High Pressure Mass Spectrometry: The Generation of Mass Spectra at Operating Pressures Exceeding 1 Torr in a Microscale Cylindrical Ion Trap

Abstract

We present the first demonstration of high pressure mass spectrometry (HPMS), which we define as mass spectrometry performed at pressures greater than 100 mTorr. Mass analysis is shown at operational pressures exceeding 1 Torr of helium buffer gas. A differentially pumped MS system was constructed for HPMS development consisting of two chambers. The first chamber (mass analysis chamber) was operated at pressures up to 1.2 Torr and contained the ionization source and a microscale cylindrical ion trap (CIT) mass analyzer. The CIT had critical dimensions of r0 = 500 μm and z0 = 650 μm. The second chamber was held at a lower pressure (≤10 mTorr) and contained an electron multiplier for detection. Mass spectra for xenon, 2-chloroethyl ethyl sulfide (CEES), and octane were acquired with helium buffer gas pressures ranging from 0.04 to 1.2 Torr in the mass analysis chamber. Full-width at half-maximum of mass spectral peaks was found to increase 143% for xenon, 40% for CEES, and 77% for octane over this pressure range, with maximum peak widths of 1.19, 1.26, and 0.82 Da, respectively. Data were fitted with an algebraic model that factors in ion-neutral collision peak broadening effects at high pressures. Experimental and theoretical peak broadening slopes showed good agreement at buffer gas pressures greater than 0.2 Torr. Experiments presented here demonstrate mass spectrometry at pressures orders of magnitude higher than conventionally practiced with any type of mass analyzer. The use of HPMS provides a way to eliminate turbo pumping requirements, leading to significant reduction in MS system size, weight, and power and facilitating a path toward compact/hand-held mass spectrometers with numerous potential applications.

High pressure mass spectrometry of volatile organic compounds with ambient air buffer gas

“…The peak broadening rates for the m/z 91 and 106 peaks in helium were 0.51 ± 0.07 and 0.41 ± 0.06 m/z units‐Torr −1 , respectively. These broadening rates agree with previous results for HPMS in helium, which found a peak broadening rate of 0.51 ± 0.05 m/z units‐Torr −1 for the m/z 41 peak of octane . In comparison, the peak broadening rates for air were 2.0 ± 0.1 and 1.6 ± 0.2 m/z units‐Torr −1 for the m/z 91 and 106 peaks, a 4‐fold increase in broadening rate over the pressure ranges studied.…”

“…If a different target pressure was required, the spacing between the plates could be changed for optimal performance. Above the maximum, the decrease in current density with increased pressure over the studied range is probably due to a decreasing mean free path and thus fewer electrons reaching the ring electrode, combined with reduced discharge current from the source . In practice, the total number of electrons available for ionization and mass analysis is a function of both current density and ionization time.…”

“…Typical axial RF voltages ranged from 5 to 10 V p‐p and were applied at approximately 6.6 MHz, about 2/3 of the drive RF frequency (corresponding to a hexapolar resonance). For experiments where the pressure was varied, the amplitude of the axial RF voltage was optimized for each pressure, consistent with previous experiments . The endcap nearest the detector was grounded to the chamber wall.…”

“…A differentially pumped system adapted from previous work was used for mass analysis . Briefly, the instrument consisted of two vacuum chambers.…”

“…High pressure mass spectrometry (HPMS) has recently been demonstrated with helium buffer gas at pressures up to 1.2 Torr . HPMS provides a path forward for the development of portable MS by reducing the overall vacuum requirements and eliminating the need for large, complex, and fragile pumping systems that include turbomolecular pumps.…”