The evolution of capillary columns for liquid chromatography

The most recent interest in liquid chromatography (LC) is the trend of column miniaturization, i.e., in reducing the diameter of the columns. Capillary columns used in LC can be classified into three types, based on the flow resistance factor ϕ defined by Knox as $$\phi = d^2 /K_0 = \Delta Pd^2 /\eta \mu L$$ where d is a characteristic dimension of the column (the internal diameter of the tube of the mean particle diameter of the packing), Ko is the specific column permeability, ΔP is the pressure drop along the column, η is the viscosity and μ the linear velocity of the mobile phase, and L is the column length. The first type of LC capillary columns have a ϕ value of about 30. These are theopen-tubular columns, with a retentive film or layer on the inside wall of the column tubing the diameter of which should be as small as possible. The second type of LC capillary columns (ϕ≃100) is made by losely packing a glass tube which is then drawn to smaller diameter. During the drawing process the particles will be partially inbedded into the tube wall. Finally the third type of LC capillary columns (ϕ≃500–1000) are made by slurry-packing 0.2–0.5 mm ID fusedsilica capillary tubes under high pressure. These columns are well packed. In the paper various aspects of these columns are reviewed and their relative performance discussed.