Ion mobility experiments

In a combined MALDI-ToF MS, ion mobility experiment and molecular modelling study, Baker and co-workers [120] studied the sequencing in copolymers of glycidyl methacrylate and butyl methacrylate, Figure 20, with different termination groups. 

Parent, D.C., Bowers, M.T., Temperature dependence of ion mobilities experiment and theory. Chem. Phys. 1981, 60, 257. 

To gather more detailed information on the serine octamer structure, ion mobility experiments (Box 4) were performed, which showed the cluster to be very com-pact. The experimental collision cross-section was determined in two separate experiments to be in the range of 187-191A. All non-zwitterionic serine octamer structures are calculated to have significantly larger theoretical cross-sections. Consequently, this experiment clearly indicates the serine octamer to contain zwitterionic serines. The additional electrostatic attraction between oppositely charged sites results in the quite compact size. A structure that is in line with these results is shown in Figure 7. 

Box 4 Ion Mobility Experiments An Approach to the Serine Octamer Structure... 

On the other extreme of the spectrum, as compared to the above applications, recent ion mobility experiments in Ar with several lanthanides (Eu, Gd, Tb, Dy, Fto, and Yb ) open up the possibility of examining intrinsic effects of the electronic configurations on the interactions with neutral species (Laatiaoui et al., 2012). The results indicated that Gd had a lower mobility than the remaining Ln that was attributed to the occupation of a d-orbital instead of an f-orbital in its GS. This is an interesting extension of the seminal... 

A quick comparison of classical ion mobility experiments with those in a gas jet can be summarized as follows In the classical ion mobility tubes ions are driven by the electric field and retarded by collisions with a stationary gas. In the combination of a gas jet with a potential barrier, ions are driven by the flowing gas and retarded by the electric field. The electric force and the frictional force by collisions are in competition in both cases (Figure 3.2). 

An ion mobility experiment requires a pulse of ions, so when using a continuous ion source (such as electrospray) it is necessary to trap ions and pulse them into the cell. Here we employed the pre-cell transfer hexapole HI (see Figure 4.1). This is achieved by placing a stopping voltage on the top hat lens (THl). To pulse ions out of the hexapole it is necessary to apply a pulsed voltage to THl, which will allow the stopping potential to be rapidly lowered for short periods of time at a set frequency. 

It is useful here to provide a brief overview of the fundamental operational principles that govern separations in both the drift tube and traveling wave mobility techniques. Since there are several other excellent contributions in this book that cover the basis for the drift tube and traveling wave ion mobility experiments, only concepts important for understanding how the measurements relate to analyte structure are developed here. 

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