Effective collisional cross‐section of small ions in the gas phase: Application to ion mobility spectrometry

Abstract

Rationale The observed drift times of monoatomic ions, including alkali metal ions and halide anions, are not fully consistent with their size. When the effect of mass is included through the Mason–Schamp equation, the deviation gets worse so that the trend of the experimental collisional cross‐sections becomes completely opposite to what is expected. This is attributed to the stronger local electric field around smaller ions. The strong electric field in the vicinity of a small ion leads to strong ion–neutral interactions and creates a drag force against ion motion. The smaller the ions, the stronger the interaction, because of the higher charge density. Methods In view of this, a modified equation is introduced to describe the relationship between the observed drift times or ion mobilities and the cross‐sections of small ions. Here, for small ions with high charge density, the experimental collision cross‐section is expressed as the effective collision cross‐section, Ωeff = σi(1 + α/ri3), that takes into account both intrinsic ion size, σi, and the ion–molecule interactions through a correction term of α/ri3, which is proportional to the charge density. Results A linear fit of the drift times of alkali metal ions and halide anions to the proposed equation showed relative deviations of <8.2%. The model successfully predicted the drift time of other small diatomic ions with reasonable error. Conclusions The proposed model can be used as a simple and efficient relationship in predicting the effective cross‐section of small ions.

Ion Mobility Mass Spectrometry for the Separation and Characterization of Small Molecules

“…The different working status can be divided into atmospheric pressure and reduced-pressure DTIMS . Based on the Mason–Schamp equation (eq ), the drift time measured by DTIMS under low-field conditions (2–10 Td; 1 Td = 10 –21 V·m 2 ) can be directly related to the CCS where K 0 is reduced mobility; z , charge state of the ion; e , elementary charge; N , number density of the drift gas; μ, reduced mass of the ion-neutral drift gas pair; k B , Boltzmann constant; and T , gas temperature.…”

“…38 Based on the Mason−Schamp equation (eq 1), the drift time measured by DTIMS under low-field conditions (2−10 Td; 1 Td = 10 −21 V•m 2 ) can be directly related to the CCS. 39 = ze Nk k T…”

Collision cross section measurement and prediction methods in omics

“…corresponding CCS value. 48,70 The ability to calculate the CCS using first principles is perhaps the greatest advantage of DTIMS.…”

“…Atmospheric pressure IM instruments are used mainly in security and forensic applications 69 . Based on the Mason–Schamp equation (Equation ), the drift time measured with DTIMS under low‐field conditions (2–10 Td; 1 Td = 10 −21 V/m 2 ) can be directly related to the corresponding CCS value 48,70 . The ability to calculate the CCS using first principles is perhaps the greatest advantage of DTIMS. where is the CCS, is the ion charge, e is the elementary charge, N is the density number of the drift gas, μ is the reduced mass of the ion‐neutral drift gas pair, k B is the Boltzmann constant, K is the mobility, and T is the gas temperature.…”