Electrophoretic field gradient focusing: An investigation of the experimental parameters

Electric field gradient focusing with electro-osmotic flow to reduce analyte dispersion: Concept and numerical investigation

In proteomics, the need to precisely examine the protein compounds of small samples, requires sensitive analytical methods which can separate and enrich compounds with high precision. Current techniques require a minimal analysis time to obtain satisfactory compound separation where longer analysis time means better separation of compounds. But, molecular diffusion will create broadening of the separated compound bands over time, increasing the peak width, and thus reducing the resolution and the enrichment. Electric field gradient focusing (EFGF) is a separation technique, in which proteins are simultaneously separated and enriched by balancing a gradient electrostatic force with a constant hydrodynamic drag force. Because of this balance, analytes are continuously pushed back to their focusing point, limiting the time-dependent peak broadening due to molecular diffusion. Current EFGF techniques are however still suffering from peak broadening because of flow-profile inhomogeneities. In this paper, we propose to use AC electro-osmotic flow (AC EOF) to create a homogeneous flow in EFGF. The interference between the electric field gradient and the AC EOF was thoroughly analysed and the concept was validated using numerical simulations. The results show that a plug flow is obtained on top of a small, distorted boundary layer. While applying different DC electric fields in the electrolyte, a constant flow velocity can be obtained by including a DC offset to the electrodes generating the AC EOF. The plug flow is then maintained over the whole separation channel length, while an electric field gradient is applied. This way, the flow-induced contribution to peak broadening can be minimized in EFGF devices. By modelling the separation of green fluorescent protein (GFP) and R-Phycoerythrin (R-PE), it was shown that the peak width of separated compounds can be reduced and that the separation resolution can be improved, compared to current EFGF methods.

Recent advances in enhancing the sensitivity of electrophoresis and electrochromatography in capillaries and microchips (2008-2010)

Capillary electrophoresis has been alive for over two decades now; yet, its sensitivity is still regarded as being inferior to that of more traditional methods of separation such as HPLC. As such, it is unsurprising that overcoming this issue still generates much scientific interest. This review continues to update this series of reviews, first published in Electrophoresis in 2007, with an update published in 2009 and covers material published through to June 2010. It includes developments in the fields of stacking, covering all methods from field-amplified sample stacking and large volume sample stacking, through to ITP, dynamic pH junction and sweeping. Attention is also given to on-line or in-line extraction methods that have been used for electrophoresis.

Influence of the semi-permeable membrane on the performance of dynamic field gradient focusing

This paper is part of our continued effort to understand the underlying principles of dynamic field gradient focusing. In this investigation, we examined three problems associated with the use of a semi-permeable membrane. First, the influence of steric and ionic exclusion of current carrying ions through the membrane was examined. It was found that resistance to the transport of ions across the membrane resulted in a shallowing of the electric field profile and an increase in the size of the defocusing zone, which is where the slope of the electric field is reversed so that it disperses rather than concentrates solutes. These problems could be reduced by using a membrane with large pores relative to the size of the buffering ions and completely void of fixed charges. Next, a numerical simulation was used to investigate concentration polarization of protein onto the surface of the membrane. Due to the presence of a transverse electric field, species were pulled toward the membrane. If the membrane is restrictive to those species, a concentrated, polarized layer will form on the surface. The simulation showed that by decreasing the channel to a depth of 20 microm, the concentrated region next to the membrane could be reduced. Finally, it was found that changes in column volume due to loss of membrane structural integrity could be mitigated by including a porous ceramic support. The variation in peak elution times was decreased from greater than 20% to less than 3%.

Development of a membrane-less dynamic field gradient focusing device for the separation of low-molecular-weight molecules

Dynamic field gradient focusing uses an electric field gradient generated by controlling the voltage profile of an electrode array to separate and concentrate charged analytes according to their individual electrophoretic mobilities. This study describes a new instrument in which the electrodes have been placed within the separation channel. The major challenge faced with this device is that when applied voltages to the electrodes are larger than the redox potential of water, electrolysis will occur, producing hydrogen ions (H+) plus oxygen gas on the anodes and hydroxide (OH(-)) plus hydrogen gas on the cathodes. The resulting gas bubbles and pH excursions can cause problems with system performance and reproducibility. An on-column, degassing system that can remove gas bubbles "on-the-fly" is described. In addition, the use of a high capacity, low-conductivity buffer to address the problem of the pH shift that occurs due to the production of H+ on the anodes is illustrated. Finally, the successful separation of three, low-molecular-weight dyes (amaranth, bromophenol blue and methyl red) is described.

Simultaneous separation of negatively and positively charged species in dynamic field gradient focusing using a dual polarity electric field

Dynamic field gradient focusing (DFGF) utilizes an electric field gradient established by a computer-controlled electrode array to separate and concentrate charged analytes at unique axial positions. Traditionally, DFGF has been restricted to the analysis of negatively charged species due to limitations in the software of our voltage controller. This paper introduces a new voltage controller capable of operating under normal polarity (positive potentials applied to the electrode array) and reversed polarity (negative potentials applied to the electrode array) for the separation of negatively and positively charged analytes, respectively. The experiments conducted under normal polarity and reversed polarity illustrate the utility of the new controller to perform reproducible DFGF separations (elution times showing less than 1% run-to-run variation) over a wide pH range (3.08 to 8.5) regardless of the protein charge. A dual polarity experiment is then shown in which the separation channel has been divided into normal polarity and reversed polarity regions. This simultaneous separation of negatively charged R-phycoerythrin (R-PE) and positively charged cytochrome c (CYTC) within the same DFGF apparatus is shown.