Optimization of the Cylindrical Ion Trap Geometry for Mass Analysis at High Pressure Public Deposited

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  • March 20, 2019
  • Chernookiy, Dmitriy
    • Affiliation: College of Arts and Sciences, Department of Chemistry
  • The cylindrical ion trap (CIT) provides many advantages for implementing mass spectrometry at elevated background pressures, presenting a practical route for the realization of a hand-portable device with performance that is suitable for many critical applications. This objective can be achieved through favorable scaling properties whereby high drive frequencies combined with small trap dimensions compensate for the loss of mass resolution from higher collisional damping rates. However, the simplified geometry of the CIT from the ideal hyperbolic electrode profile gives rise to higher-order fields that must be optimized for satisfactory performance. Previous investigations of CIT geometry have addressed performance at relatively low background pressures (ca. 1 mTorr) with only a few trap configurations. In this work, a practical range of CIT geometry parameters was experimentally evaluated at pressures between ca. 20 to 1000 mTorr of helium at a drive frequency of 9 MHz for ring electrodes with a 0.500 mm radius. Mesh-covered endcap electrodes were substituted for the traditional aperture-style design to mitigate alignment concerns, which proved to be robust and not inherently limiting to resolution. The study focused on the major parameters of ring electrode thickness (0.600, 0.650, 0.700, 0.750 and 0.800 mm) and ring-to-endcap spacing (varied between 0.075 to 0.300 mm) for a total of 20 unique traps. The optimization results are reported in terms of these dimensions and are also generalized by correlating performance changes to specific multipole fields. The octopole was found to be strongly correlated with trends in resolution at all pressures, with the dodecapole serving a minor role; the effects of all higher terms were confirmed to be inconsequential on a first-approach basis. At lower pressures resolution was primarily improved through an extension of the overall field linearity and the enhancement of an octopolar nonlinear resonance for double-resonance ejection, while at higher pressures it appears to benefit from the dynamic stabilization offered by more-than-linear fields. Thus, the optimal trap geometry was found to vary with the degree of damping and the mode of mass analysis. However, the spectral peak width was minimized over the widest pressure range with octopole strengths near 10% relative to the quadrupole.
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Rights statement
  • In Copyright
  • Glish, Gary
  • Mitran, Sorin
  • Jorgenson, James
  • Ramsey, J. Michael
  • Baer, Tomas
  • Doctor of Philosophy
Degree granting institution
  • University of North Carolina at Chapel Hill Graduate School
Graduation year
  • 2016

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