Results with an Apparatus for Pressurized Planar Electrochromatography

Abstract

Pressurized planar electrochromatography (PPEC) is a fast and efficient planar chromatographic technique. The mobile phase is driven by electroosmotic flow, while the system is pressurized in a manner that allows heat to flow between the sorbent layer and the pressurizing medium. The reproducibility of solute retention was not satisfactory in the initial report describing PPEC. In the current report, this reproducibility is improved by better control of several experimental variables. The pressure at which PPEC is performed is now free of drift, and the temperature at which the layer is preconditioned is maintained to within +/-1 degrees C. The best reproducibility of retention is obtained when the plate is soaked in the mobile phase for a defined time before each run. In the original prototype, the temperature of the sorbent layer was not controlled. In the present apparatus, water, at a constant temperature between 3 and 60 degrees C, is circulated through channels in the two die blocks that pressurize the layer. The highest efficiency is obtained at an intermediate temperature. This behavior is ascribed to high resistance to mass transfer at the lower temperatures and increased diffusion at higher temperatures. Efficiency, as measured by the number of theoretical plates, increases with increasing migration distance. The height equivalent of a theoretical plate diminishes with increasing migration distance, and values as low as 0.0106 mm are obtained under appropriate conditions. This extrapolates to 94 000 plates/m. Manual spotting was used in this report. Evidence is presented that substantially better efficiency would be obtained if the initial spot size were smaller. The efficiency of PPEC in its current form is illustrated by a chromatogram showing the separation of nine solutes in 2 min. PPEC was also performed with TLC plates in a back-to-back configuration, and this doubles the number of samples that can be simultaneously separated.

High performance stationary phases for planar chromatography

“…The plate height generally declines with increasing migration distance reaching a plateau or shallow region between about 35 and 75 mm [57,61,63,64]. The lowest observed plate heights for the plateau region fall into the range 10-15 m corresponding to H ≈ 2d p for high performance pre-coated octadecylsiloxane-bonded silica layers.…”

“…Electroosmosis is used to drive the mobile phase flow through the layer in pressurized planar electrochromatography and requires an aqueous buffer as a component of the mobile phase [23,[57][58][59][60][61][62]]. An electric field applied across the layer is used to control the mobile phase velocity which is independent of the solvent front migration distance.…”

“…The mobile phase velocity depends on the physicochemical properties of the mobile phase and the mobile-stationary phase interface and is generally higher for silica-based chemically bonded phases with a lower bonding density and is independent of the particle size. Problems associated with either excess heating of the layer or accumulation of liquid on the layer surface can be avoided by applying pressure to the layer, in a manner that allows heat to flow between the layer and the pressurizing mechanism, typically a special metal block and hydraulic press with some mechanism, such as circulating liquid, to control temperature [57]. In an open system liquid accumulates on the surface of the layer causing distortion of sample bands, and if too much heat is generated, the separation is terminated due to drying of the layer.…”

“…There are specific problems associated with sample application since the layer must be completely impregnated with the mobile phase at the start of the separation while samples are typically applied to the dry layer [57,63]. In addition, sample zones applied to the layer are generally too large to obtain optimum performance and cited plate height measurements are usually calculated by subtraction of the variance due to the sample application zone.…”

Microchip free flow planar reversed phase electrochromatography with monolithic stationary phase

“…With the further introduction of pressure in PEC, the above mentioned problems could be partially solved [8,9]. However, the efficiency might be lost since the flat mobile phase profiling is changed to parabolic one.…”