Performance of monolithic silica capillary columns with increased phase ratios and small-sized domains

Monolithic silica capillary columns for HPLC were prepared from tetramethoxysilane to have smaller sized domains and increased phase ratios as compared to previous materials, and their performance was evaluated. The monolithic silica columns possessed an external porosity of 0.65-0.76 and a total porosity of 0.92-0.95 and showed considerably higher performance and greater retention factors in a reversed-phase mode after chemical modification than columns previously reported. An octadecylsilylated monolithic silica column with the smallest domain size (through-pores of similar to 1.3 mu m and silica skeletons of similar to 0.9 mu m) showed a plate height of less than 5 mu m at optimum linear velocities (u) of 2- 3 mm/s in 80% acetonitrile for a solute having retention factors of similar to 1, and similar to 7 mu m at u) 8 mm/s. With a permeability similar to that of a column packed with 5-mu m particles, the monolithic silica columns were able to attain column efficiencies comparable to that of particulate columns packed with 2- 2.5-mu m particles, and showed performance in the "forbidden region" for the previous columns. The performance of the monolithic column can be compared favorably with that of a particle-packed column when 15 00030 000 or more theoretical plates are desired at a pressure drop of 20-40 MPa or lower. The increased homogeneity of the co-continuous structures, in addition to the small-sized domains, contributed to the higher performance as compared to previous monolithic silica columns.