Separation using planar chromatography with electroosmotic flow

Planar chromatography with electroosmotic flow is used to separate either a mixture of dyes using 80% aqueous ethanol as the mobile phase or a mixture of miscellaneous compounds using 45% aqueous acetonitrile as the mobile phase. Both mobile phases are 1.0 mM in N-[tris(hydroxymethyl)methyl]-3-aminopropanesulfonic acid (TAPS) buffer. Separations using this technique are faster and more efficient than the same separations by conventional TLC. The respective relationships between migration velocity and applied potential, and between analysis time and distance migrated, are presented.     

The efficient analysis of neutral and highly polar pharmaceutical compounds using reversed-phase and ion-exchange electrochromatography

Summary Highly polar compounds, such as tricyclic antidepressants are very difficult to analyse by electrochromatography with conventional reversed-phase silica-based chromatography packings. At high pH (high electroosmotic flow) the test compounds were not eluted from a Spherisorb ODS-1 column, as a result of strong interactions between the analyte and residual silanol groups on the packing material. By lowering the pH of the mobile phase, whereby the highly basic test compounds become positively charged, it was possible to elute the samples but only with severe peak tailing. Because the electroosmotic flow was greatly reduced, the elution time for neutral species became prohibitively long. By use of a strong cation exchanger in place of C₁₈-silica it was found possible to resolve a series of highly basic compounds with very high efficiencies, with very little evidence of peak tailing. Plate numbers in excess of 8 million per metre were observed.     

Variables that Affect Performance in Planar Chromatography with Electroosmotic Flow

JPC – Journal of Planar Chromatography – Modern TLC - Planar electrochromatography (PEC) is a mode of planar chromatography in which the mobile phase is driven by electroosmotic flow....     

Planar chromatography with a forced flow of the mobile phase produced by an electroosmotic pump on a plate with a closed sorption layer

A new variant of thin-layer chromatography (TLC) with a forced flow of the mobile phases produced by an electroosmotic pump on a plate with a closed sorption layer is proposed. The new variant of TLC is tested for separation of dyes with ethanol and acetonitrile as mobile phases. It is established that the new method is 15–33% faster compared to traditional TLC and provides the same or higher efficiency.     

A variant of circular thin-layer chromatography with a forced flow of the mobile phase produced by an electroosmotic pump

A new variant of circular thin-layer chromatography (TLC) with a closed sorption layer in which a forced flow of the mobile phase was produced and controlled by an external electroosmotic pump placed at the center of a TLC plate was suggested. It was shown experimentally that the method operated properly and could be used in planar chromatography.     

Evaluation of ODS-AQ stationary phase for use in capillary electrochromatography

The aim of this study was to evaluate the applicability of ODS-AQ packing material as a stationary phase in capillary electrochromatography (CEC). The electroosmotic flow created on an ODS-AQ stationary phase was measured at different mobile phase compositions and at different column temperatures. It was observed that the electroosmotic flow generated in the column increased by 50% when the temperature of the system was raised from 20 degrees C to 60 degrees C, while all other conditions were kept constant. The electroosmotic flow produced by the ODS-AQ stationary phase was found to be comparable to the flow generated in a column packed with Nucleosil bare-silica material. In addition, a set of polar compounds (D-lysergic acid diethylamide derivatives) was utilized to determine the influence of temperature and mobile phase composition on their chromatographic behavior on an ODS-AQ stationary phase in a CEC mode. A linear relationship between the solute retention factor and column temperatures was seen over the temperature range studied (20 degrees C to 60 degrees C). A quadratic function was used to describe the changes in the solute retention factors with variation of acetonitrile concentration in the mobile phase.     

