Drift Tube Ion Mobility: How to Reconstruct Collision Cross Section Distributions from Arrival Time Distributions?

Abstract

Ion mobility spectrometry allows one to determine ion collision cross sections, which are related to ion size and shape. Collision cross sections (CCS) are usually discussed based on the peak center, yet the width of each peak contains further information on the distribution of collision cross sections of each conformational ensemble. Here, we analyze how to convert arrival time distributions (ATD) to CCS distributions (CCSD). With a calibration curve taking into account the CCS dependence of the time spent outside the mobility region, one can reconstruct CCS distributions with correct peak center values. However, the peak widths are incorrectly rendered because ion diffusion, which affects the peak width in the time domain, is irrelevant to collision cross sections. For drift tube ion mobility, we describe a new method, coined "FWHMstep", using a step-field experiment and processing the peak's full width at half-maximum to reconstruct CCSDs. The width of the CCS distribution helps to characterize the analyte's structural heterogeneity, and/or its flexibility (i.e., the variety of ways the analyte ions can rearrange following electrospray into kinetically stable gas-phase conformations).

Recommendations for reporting ion mobility Mass Spectrometry measurements

“…In the framework of structural studies, reporting collision cross section distributions (CCSD) becomes increasingly popular. An outline and discussion of the different procedures possible for drift tube ion mobility is provided by Marchand et al (). It should also be noted that these procedures introduces additional uncertainty and biases depending on the data processing method (using the term “apparent CCSD” would convey this notion), and should not be used when ions of different charges coexist within the given m / z range.…”

“…Most of the drift contributing to t 0 occurs in the ion funnel containing the same gas as the drift region, but drift occurs in the collision cell as well. Small analyte‐dependent effects on t 0 were found when a different gas was used in the collision cell compared to the drift region (Marchand et al, ). If the same gas is used (typically N 2 ), the calibrant and analyte need not be of the same chemical class.…”

Native Ion Mobility Mass Spectrometry: When Gas-Phase Ion Structures Depend on the Electrospray Charging Process

“…The absorbance was recorded at 260 nm on a Uvikon XS, and molar extinction coefficients calculated using the IDT website by applying the Cavaluzzi-Borer Correction [35]. dT6 (MM = 1763.2) and dTG4T (MM = 1863.3) were used to verify the instrument each day for the determination of collision cross section (CCS) values ( DT CCSHe = 306 Ų for dT6 2-, see supporting information of [36], and 788 Ų for [(dTG4T)4(NH4)3] 5- [37]). The for the native IM-MS study.…”

Importance of collision cross section measurements by ion mobility mass spectrometry in structural biology

“…The CCS distribution (reflected by the full width at half maximum of the mobility measurement, provided due consideration has been given to ion diffusion in the drift time domain) can often be another important experimental indicator not just of the structure, but rather of the structural heterogeneity of proteins and their complexes. Narrow CCS distributions tend to indicate a single predominant conformation, while broader CCS ranges are consistent with conformational flexibility in solution which allows the protein to sample multiple states.…”

“…However, the ion charge density is intimately linked to the contribution of glancing collisions, and a recent investigation suggests accurate structure elucidation on the basis of IM-MS measurements must account for charge distribution when localized charges are present, even for systems as large as 12 kDa 45. This work also provides evidence that accurate structure elucidation is unlikely if CCS values recorded in one buffer gas are converted into other buffer gases when the electronic properties of the two gases differ.The CCS distribution (reflected by the full width at half maximum of the mobility measurement, provided due consideration has been given to ion diffusion in the drift time domain46 ) can often be another important experimental indicator not just of the structure, but rather of the structural heterogeneity of proteins and their complexes. Narrow CCS distributions tend to indicate a single predominant conformation, while broader CCS ranges are consistent with conformational flexibility in solution which allows the protein to sample multiple states.…”