Synthesis of spherical porous silicas in the micron and submicron size range: challenges and opportunities for miniaturized high-resolution chromatographic and electrokinetic separations

Classical silica technology has reached its limit with respect to an ultima te minimum particle size of about 2 mu m in diameter. Here, a novel process is presented which allows one to synthesize porous silica beads and contro l their particle diameter in situ, within the range of 0.2-2.0 mu m. As a r esult, no sizing is required and losses of silica are avoided. Furthermore, the process enables one to control in situ the pore structural parameters and the surface chemistry of the silica beads. Even though surface funtiona lized silicas made according to this process can principally be applied in fast HPLC the column pressure drop will be high even for short columns. In addition, the column efficiency, expressed in terms of the theoretical plat e height is about H similar to 2d(p) in the best case and limited by the A and C term of the Van Deemter equation. In other words the gain in total pr ate number when using 1-2 mu m silica beads in short columns is minimal as compared to longer columns packed with 5 mu m particles. Capillary electroc hromatography (CEC) as a hybrid method enables the application of micron si ze as well as submicron size particles. This consequently enhances column e fficiency by a factor of 5-10 when compared to HPLC. The use of short CEC c olumns packed with submicron size silicas provides the basis for fast and e fficient miniaturized systems. The most significant feature of CEC as compa red to HPLC is that the former allows one to resolve polar and ionic analyt es in a single run. An alternative method for miniaturization is capillary electrophoresis (CE) which generates extremely high efficiencies combined w ith fast analysis. Its application, however, is limited to ionic substances . (C) 2000 Elsevier Science B.V. All rights reserved.