Unstable High-Field Mobility of Runaway Ions

8 UNSTABLE HIGH-FIELD MOBILITY OF RUNAWAY IONS 

This book has started from the description of steady-state ion drift induced by constant as the key feature of IMS (1.3). This was contrasted with the ion acceleration in MS, and the boundary between the two regimes was drawn in terms of the number of ion-molecule collisions during the measurement that depends on the gas pressure and experimental timescale (1.3.2). Whereas continuous acceleration of 

Thus far, mnaway was predicted or observed only for atomic ions in atomic gases. For polyatomic ions and/or gases, the decrease of sD,(s) at high s required for mnaway appears less likely because of vibrational and/or rotational energy loss channels that are usually effective in the relevant s range (2.5 and 2.6.1). For polyatomic ions, field heating may cause dissociation at E/N below the mnaway threshold, leaving the issue moot. 

Runaway has not yet been seen in differential IMS and might not happen at P = 1 atm because of the electrical breakdown constraints. For example, even the lowest E/N for mnaway in known systems ( 40 Td) is somewhat above the breakdown thresholds for pertinent gases (He and Ne) at STP. However, mnaway that occurs at a reduced P in high-field IMS will certainly occur in differential MS. That would be manifested as a differential effect of exceptional strength and sensitivity to the maximum E/N used and some other instrument parameters. The potential utility of that regime makes it worthwhile to explore whether mnaway might still be possible for polyatomic ions despite the contrary factors outlined above. 

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