Planar electrochromatography

Recent developments in planar electrochromatography (PEC) in both the normal-phase and the reversed-phase modes, and at both atmospheric and elevated pressure, are reviewed. Other forced-flow techniques in planar chromatography are also briefly covered. Mobile phase migration in PEC is primarily due to electroosmotic flow, which is controlled by the applied electric field. Capillary mediated flow is an important secondary contributor to migration, and occurs because the layer is unsaturated as a consequence of liquid evaporating from the layer due to Joule heating. The magnitude of the electric field and the concentration of ions in solution are important variables that control both electroosmotic flow and Joule heating. Separations are faster and more efficient than those obtained by conventional planar chromatography, provided appropriate experimental conditions are selected. With inappropriate conditions, either mobile phase accumulates on the surface of the sorbent layer, or Joule heating causes excessive evaporation. The former results in poor spot shape, and the latter can cause the layer to dry. Good separations are obtained when there is a balance between these two effects. The problems associated with mobile phase accumulating on the surface of the sorbent layer, and with excessive evaporation of mobile phase, do not occur with pressurized planar electrochromatography. This technique is performed at high pressure, under conditions that allow heat to be removed form the sorbent layer. This allows the use of a substantially higher electric field than in PEC, and results in a high mobile phase flow rate.     

Polyrotaxane approach for synthesis of continuous beds for capillary electrochromatography

The polyrotaxane formation approach was evaluated for synthesis of continuous beds for capillary electrochromatography. This approach has the advantage of generating diverse electroosmotic and chromatographic properties without chemical reactions. The polyrotaxane derivatized continuous beds were formed adding the macrocyclic compounds to the solution of neutral acrylic monomers and crosslinker prior to the initiation of the polymerisation. Cationic and anionic derivatives of beta-cyclodextrin were used as macrocyclic compounds. Investigation of the electroosmotic properties indicated a template directed and enthalpy controlled self-assembly of the polyrotaxanes during the polymerisation of the continuous beds. This process was monomer-composition dependent and favored by the hydrophobicity of the polymeric skeleton. The morphology of the continuous beds was evaluated using high-resolution optical microscopy with CCD camera and atomic force microscopy. Reversed-phase capillary chromatography driven by electroosmosis, originating from the polyrotaxane structure, was performed using several test mixtures. Not primarily designed for the chiral chromatography the polyrotaxane derivatized continuous beds demonstrated enantioselective separation of D,L-metoprolol. The stability of the polyrotaxane derivatized continuous beds was tested. The beds demonstrated reproducible electroosmotic properties in the range from pH 4 to pH 9 (RSD=0.69%).     

十二烷基键合氧化锆填充电色谱柱的研究

以十二烷基键合氧化锆(C12-ZrO2)作为固定相,制备了填充毛细管电色谱(CEC)柱,较为系统地研究了流动相条件对电渗流的影响、填充CEC柱的稳定性、碱性与中性化合物的保留与流动相pH值和有机溶剂含量的关系。C12-ZrO2固定相填充CEC柱在pH3～11．7范围内具有极好的稳定性;利用磷酸盐与氧化锆表面之间较强的相互作用,能够有效解决传统硅胶键合烷基固定相在有机溶剂含量低的流动相条件下不稳定的问题;同时吸附磷酸盐的固定相表面使得在更宽的流动相pH值范围内CEC柱有足够的电渗流,进一步拓宽CEC的应用领域。     

Capillary Electrochromatography with Monolithic Poly(styrene‐co‐divinylbenzene‐co‐methacrylic acid) as the Stationary Phase

A new kind of monolithic capillary electrochromatography column with poly(styrene‐co‐divinylbenzene‐co‐methacrylic acid) as the stationary phase has been developed. The stationary phase was found to be porous by scanning electron microscopy and the composition of the continuous bed was proved by IR spectroscopy to be the ternary polymer of styrene, divinylbenzene, and methacrylic acid. The effects of operating parameters, such as voltage, electrolyte, and organic modifier concentration in the mobile phase on electroosmotic flow were studied systematically. The retention mechanism of neutral solutes on such a column proved to be similar to that of reversed‐phase high performance liquid chromatography. In addition, fast analyses of phenols, chlorobenzenes, anilines, isomeric compounds of phenylenediamine and alkylbenzenes within 4.5 min were achieved.     

