Improving the Sensitivity and Linear Range of Photoionization
Ion Mobility Spectrometry via Confining the Ion Recombination and
Space Charge Effects Assisted by Theoretical Modeling
posted on 2024-02-23, 15:03authored byYiqian Xu, Qimu Yang, Manman Pan, Dandan Jiang, Yi Yu, Chuang Chen, Haiyang Li
Photoionization
(PI) is an efficient ionization source for ion
mobility spectrometry (IMS) and mass spectrometry. Its hyphenation
with IMS (PI-IMS) has been employed in various on-site analysis scenarios
targeting a wide range of compounds. However, the signal intensity
and linear dynamic range of PI-IMS at ambient pressure usually do
not follow the Beer–Lambert law predictions, and the factors
causing that negative deviation remain unclear. In this work, a variable
pressure PI-IMS system was developed to examine the ion loss effects
from factors like ion recombination and space charge by varying its
working pressure from 1 to 0.1 bar. Assisted by theoretical modeling,
it was found that ion recombination could contribute up to 90% of
signal intensity loss for ambient pressure PI-IMS setups. Lowering
the pressure and increasing the electric field in PI-IMS helped suppress
the ion recombination process and thus an optimal pressure Poptimal appeared for best signal intensity,
despite the decreased net ion number density and the increased space
charge effect. A simplified theoretical equation taking ion recombination
as the primary ion loss factor was derived to link Poptimal with analyte concentration and electric field
in PI-IMS, enabling a swift optimization of the PI-IMS performance.
For example, compared to ambient pressure, PI-IMS at a Poptimal of 0.4 bar provided a signal intensity increment
of more than 400% for 0.716 ppmv toluene and also expanded the linear
dynamic range by more than two times. Revealing factors influencing
the PI-IMS response would also benefit the applications of other chemical
ionization sources in IMS or mass spectrometry (MS).