Ion Radii and the Mass-Mobility Correlation in Homologous Series

1 Ion Radii and the Mass-Mobility Correlation in Homologous Series 

One test of the three models is to compare the experimentally measured reduced mobility values and cross sections of ions in a linear IMS within a homologous series with the predicted values. In such a series, the ion radius strongly affects the distance of approach to the neutral gas molecule and, correspondingly, the interaction parameters. In such a homologous series of ions, the ion radius is assumed to vary approximately as the cube root of its mass. 

In the rigid sphere model, the sum of the radii of the ion and the neutral molecule d will increase slightly as the chain length and ion mass in the homologous series increase. In the polarization limit model, the ion size is totally neglected, whereas in the hard-core potential model, (the minimum in the interaction potential) depends on the ion mass, as shown in Equation 10.22  

Another factor that affects the ion radius and the ion mass arises from the observation that ions drifting in polarizable gases, especially at low temperatures, tend to form clusters with the drift gas molecules. Thus, the mass of the ion may be incremented from its original mass m by clustering with n neutral molecules, each having a mass of M, such that its effective mass m fis altered per Equation 10.24  

It should be noted that n here is the average of the number of drift gas molecules clustered on the core ion drifting through the buffer gas and is not necessarily a natural number. This mass increment will also affect r obtained in Equation 10.23 when the effective mass is substituted in the equation, giving Equation 10.25  

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