Assessing the Impact of Drift Gas Polarizability in Polyatomic Ion Mobility Experiments

Due to the core assumptions
of kinetic theory and the drive toward
realizing reproducible gas-phase measurements, ion mobility experiments
are commonly conducted in the presence of an inert, neat buffer gas,
usually nitrogen or helium. Mixing drift gases in defined, static
ratios can provide useful information not only for optimizing the
separation of analytes but also for defining the interaction between
the ion and neutral particle. In a foundational effort, we seek to
validate the role of the drift gas polarizability on the observed
mobility of the ions by systematically mixing drift gases to discretely
access a range of bulk gas polarizabilities not given by pure drift
gases. Compared to historical efforts to probe the role of polarizability
on the ion–neutral collisional cross section where a linear
relationship was assumed, the data collected in the present effort
clearly illustrate a quadratic dependency of the ion–neutral
particle collision cross section and polarizability (R2 > 0.999). When translating these data into the mobility
dimension, we illustrate that the gas-phase mobility of polyatomic
ions conforms to Blanc’s law. These observations combined with
considerations related to Langevin’s polarization limit provide
an experimental mechanism to estimate to what degree an ion–neutral
interaction conforms to either the hard-sphere or induced-dipole model.
To support these observations, additional comparisons are made with
the respective reduced masses, polarizabilities, and mobilities of
ions in mixtures where different degrees of hard-sphere interactions
are present.