Models for Ion-Neutral Interactions

In addition to the diffusive forces and effects of an external electric field mentioned, ion motion in an IMS drift tube is affected by the electrostatic interactions between the ion and the gas molecules of the supporting atmosphere. The electron cloud surrounding the neutral gas molecule is polarized by the ion, thus inducing a dipole moment in the neutral molecule. This results in an electrostatic interaction between the ion and the neutral molecule the ion-induced dipole effect. Furthermore, molecules that have permanent dipole or quadrupole moments will be attracted to the ion through ion-dipole or ion-quadrupole interactions. 

Several models have been proposed to account for the overall effect of these three forces on the motion of the ion, and some of the classical models are discussed here in brief, and their usefulness in predicting the mobility of polyatomic ions in different drift gases is examined. Two simple models are considered first the rigid sphere model and the polarization limit model. Next, a more refined yet relatively simple-to-use model is described in which a 12,4 hard-core potential represents the ion-neutral interaction. The more complex three-temperature model is not discussed because ions in linear IMS are traditionally regarded as thermalized. This is the one-temperature assumption, in which ion temperature is assumed to be equal to the temperature of the drift gas. 

The critical test of any model is the capability to reproduce experimentally observed correlations between the mass of an ion and its mobility and between temperature and mobility, as well as to predict the mobility coefficients of ions in different gas atmospheres accurately. Normally, the success of a model is examined with relatively simple systems such as a homologous series of ions. For example, one such series is that in which the only changes arise from the addition of methylene groups in the ion with minimal changes in ion size and internal charge distribution. A complication in this discussion is the limited number of such tests with ions and gases representative of analytical IMS conditions. 

Most of the early published investigations on models of ion mobility were made by physicists in relatively simple systems, mainly those in which monoatomic ions drifted through an inert monoatomic or diatomic neutral gas. - - - This is evident in Table 1 given in Appendix 1 of Reference 4. As this chapter is concerned with polyatomic ions drifting through polar or polarizable gases, especially air, there are not many detailed experimental and theoretical studies that can be cited as relevant.  

FRAME 10.1 SOME EXAMPLES FOR NEUTRAL-NEUTRAL COLLISION THEORY  

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