Electric field, separations based concepts

The present chapter is devoted mainly to one of these new theories, in particular to its possible applications to photon physics and optics. This theory is based on the hypothesis of a nonzero divergence of the electric field in vacuo, in combination with the condition of Lorentz invariance. The nonzero electric field divergence, with an associated space-charge current density, introduces an extra degree of freedom that leads to new possible states of the electromagnetic field. This concept originated from some ideas by the author in the late 1960s, the first of which was published in a series of separate papers [10,12], and later in more complete forms and in reviews [13-20]. 

Hardness and softness as chemical concepts were presaged in the literature as early as 1952, in a paper by Mulliken [138], but did not become widely used till they were popularized by Pearson in 1963 [139]. In the simplest terms, the hardness of a species, atom, ion or molecule, is a qualitative indication of how polarizable it is, i.e. how much its electron cloud is distorted in an electric field. The adjectives hard and soft were said to have been suggested by D.H. Busch [140], but they appear in Mulliken s paper [138], p. 819, where they characterize the response to spatial separation of the energy of acid-base complexes. The analogy with the conventional use of these words to denote resistance to deformation by mechanical force is clear, and independent extension, by more than one chemist, to the concept of electronic resistance, is no surprise. The hard/soft concept proved useful, particularly in rationalizing acid-base chemistry [141]. Thus a proton, which cannot be distorted in an electric field since it has no electron cloud (we ignore the possibility of nuclear distortion) is a very hard acid, and tends to react with hard bases. Examples of soft bases are those in which sulfur electron pairs provide the basicity, since sulfur is a big fluffy atom, and such bases tend to react with soft acids. Perhaps because it was originally qualitative, the hard-soft acid-base (HSAB) idea met with skepticism from at least one quarter Dewar (of semiempirical fame) dismissed it as a mystical distinction between different kinds of acids and bases [142]. For a brief review of Pearson s contributions to the concept, which has been extended beyond strict conventional acid-base reactions, see [143],... 

Several recent reviews have discussed the fundaments and applications of PI concepts. Doble [16] has discussed the concept of a green reactor, for example, how process intensification could be achieved by microreactor technology using very high forces, ultra-high pressures, electrical fields, ultrasonics, surfactant-based separations, shorter diffusion and conduction pathways, flow field and fluid microstmcture interactions, and/or size-dependent phenomena. 

An entirely different concept in analytical separations is provided by capillary electrophoresis (CE) in which the flow of liquid is generated by electro-osmotic flow (EOF) driven by an external electric field. The major advantage of this approach is the essentiaUy flat plug flow profile that leads to intrinsically more narrow elution peaks than the parabolic flow profiles characteristic of pressure-driven viscous flows. In capillary zone electrophoresis (CZE) separation is achieved by superimposing the different electrophoretic mobihties of the solutes on to the EOF. In electrochromatography the separation is achieved as in packed column HPLC but using an EOF to generate flow of the mobile phase past the stationary phase particles. The importance of these EOF-based techniques is their application to miniaturized devices, lah-on-a-chip or micro total analysis systems. Such devices that can be directly interfaced to a mass spectrometer via an ESI source are currently under intense development. 

The working principle of a third type of mass filter, the ion trap detector (ITD), is illustrated in Fig. 6. It operates like the quadrupole based on the concept that the trajectories of ions, traveling in an oscillating electric field, become influenced by the wave frequency. Dissimilar to the quadrupole filter, however, the ion separation of the ITD occurs in a closed cavity, where the dc and ac currents applied to a ring electrode and an end cap of the cell set up the electric field. The molecules enter the cell in which they are ionized by an electron beam, and under the control of the given electric field the ions are forced to move in an orbit within the space of the cell. When the ac or dc potential is changed, the motion of some ions becomes unstable. 

Some arguments that imply limitations on the concept of molecular multipole moments due to the requirements of gauge invariance are presented. The discussion is based on a pair of polarization field vectors which are natural generalizations of multipole series. A class of transformations that mix some of the components of the polarization field vectors, so spoiling a separation into electric and magnetic types, is identified. The results are related to the gauge-invariant transition amplitudes. 

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