Some considerations concerning the composition of the mobile phase in capillary electrochromatography

In capillary electrochromatography (CEC) the propulsion of the mobile phase is effected by electroosmosis. The velocity of the electroosmotic flow is dependent on surface properties of the stationary phase and on bulk properties of the mobile phase. Therefore, in CEC the optimization of the mobile phase composition must take more factors into account than in pressure-driven LC. In this paper, the impact of the electrolyte concentration in the mobile phase and of the volume fraction of the organic mobile phase constituent on the velocity of the electroosmotic flow and on the chromatographic efficiency is investigated for CEC with capillaries packed with octadecylsilica gel. Bias of the data by an open section of the capillary has been excluded by employing completely packed capillaries and detection in a packed section. Acetonitrile as organic constituent of the mobile phase is compared to other possible organic modifiers (polar organic solvents) concerning influence on velocity of the electroosmotic flow and retention of solutes.     

Separation of enantiomers by nanoliquid chromatography and capillary electrochromatography using a bonded cellulose trisphenylcarbamate stationary phase

A cellulose trisphenylcarbamate-bonded chiral stationary phase was applied to nano-liquid chromatography (nano-LC) and capillary electrochromatography (CEC) with nonaqueous and aqueous solutions as the mobile phases. Several chiral compounds were successfully resolved on the prepared phase by nano-LC. The applicability of nonaqueous CEC on a cellulose derivative stationary phase was investigated with the organic solvents methanol, hexane, 2-propanol, and tetrahydrofuran (THF) containing acetic acid, as well as triethylamine as the mobile phases. Enantiomers of warfarin and praziquantel were baseline-resolved with plate numbers of 82,300 and 38,800 plates/m, respectively, for the first eluting enantiomer. The influence of applied voltage, concentration of nonpolar solvent, apparent pH, and buffer concentration in the mobile phase on the electroosmotic flow (EOF) and the mobility of the enantiomers was evaluated. Enantioseparations of trans-stilbene oxide and praziquantel were also achieved in aqueous CEC with plate numbers of 111,100 and 107,400 plates/m, respectively, for the first eluting enantiomer. A comparison between nonaqueous CEC and aqueous CEC based on a cellulose trisphenylcarbamate stationary phase was discussed. Pressure-assisted CEC was examined for the chiral separation of praziquantel and faster analysis with high enantioselectivity was acquired with the proper pressurization of the inlet vial.     

A Modified Device for Pressurized Planar Electrochromatography and Preliminary Results with On-Line Sample Application

Pressurized planar electrochromatography (PPEC) is a separating technique in which an electric field is applied to force the mobile phase movement through a porous media (electroosmotic effect). High separation efficiency, fast separations and changes in separation selectivity in comparison to liquid chromatography, especially thin layer chromatography (planar chromatography, TLC), are features of this technique. Constructional methodological challenges to PPEC are obstacles to its development and application in laboratory practice. In this article, an attempt to overcome the challenges related to device construction and sample application/injection is described. The introduced device enables both prewetting of the adsorbent layer and electrochromatogram development with a single PPEC device. It also enables simultaneous application/injection of six samples on a chromatographic plate in a stream of the mobile phase (on-line application/injection). In addition, the PPEC chamber was equipped with a thermostat. The device is characterized by an impressive throughput in comparison to the other planar technique, TLC/HPTLC. Although the developed device still needs improvement, it is, in our opinion, a considerable step toward possible automation of this planar separation technique.     

Enantiomeric separation of acidic compounds of pharmaceutical interest by capillary electrochromatography employing glycopeptide antibiotic stationary phases

Enantiomeric separation of some selected acidic compounds of pharmaceutical interest belonging to the group of non-steroidal anti-inflammatory drugs were separated by capillary electrochromatography employing silica based glycopeptide antibiotic stationary phases, namely vancomycin or a teicoplanin derivatives (Hepta-Tyr). The vancomycin stationary phase allowed to achieve the chiral resolution of some racemic studied compounds only using mobile phases containing ammonium formate at a relatively low pH 2.5-3.5 and acetonitrile. Employing the teicoplanin derivative stationary phase, good enantiomeric resolution was achieved eluting with mobile phases containing sodium phosphate pH 6-acetonitrile. Enantiomers were moved to the detector because a relatively high reversed electroosmotic flow (due to the positive charge of the stationary phase) and to the electrophoretic mobility of analytes.     

