Asymmetric electric field

Fig. 6. Schematic of a differential mobility spectrometer showing the principles of ion separation in a differential mobility spectrometry (DMS) drift tube. Ion paths are governed by both the asymmetric electric field and field dependence of mobility for an ion. The inset displays the asymmetric waveform of separation electric field used in the DMS drift tube. The waveforms shown are theoretical (top part) and actual or experimental (bottom part) used in these experiments.

The absence of splittings of X—F spin-spin couplings in XF4 species is similar to the observations in XF2 species (3.1.4) and is due to rapid relaxation caused by interaction of the quadrupole moment of Cl or Br, with the asymmetric electric field gradient about the central halogen. 

The first configuration of field-dependent mobility released from the former Soviet Union and introduced initially into North America was a cylindrical design as shown in Figure 6.5a in this early configuration, sample molecules were ionized and passed with the gas flow between two concentric tubes (the inner and outer electrodes). The asymmetric electric field, or separation field, was applied between these inner and outer electrodes. In later designs, the ions were separated from sample flow into a flow of purified air. 

An, X. Eiceman, G.A. Rasanen, R.-M. Rodriguez, J.E. Stone, J.A. Dissociation of proton bound dimers of ketones in asymmetric electric fields with differential mobility spectrometry and in uniform electric fields with ion mobility spectrometry, 2012. (submitted)... 

In 3.1, we discussed using asymmetric electric field to disperse ions in space by the difference between mobilities at high and low E. However, constructing a dispersive FAIMS (Figure 3.17) in parallel to TOF MS or DT IMS presents a daunting engineering challenge. 

This concludes our review of the fundamentals of FAIMS technology, narrowly defined as the use of asymmetric electric field to filter ions with a given difference between mobility in a gas at two unequal field intensities. In Chapter 5, we discuss several concepts falling within the broad definition of differential or nonlinear IMS (1.1) but distinct from FAIMS as currently practiced. 

While the FAIMS separation power for macroions may be greatly raised by reversible alignment of ion dipoles in asymmetric electric field (3.3.5), the extraction of absolute collision cross sections and thus ion geometries from such data has... 

In the one-body model it is supposed that the nucleus is spherical and the potential barrier has a simple form (O Fig. 2.35). In an exact theory, especially in the case of deformed nuclei, more realistic potential barriers must be taken into account. Furthermore the interaction of the emitted a particle with the asymmetric electric field of the deformed nucleus can also modify the a spectrum. 

Particles dispersed in an aqueous medium invariably carry an electric charge. Thus they are surrounded by an electrical double-layer whose thickness k depends on the ionic strength of the solution. Flow causes a distortion of the local ionic atmosphere from spherical symmetry, but the Maxwell stress generated from the asymmetric electric field tends to restore the equilibrium symmetry of the double-layer. This leads to enhanced energy dissipation and hence an increased viscosity. This phenomenon was first described by Smoluchowski, and is now known as the primary electroviscous effect. For a dispersion of charged hard spheres of radius a at a concentration low enough for double-layers not to overlap (d> 8a ic ), the intrinsic viscosity defined by eqn. (5.2) increases... 

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