Monolithic silica rod columns for high-efficiency reversed-phase liquid chromatography

Chromatographic properties of a new type of monolithic silica rod columns were examined. Silica rod columns employed for the study were prepared from tetramethoxysilane, modified with octadecylsilyl moieties, and encased in a stainless-steel protective column with two polymer layers between the silica and the stainless-steel tubing. A 25 cm column provided up to 45,000 theoretical plates for aromatic hydrocarbons, or a minimum plate height of about 5.5 μm, at optimum linear velocity of ca. 2.3 mm/s and back pressure of 7.5 MPa in an acetonitrile–water (80/20, v/v) mobile phase at 40 °C. The permeability of the column was similar to that of a column packed with 5 μm particles, with KF about 2.4 × 10−14 m2 (based on the superficial linear velocity of the mobile phase), while the plate height value equivalent to that of a column packed with 2.5 μm particles. Generation of 80,000–120,000 theoretical plates was feasible with back pressure below 30 MPa by employing two or three 25 cm columns connected in series. The use of the long columns enabled facile generation of large numbers of theoretical plates in comparison with conventional monolithic silica columns or particulate columns. Kinetic plot analysis indicates that the monolithic columns operated at 30 MPa can provide faster separations than a column packed with totally porous 3-μm particles operated at 40 MPa in a range where the number of theoretical plates (N) is greater than 50,000.