Traveling Wave IMS

The transit time t, needed for the ion to traverse the length of the drift tube L in the TW-IMS when the ratio c is low is proportional to the squares of mobility K and electric field intensity E, as opposed to linear scaling of the mobility with the electric field in linear IMS and DMS. 

As the propagating wave speed decreases, the ion transit velocity asymptotically approaches the wave speed. The resolving power of TW-IMS depends on mobility, scaling as in the low-c limit and less at higher c. This nonlinear dependence of the transit time on mobility affects the resolving power of TW-IMS, and it is claimed that near-optimum resolution is achievable over a approximately 300-400% range of mobilities.  

Bearing in mind that TW-IMS is usually operated at low drift gas pressure P, the mobility can be expressed in the low-field regime as 

The motion of ions in a buffer gas is governed by diffusive forces, the external electric field and the electrostatic interactions between the ions and neutral gas molecules. Ion-dipole or ion-quadrupole interactions, as well as ion-induced dipole interactions, can lead to attractive forces that will slow the ion movement, mainly due to clustering effects. The interaction potential can be calculated according to different theories, and three such approaches—the hard-sphere model, the polarization limit model, and the 12,4 hard-core potential model— were introduced here. Under 

An attempt to comprehensively, yet concisely, present an overview of the theory underlying the mobility of ions with regard to the linear IMS and DMS was made by Spangler. However, here we present the fundamental aspects of the forces that govern the motion of an ion under conditions that pertain to the practical IMSs. 

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Pringle SD, Giles K, Wildgoose JL, et al., An investigation of the mobility separation of some peptide and protein ions using a new hybrid quadrupole/travelling wave IMS/oa-ToF instrument, Int. J. Mass Spectrom. 2007 261(1) 1-12. 

Conventional IMS is often called drift tube IMS (DTIMS) because the constant electric field is commonly established in tubes where ions drift along the axis. However, implementations of conventional IMS vary and other designs have emerged in both research and commercial systems. Some, such as traveling wave IMS (TWIMS), acmally employ a time-dependent field, but that is for instrumental reasons and does not affect the separation parameters. 

Recently, two other approaches to IMS have been introduced commercially these are high field asymmetric waveform IMS (FAIMS) [2] and traveling wave IMS (TWIMS) [3]. 

Whereas high-field asymmetric waveform ion mobility spectrometry (FAIMS) is able to separate ions in a manner similar to IMS, the method cannot be considered an alternative to IMS in the context (elucidation of ion structure) discussed here, since electric fields employed in FAIMS are way above the low-field limit. " - However, two other newer developments, traveling-wave IMS and overtone ion mobility spectrometry (OMS), are worth mentioning here briefly. 

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