PREPARATION OF OPEN-TUBULAR COLUMNS FOR REVERSED-PHASE HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY

A procedure for the preparation of a thin layer of silica gel with chemically bonded C18 moieties on the internal wall of fused-silica capillaries is developed. The preparation of the silica layer is based on the hydrolytic polycondensation and gelling of tetraethyl orthosilicate (TEOS) in a pH-fixed water-methanol solution. It is demonstrated that the most critical step of this method is the formation of a thin, homogeneous film of the pregelling solution of TEOS on the capillary wall, which is achieved by using a static technique. In a subsequent step, complete gelling of the film is achieved in a stream of nitrogen at 100-degrees-C. Finally, the bonding the C18 is carried out at elevated temperature. Some of the parameters controlling the porosity and thickness of the silica layer (such as preconditioning of the fused-silica wall, time and temperature of the pregelling process, and pH at which hydrolysis takes place) and the yield of the C18 bonding reaction are studied. It is demonstrated that although the method is developed on 50-mum-i.d. capillaries, it is easily modified for achieving 10- and 5-mum-i.d. columns. Values of reduced plate height around 0.4 were obtained for 10- and 5-mum-i.d.columns. Using 5-mum columns 0.5 X 10(6) plates m-1 and 1000 plates s-1 were obtained. The reproducibility of the method in terms of k' (RSD = 12-14%) and reduced plate height (RSD = 4-5%) from column to column is quite good. Some applications of these open capillary columns for the separation of PAHs are presented.