Silicate polymerization for the preparation of bed-retention frits in capillary electrochromatography

Bubble formation, which is associated with bed-retention frits, is a critical experimental problem in capillary electrochromatography systems. In this investigation, porous silica frits were prepared via spot-heating of a silicate solution, and the effects of several experimental parameters on their performance were studied. The optimal sodium silicate concentrations were 10.8% and 5.4% (w/v) for outlet and inlet frits, respectively. The heating times were 5-6 s for outlet frits and < 1 s for inlet frits. Under optimized conditions, outlet frits were 75 microns (+/- 12 microns) and the heat treatment did not make the capillary fragile at the frit location. Bubble formation was affected by frit length, density, and silanization of the frits with trimethylchlorosilane. Packed capillaries with optimized frits were used successfully in a commercial CE instrument over a normal working day without pressurization, at relatively high ionic strengths (10 mM), and over a wide range of acetonitrile compositions (20%-80%). Currents were also stable for > or = 3 h under very high current (27 microA) conditions. As part of this study, the efficiency and reproducibility of packed capillaries were also briefly evaluated.