Investigation of Zero-/High-Field Ion Mobility Orthogonal Separation Using a Hyphenated DMA–FAIMS System and Validation of the Two-Temperature Theory at Arbitrary Field for Tetraalkylammonium Salts in Nitrogen

Abstract

Toward greater separation techniques for ions, a differential mobility analyzer (DMA) has been coupled with field asymmetric waveform ion mobility spectrometry (FAIMS) to take advantage of two mobility-related but different methods of separation. The filtering effect of the DMA allows ions to be selected individually based on low-field mobility and studied in FAIMS at variable electric field, yielding mobility separations in two dimensions. Because spectra fully describe ion mobility at variable field strength, results are then compared with a twotemperature theory-predicted mobility up to the fourth-order approximation. The comparison yields excellent results up to at least 100 Td, beyond which the theory deviates from experiments. This is attributed to two effects, the enlargement of the structure due to ion heating and the inelasticity of the collisions with the nitrogen bath gas. The corrected mobility can then be used to predict the dispersion plot through a newly developed implicit equation that circumvents the possible issues related to the more elaborate Buryakov equation. Our results simultaneously show that the DMA−FAIMS coupling yields complete information on ion mobility versus the field-strength to gas-density ratio and works toward predicting such spectra from ion structures and gas properties.

Ion mobility calculations of flexible all-atom systems at arbitrary fields using two-temperature theory

“…3. 35,37 In the same figure, we can observe over-imposed numerical results obtained through the fourth approximation of the two-temperature theory, which exhibit reasonable agreement with experiments, 17 with noticeable deviations occurring at reduced field strengths exceeding 100 Td. Notably, in all instances, the experimental data indicate lower mobilities when contrasted with the calculated values.…”

“…To investigate this, one can refer to previously obtained experimental mobilities of tetraalkylammonium ions, measured as a function of reduced field strength with a DMA-FAIMS hybrid system in ambient air conditions. 35 Methodology of this experimental setup, detailed elsewhere, 35 involves a DMA acting as a primary filter and measuring mobility in a uniform, low electric field. FAIMS, on the other hand, creates an asymmetrical waveform between parallel plates where the ions are passing through, separating them based on the ratio of mobility at high and low field.…”

“…The resulting mobility to reduced field strength values can then be used to obtain a dispersion plot using the method shown in ref. 35 and 37. In this study, a typical bisinusoidal dispersion field was used, however, any waveform can be analyzed.…”

“…In a previous manuscript, 35 we obtained the experimental mobilities of tetraalkylammonium salts at arbitrary fields using a High Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS) 9,36–39 instrument coupled to a Differential Mobility Analyzer (DMA). 40,41 The experimental results were tested against our numerical two-temperature theory prediction.…”

MobCal-MPI 2.0: an accurate and parallelized package for calculating field-dependent collision cross sections and ion mobilities

“…Such a model requires exact knowledge of how internal energy is exchanged between the collision gas and the ion's rovibrational states. As advances are made in treating the inelasticity problem, 78,79 we expect that the width of the distributions shown in Fig. 7C will decrease.…”