How to utilize the true performance of monolithic silica columns

Ways of utilizing the true separation efficiency of monolithic silica (MS) columns were studied. The true performance of MS columns, both regular‐sized (rod‐type clad with PEEK resin, 4.6 mm ID, 10 cm) and capillary sized (in 100 or 200 μm fused silica capillary, 25–140 cm) was evaluated by calculating the contribution of extra‐column effects. HETP values of 7–9 μm were observed for solutes having retention factors (k values) of up to 4 for rod columns and up to 15 for a capillary column. The high permeability of MS columns allowed the use of long columns, with several connected together in the case of rod columns. Narrow‐bore connectors gave good results. Peak variance caused by a column connector ranges from 50 to 70% of that caused by one rod‐type column for up to three connectors or four columns in 80% methanol, but the addition of a 4th or 5th connector to add a 5th and 6th column, respectively, caused a much greater increase in peak variance, especially for long‐retained solutes, which is greater than the variance caused by one rod column. Rod columns seem to show slightly lower efficiency at a pressure higher than 10 MPa or so. The use of acetonitrile‐water as a mobile phase better preserved the ability of individual rod columns to generate up to 100,000 theoretical plates with 14 columns connected. Methods for eliminating extra‐column effects in micro‐HPLC were also studied. Split injection and on‐column detection resulted in optimum performance. A long MS capillary measuring 140 cm produced 160,000 theoretical plates. The column efficiency of a capillary column was not affected by the pressure, showing advantages over the rod columns that exhibited peak broadening caused by connectors and pressure.