Electric-Field Driven Separations

In electric-field driven separations an electric field causes ions to travel through a matrix, such as a gas, liquid, or gel. The movement is retarded by frictional forces from interaction with the matrix and the ions almost instantly reach a steady-state velocity. This velocity depends on properties of both the sample molecules and the surrounding matrix. The two main types of electric-field driven separations are ion mobility spectrometry where the matrix is a gas and electrophoresis where the matrix is a liquid or gel. 

Electric Field driven Separations Phenomena and Applications, Sep. 

Electric-field-driven transport in media made of hydrophilic polymers with nanometer-size pores is of much current interest for applications in separation processes. Recent advances in the synthesis of novel media, in experimental methods to study electrophoresis, and in theoretical methodology to study electrophoretic transport lead to the possibility for improvement of our understanding of the fundamentals of macromolecular transport in gels and gel-like media and to the development of new materials and applications for electric-field-driven macromolecular transport. Specific conclusions concerning electrodiffusive transport in polymer hydrogels include the following. 

Similar chip design was also used for the offline interface with matrix-assisted laser desorp-tion/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). In contrast to inline ESI, MALDI provides for time flexibility. Separated compounds are collected on the plate and archived, thus enabling reanalyzing the sample later if so desired. Also, MS or MS/MS performance can be optimized independently in this implementation. An important part of the system is the noncontact electric field-driven deposition system mounted on an x,y,z stage, enabling computer-controlled deposition of sample spots across the plate. 

In addition to the wet and optical spectrometric methods, which are often used to analyse elements present in very small proportions, there are also other techniques which can only be mentioned here. One is the method of mass spectrometry, in which the proportions of separate isotopes can be measured this can be linked to an instrument called a field-ion microscope, in which as we have seen individual atoms can be observed on a very sharp hemispherical needle tip through the mechanical action of a very intense electric field. Atoms which have been ionised and detached can then be analysed for isotopic mass. This has become a powerful device for both curiosity-driven and applied research. 

While the external electrical field approach is a method directly modifying the zeta-potential of the capillary wall, it is not applicable with commercial apparatuses. The back-pressure technique, on the other hand, has the disadvantage that the flat electroosmotic flow profile is disrupted by superposition of a pressure-driven laminar flow profile hence, the efficiency of separation deteriorates. 

A variety of microscale separation methods, performed in capillary format, employ a pool of techniqnes based on the differential migration velocities of analytes under the action of an electric field, which is referred to as capillary electromigration techniques. These separation techniques may depend on electrophoresis, the transport of charged species through a medium by an applied electric field, or may rely on electrically driven mobile phases to provide a true chromatographic separation system. Therefore, the electric field may either cause the separation mechanism or just promote the flow of a solution throughout the capillary tube, in which the separation takes place, or both. 

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