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  • Author or Editor: Ștefan Kreibik x
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To simulate front displacement through porous media (TLC plates) the effect of the axisymmetric alternating electric fields on liquids confined in capillary tubes was studied. This electric field causes liquid displacement through capillary tubes with great velocity, the main action being on the liquid column meniscus. Capillary tubes were used to enable understanding of the flow phenomena which occur in porous media. It was observed that the displacement velocity of the liquids confined in capillary tubes depended on the magnitude of the alternating electric field, the electric properties (conductivity, permittivity) of the liquids, the frequency of the current, and the tube material.In normal TLC practice, migration of the mobile phase through the layer is controlled by capillary forces. The velocity and migration distance of the mobile-phase front in prous media can be increased by application of an external electric field, an effect called by us ‘dielectroosmotic flow’ (DEOF), because of its similarity with electrokinetic phenomena. DEOF is observed on different TLC plates and paper strips developed with non-polar and polar solvents. As a result the separation of some compounds was improved. This method is a hybrid of electric forced flow and classical TLC; we have named it ‘planar dielectrochromatography’ (PDEC).A horizontal chromatographic chamber has been constructed and has been used to investigate the principles of planar dielectrochromatography, specifically the increase in front velocity and the dielectrophoretic force generated at granule level on different ready-to-use plates.

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Drawn by capillary forces the developing solvent (mobile phase) migrates through the thin layer (stationary phase) over a defined distance. During this process the sample is separated into fractions. In TLC the flow velocity of solvent is inversely related to the distance migrated by the solvent front. For this reason classical TLC is time-consuming and usually of low chromatographic efficiency. To improve the separation selectivity suitable transverse alternating electric fields have been used to modify the mobile phase front velocity and the migration distance of solutes. In this paper, a vertical chromatographic chamber has been constructed and has been used to study experimentally the principles of planar dielectrochromatography, particularly the increase of the mobile phase front velocity and of the dielectrophoretic force generated at granule level on alumina plates. The equicurrent and countercurrent arrangements of armatures were taken into consideration. Good results were obtained in the countercurrent arrangement, the resolution of solute separation being improved.

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Our previous work on planar dielectrochromatography (PDEC) showed that application of transverse electric fields to porous media can affect the separation. In experiments with a vertical PDEC (V-PDEC) chamber we observed that when a potential is applied some of the metal components used to construct the chamber generate electrical fields that can cause deviation of the spots from the vertical, disturbing the separation process. Starting from this statement we changed the geometry of the electric field used in the V-PDEC chamber to obtain the most favorable chromatographic results. For this we conceived and experimented with five types of armature (periodic lamellar strips, lamellar strips of variable widths, and spherical, conical, and Félici shape). The shapes of the armatures generate electric fields with different geometry that have characteristic effects on the chromatographic process. The experiments were performed on alumina plates with toluene as mobile phase and a lipophilic dye mixture as solutes. The tests showed that different electric field geometries which do not completely cross the glass support of the TLC plates can be used. This results in enhancement of the intensity of the electric field in the chromatographic layer, especially for the spherical and conical armatures. With these two armatures we achieved the best resolution of the solutes.

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JPC - Journal of Planar Chromatography - Modern TLC
Authors: Stefan Kreibik, Virginia Coman, Constantin Măruțoiu and Gheorghe Mihăilescu

Some of the physical properties of dielectric liquids are influenced by electric fields. Interesting results were obtained in this work by applying external alternating electric fields to chromatographic media (paper or TLC plates) filled with dielectric liquids. Capillary tubes were used to enable understanding of the flow phenomena which occur in porous media. It was observed that the displacement velocity of the liquids confined in the capillary tubes depended on the magnitude of the alternating electric field, the electric properties (conductivity, permittivity) of the liquids, the frequency of the current, the tube diameters, the shapes of the molecules, and the tube material. Evident electrokinetic effects (dielectroosmotic flow) were observed on silica gel layers and paper strips developed with non-polar and polar solvents. As a result the separation resolution of non-ionic compounds was improved. This method is a hybrid of electric forced flow and classical diffusion; we have named it planar dielectrochromatography.

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Drawn by capillary forces the developing solvent (mobile phase) migrates through the thin layer (stationary phase) over a defined distance. During this process the sample is separated into fractions. To improve separation selectivity suitable transversal electric fields have been used to modify the mobile phase front velocity and the migration distance. In this paper, a horizontal chromatographic chamber has been constructed and has been used to study experimentally the principles of planar dielectrochromatography, in particular the increase of the mobile phase front velocity and the dielectrophoretic force generated at granule level on different ready-to-use Macherey-Nagel plates.

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JPC - Journal of Planar Chromatography - Modern TLC
Authors: Virginia Coman, Ştefan Kreibik, Cristian Tudoran, Ocsana Opriş and Florina Copaciu

The solvent movement under the influence of different forces (hydrostatic, centrifugal, gravitational, electrical, etc.) is essential in most of the industrial and laboratory processes. In our previous works, we used an electrohydrodynamic device to simulate the movements of some chromatographic solvents that are utilized in dielectrochromatography. In those experiments, we used sinusoidal current as high voltage alternating current. The most significant results were obtained for the movement of non-polar solvents in comparison with the polar ones. The aim of this paper is to study the behavior of non-polar and polar solvents in the pulsating electric field. A special attention was paid to some physical properties of polar solvents under electric current pulse.

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Thin-layer chromatography (TLC) is a frequently used technique for the separation of polar and non-polar compounds, with a rich literature in this field. The aim of this paper is to show the TLC adsorbent quality of the Nevşehir volcanic tuff from Turkey that is compared with the Mirșid volcanic tuff from Romania. Our experimental investigations have shown that it is possible to achieve TLC plates coated with Nevşehir volcanic tuff, in natural occurrence, excepting grinding and sieving processes, which can be used for the separation of polar compounds, such as amino acids or food dyes. In the present article, modern techniques for characterizing the volcanic tuff powders, namely, X-ray diffraction, inductively coupled plasma–optical emission spectrometry, scanning electron microscopy, specific surface area and porosity measurements, and infrared spectroscopy, were used in addition to the chromatographic testing. TLC plates coated with Nevşehir tuff were prepared and tested at the separation of some amino acids, obtaining good results. Also, the impregnation of Nevşehir tuff with NaOH and NaCl, respectively, allowed the separation of some hydrophilic dyes. The obtained results have contributed to understand the chromatographic properties of the Nevşehir volcanic tuff. The TLC capability of the Nevşehir tuff can enlarge the pallete of inexpensive adsorbents with possible applications in the field of layer chromatographic separations.

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In our previous papers, we defined layer (planar) dielectrochromatography and we presented its fundamentals (theoretical aspects) such as the displacement of dielectric liquids under nonuniform external alternating electric fields (dielectroosmotic flow [DEOF] effect), the displacement of solute particles or polarized granules (dielectrophoresis [DEP] effect) in electric fields generated by armatures, the theoretical evaluation of the electric intensity generated in the stratified dielectrics, etc. Ready-to-use plates of alumina, silica gel, and cellulose were used for experiments. The obtained results have encouraged us to create our own TLC plates based on alumina enriched with compounds of high dielectric constants like barium sulfate, barium titanate, and titanium dioxide. In this paper, we present the preparation of seven plates containing increasing amounts (1, 2, 3, 4, 6, 8, 10) g of barium titanate in 35 g alumina, the methodology used in their characterization, as well as the obtained results and the perspectives of using this ingredient.

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