How Hot Are Your Ions in Differential Mobility Spectrometry?

Abstract

Ions can experience significant field-induced heating in a differential mobility cell. To investigate this phenomenon, the fragmentation of several para-substituted benzylpyridinium “thermometer” ions (R = OMe, Me, F, Cl, H, CN) was monitored in a commercial differential mobility spectrometer (DMS). The internal energy of each benzylpyridinium derivative was characterized by monitoring the degree of fragmentation to obtain an effective temperature, T eff, which corresponds to a temperature consistent with treating the observed fragmentation ratio using a unimolecular dissociation rate weighted by a Boltzmann distribution at a temperature T. It was found that ions are sufficiently thermalized after initial activation from the ESI process to the temperature of the bath gas, T bath. Once a critical field strength was surpassed, significant fragmentation of the benzylpyridinium ions was detected. At the maximum bath gas temperature (450 K) and separation voltage (SV; 4400 V) for our instrument, T eff for the benzylpyridinium derivatives ranged from 664 ± 9 K (p-OMe) to 759 ± 17 K (p-H). The extent of activation at a given SV depends on the ion’s mass, degrees of freedom, (N DoF), and collision frequency as represented by the ion’s collision cross section. Plots of T eff vs the product of ion mass and N DoF and the inverse of collision cross section produce strong linear relationships. This provides an attractive avenue to estimate ion temperatures at a given SV using only intrinsic properties. Moreover, experimentally determined T eff correlate with theoretically predicted T eff using with a self-consistent method based on two-temperature theory. The various instrumental and external parameters that influence T eff are additionally discussed.

Action spectroscopy of the isolated red Kaede fluorescent protein chromophore

“…One instrument offers isomer selectivity using differential mobility spectrometry (DMS), although heats the ions to T ≈ 450 − 500 K during the isomer separation stage before they are trapped and collisionally cooled to some degree. 40 The second instrument probes ions at T ≈ 298 K, although does not offer isomer/deprotomer selectivity and requires judicious choice of electrospray conditions (including solvent) to achieve a near pure deprotomer yield. Independent measurements recorded from both of these platforms reveal that at least three gas-phase forms of the deprotonated rKFP chromophore are generated using electrospray ionization, and that the principal phenoxide and imidazolate deprotomers have distinct photodissociation action spectra over the S 1 ←S 0 band.…”

“…Ion temperatures in the DMS cell are approximated at T ≈471 K (peak 1) and T ≈474 K (peak 2), based on earlier studies with thermometer ions. 40 However, even though the ions are heated in the DMS cell, the selected ion populations are thermalized to some degree during their passage and storage in Q3. The temperature of the ions in the photodissociation region is therefore expected to be closer to room temperature.…”

Clustering and Nonclustering Modifier Mixtures in Differential Mobility Spectrometry for Multidimensional Liquid Chromatography Ion Mobility–Mass Spectrometry Analysis

“…Thus, the difference in CoV is related to the rate of dynamic clustering/declustering, and analytes with relatively strong interaction potentials will exhibit a higher rate of clustering at the low field and a lower rate of declustering at the high field than those with relatively weak interaction potentials, which results in a resolution dependence on modifier concentration. We assume that both diastereoisomers exhibit negligible differences in collision cross section, which would otherwise affect the rate of collision and local ion temperature . The CoV values of Figures and are extracted from the heat maps generated from the raw data, which are provided in Figure S2.…”

“…We assume that both diastereoisomers exhibit negligible differences in collision cross section, which would otherwise affect the rate of collision and local ion temperature. 27 The CoV values of Figures 2 and 3 are extracted from the heat maps generated from the raw data, which are provided in Figure S2. Additional data of CoV shift with different analytes and different isopropanol modifier concentrations are presented in Figure S3.…”

Determining Collision Cross Sections from Differential Ion Mobility Spectrometry

“…DMS experiments to map ion differential mobility were conducted at 150 °C, which corresponded to a bath gas temperature of 100 °C. , DMS measurements consisted of stepping the SV from 1500 to 3000 V in 500 V increments and in 250 V increments thereafter up to SV = 4000 V. At each SV, the ion current was monitored as the CV was scanned from −10 to 30 V in increments of 0.1 V to produce an ionogram. Each ionogram is fit with a Gaussian distribution, for which the centroid is taken as the CV that corresponds to maximum ion transmission.…”