The dependence of reduced mobility, ion-neutral collisional cross sections, and alpha values on reduced electric field strengths in ion mobility

As ion mobility spectrometry (IMS) is used with mass spectrometry in more applications, increased emphasis is placed on the ion-neutral collisional cross sections (CCS) to identify unknown analytes in complex matrices. While CCS values can provide useful information about relative analyte size, several critical assumptions are inherent in the most common method of calculating CCS values, the Mason-Schamp equation. The largest source of error in the Mason-Schamp equation originates from not accounting for higher reduced electric field strengths, which are present in low-pressure instruments that require calibration. Previous corrections based on field strength have been proposed in literature, but their data used atomic ions in atomic gases, whereas most applications examine molecules measured in nitrogen. Here, we use a series of halogenated anilines measured in air and nitrogen between 6-120 Td on a first principles ion mobility instrument (HiKE-IMS). With this series of measurements, the average velocity of the ion packet is known allowing for direct calculation of reduced mobilities (K-0), alpha functions, and finally, a detailed examination of CCS as a function of E/N. In the worst-case scenario, there is over a 55% difference in CCS values for molecular ions measured at high fields depending on the method used. When comparing CCS values to those in a database for unknown identification, this difference can lead to misidentification. To immediately alleviate some of the error in calibration procedures, we propose an alternative method using K-0 and alpha functions that simulate first principles mobilities at higher fields.

Automated summary

As IMS is increasingly used in combination with mass spectrometry, understanding the ion-neutral collisional cross sections (CCS) becomes crucial for identifying unknown analytes in complex samples. However, the commonly used method of calculating CCS values, the Mason-Schamp equation, has inherent limitations and errors, particularly when applied to low-pressure instruments that require calibration. Existing corrections based on field strength are inadequate due to the use of atomic ions in atomic gases, while most applications involve molecules in nitrogen. In this study, halogenated anilines were measured in air and nitrogen using a first principles ion mobility instrument (HiKE-IMS), allowing for a direct calculation of reduced mobilities (K-0) and alpha functions, and a detailed analysis of CCS as a function of E/N. It was found that the method used could lead to over a 55% difference in CCS values for molecular ions measured at high fields, potentially leading to misidentification when comparing with a database.