Traveling Wave Drift Tubes

Li, H. Giles, K. Bendiak, B. Kaplan, K. Siems, W.E. HiU, H.H., Resolving structural isomers of monosaccharide methyl glycosides using drift tube and traveling wave ion mobihty mass spectrometry. Ana/. Chem. 2012, 84, 3231—3239. 

As described in previous chapters, there are many different types of ion mobility methods. These include drift tube ion mobility spectrometry (DTIMS), traveling wave ion mobility spectrometry (TW-IMS), differential mobility spectrometry (DMS), differential mobility analysis (DMA), and aspiration ion mobility spectrometry (alMS). All of these IMS methods have been interfaced to MSs. 

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. 

Wildgoose, J.L. Giles, K. Pringle, S.D. Koeniger, S.L. Valentine, S.J. Bateman, R.H. Clemmer, D.E. A comparison of travelling wave and drift tube ion mobility separations. Proc. 54th ASMS Conference on Mass Spectrometry and Allied Topics, Seattle, WA, May 28-June 1, 2006, ThP 64.AQ... 

If there are no microwaves on the slow-wave transmission Hne of a traveling wave tube, the velocity of an electron in the electron beam emitted from the electron gun is constant and uniform. As seen from Fig. 6.1, there is practically no longitudinal DC electric field in the transmission line section. Therefore, the electrons are just drifting with the speed given by the electron gun, which is... 

The catcher of a two-cavity klystron amplifier can be replaced by the slow-wave structure of a traveling wave tube, as shown in Fig. 6.24(b). This type of tube is termed the twystron (Ishii, 1989). The slow-wave structure provides a broader frequency bandwidth than a regular two-cavity klystron. A microwave input fed into the buncher cavity produces velocity modulation to the electron beam. The electron beam is bunched while drifting, and the bunched electrons induce a microwave voltage in the microwave slow-wave structure. The electron beam is focused by the use of longitudinally applied magnetic flux density B. 

Ion-mobility may be implemented in IMS-MS systems in the form of (a) drift tubes, as already discussed [138, 139, 144], (b) differential ion-mobility or high-field asymmetric waveform ion mobility spectrometiy (FAIMS) devices [145-147], and (c) traveling-wave ion guide devices [44,148]. Tandem IMS-IMS-MS systems... 

The initial setup was developed into a hybrid quadrapole-ion-mobility-TOF instrument [148], The collision cell region of this instrument features three traveling-wave stacked ring ion guides, of which the middle one is used as ion-mobility drift tube and the other two may be used as collision cell, when applicable. The 185 imn long IMS part is operated at pressures up to 1 mbar with up to 200 ml/min Ar whereas the collision cells are 100 mm long and operated at 10 mbar with up to 10 ml/min gas [148]. The system can be used for a wide variety of applications. 

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. 

In the introduction to this chapter a combustion wave was considered to be propagating in a tube. When the cold premixed gases flow in a direction opposite to the wave propagation and travel at a velocity equal to the propagation velocity (i.e., the laminar flame speed), the wave (flame) becomes stationary with respect to the containing tube. Such a flame would possess only neutral stability, and its actual position would drift [1], If the velocity of the unbumed mixture is increased, the flame will leave the tube and, in most cases, fix itself... 

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