Capillary electrophoresis (CE) is a separation system based on the migration differences of charged analytes though a buffer-filled capillary. Microchip capillary electrophoresis (μCE) is based on the same principles as CE, but on a microchip. Microchip systems provide specific advantages when compared to traditional separation methods that make this a potentially valuable tool for pharmaceutical analysis.
A novel method is described for estimating pKa using indirect fluorescence detection on a glass microfluidic device. The method is based on the differences in electrophoretic mobility of the analyte as a function of the pH of the running buffer. Nine compounds were tested, including several of pharmaceutical importance, with pKa values from 10.3 to 4.6. Calculated pKa values agreed well with literature values obtained by traditional methods, differing not more than 0.2 from the literature value.
In an effort to transfer this system to a polymer microchip platform, a polyelectrolyte multilayer (PEM) coating system was developed for poly (methylmethacrylate) (PMMA) microchips. The multilayer system was prepared by layer-on-layer depositon of poly(diallydimethylammonium) chloride (PDAD) and polystyrene sulfonate (PSS). The performance of the PEM-PMMA microchip was compared to those of a standard glass microchip and a PEM-glass microchip in terms of electroosmotic flow and separating two fluorescent dyes. It was found that, with careful preparation, a PEM-PMMA microchip can be prepared that has properties comparable, and in some cases superior, to those of a standard glass microchip. However, transferring the pKa method to the PEM-PMMA microchip was limited due to detection problems.
The effect of a polyelectrolyte multilayer (PEM) coated capillary on the chiral separation of several pharmaceutical compounds using heparin as a chiral additive was also investigated. The effect of pH, ionic strength, and heparin concentration on the migration of pheniramine was investigated. In addition, the chiral separation of six other antihistamines and antimalarial compounds was examined. It was found that a PEM-coated capillary reduced analysis times for these compounds because of the increase in electroosmotic flow and improved the reproducibility of the method to < 1%.