Liquid Chromatography in Helically Coiled Open Tubular Columns

Abstract

Limitations on pressure, packing procedures, and extra-column broadening make it unlikely that the use of particles smaller than 2 μm in packed columns for liquid chromatography will be effective. We have therefore investigated the potential of alternative column types, in particular open tubular columns, for LC. When tubes are tightly coiled into helices, the radial mass transfer in the mobile phase is greatly enhanced by radial convection, called secondary flow, originating from centrifugal forces. A criterion is developed for the velocity up to which the classical description of axial dispersion in straight columns, after Golay, can be expected to hold. A correction for curvature effects is given for intermediate and high velocities. In principle, the corrected equations enable one to determine molecular diffusion constants from peak broadening within short times, even for macromolecular solutions. At high velocities a drastic decrease in peak broadening is expected, and this was experimentally confirmed for nonretained solutes. The separation speed increases at least quadratically with velocity such that 10³ to 10⁴ plates/sec can easily be obtained at pressures not higher than those presently needed for packed columns. The coating of a stable homogeneous layer of stationary phase onto the column wall has been only partially solved. Experience gained with various types of coating is discussed.