlling ions. Ion isolation was demonstrated at 1 Torr of air via apex isolation, multifrequency isolation, and partial instability scan isolation. Fragmentation via high pressure collision-induced dissociation (HPCID) was demonstrated for several compounds. To maximize the CID iv efficiency, several variables were investigated. Increasing drive RF frequency was shown to be beneficial over the range tested (up to 25 MHz). Reductions in trap size, as expected, lowered pseudopotential well depth and hindered conversion efficiency. Increased pressure improved conversion efficiency, as did the use of air instead of helium. The coupling of GC to HPMS was also demonstrated with both helium and nitrogen carrier gases. A complex mixture of six chemical warfare agent simulants, which was unidentifiable with only a HPMS system, was separated and detected.
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  • In Copyright
Advisor
  • Jorgenson, James
  • Atkin, Joanna
  • Rosen, Elias
  • Hicks, Leslie
  • Ramsey, J. Michael
Degree
  • Doctor of Philosophy
Degree granting institution
  • University of North Carolina at Chapel Hill Graduate School
Graduation year
  • 2017
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