毛细管反相电色谱法分离行为的研究

对乙睛-水-磷酸二氢销体系毛细管反相电色谱分离行为进行了研究。采用柱上紫外检测,在75μmi.d.×30cm的毛细管ODS（3μm）填充柱上获得了小于2.0的折合培板高度。同时还研究了乙睛的比例、电解质的浓度和电场强度等因素对电渗流和往效的影响。     

混合固定相电中溶质的输运特征

 以离子交换和反相固定相构成的混合固定相电中 ,溶质迁移在受到疏水、离子交换作用的同时 ,对于带电溶质而言 ,还受到电泳迁移的影响。根据离子独立迁移原理 ,结合过程中的多种相互作用 ,得到了描述溶质表观迁移速率与其各形态迁移速率、各种相互作用之间相互关系的理论表达式 ;讨论了混合模式电中流动相的 pH及其中的有机调节剂浓度、混合固定相配比等对电渗流的影响及不同形态溶质在柱内的输运特征。结果表明 ,在电中采用混合固定相可以在较大的 pH和有机调节剂浓度范围内得到较强且稳定的电渗流。     

Electroosmotic Circular Thin-Layer Chromatography

A new version of electroosmotic thin-layer chromatography, circular electroosmotic chromatography, was suggested and a prototype apparatus for its implementation was designed.     

电渗泵中电渗流的控制

电渗泵是利用载流的电渗驱动原理，结合电色谱(EC)、毛细管电泳(CE)、液相色谱柱技术制作的输液微泵，是新颖的流体和样品输送技术。电渗泵中电渗流(EOF)控制方法与EC和CE等文献中的电渗流控制方法是相同的。本文对EC和CE等文献中有关EOF控制方法作了总结，并对电渗泵的研究现状和应用作一些前瞻分析。     

Separation of basic, acidic and neutral compounds by capillary electrochromatography using uncharged monolithic capillary columns modified with anionic and cationic surfactants

A mode of capillary electrochromatography (CEC), based on the dynamical adsorption of surfactants on the uncharged monolithic stationary phases has been developed. The monolithic stationary phase, obtained by the in situ polymerization of butyl methacrylate with ethylene dimethacrylate, was dynamically modified with an ionic surfactant such as the long-chain quaternary ammonium salt of cetyltrimethylammonium bromide (CTAB) and long-chain sodium sulfate of sodium dodecyl sulfate (SDS). The ionic surfactant was adsorbed on the surface of polymeric monolith by hydrophobic interaction, and the ionic groups used to generate the electroosmotic flow (EOF). The electroosmotic mobility through these capillary columns increased with increasing the content of ionic surfactants in the mobile phase. In this way, the synthesis of the monolithic stationary phase with binary monomers can be controlled more easily than that with ternary monomers, one of which should be an ionic monomer to generate EOF. Furthermore, it is more convenient to change the direction and magnitude of EOF by changing the concentration of cationic or anionic surfactants in this system. An efficiency of monolithic capillary columns with more than 140000 plates per meter for neutral compounds has been obtained, and the relative standard deviations observed for to and retention factors of neutral solutes were about 0.22% and less than 0.56% for ten consecutive runs, respectively. Effects of mobile phase composition on the EOF of the column and the retention values of the neutral solutes were investigated. Simultaneous separation of basic, neutral and acidic compounds has been achieved.     

非电荷型毛细管电色谱原位柱的色谱行为研究──阴离子表面活性剂的动态改性

 采用原位聚合的方法在毛细管中合成了非电荷型连续床电色谱原位柱 ,通过在电色谱流动相中加入阴离子表面活性剂十二烷基硫酸钠 (SDS)进行动态改性使其产生电渗流 ,考察了SDS浓度及有机改性剂浓度等因素对电渗流的影响。此类连续床柱制备容易 ,柱效可达 14万理论塔板 /m ,在不同的操作条件下有良好的稳定性 ,连续 10次运行 ,其死时间t0 与保留时间的精密度分别为 0 .2 2 %和 <0 .5 6 %。