INTRAPARTICLE FLOW AND PLATE HEIGHT EFFECTS IN LIQUID-CHROMATOGRAPHY STATIONARY PHASES

Velocity independent plate heights were apparently first recognized fo r hydrodynamic chromatography columns, packed with nonporous, 115 micr on glass beads which were run at reduced mobile phase velocities of 10 to 10,000. Hydrodynamic chromatography separates based on the tendenc y of small molecules (or particles) to associate with slower moving fl uid streamlines near the surfaces of particles, compared to larger mol ecules which seek faster streamlines. Consequently, the larger molecul es elute first. Velocity independent plate heights in liquid chromatog raphy have also been observed for nonadsorbed solutes in particulate a nd fibrous stationary phases. These stationary phases have pores which exceed 10(-4) to 10(-5) cm in dimension. The flat plate height is att ributed to flow in the channels formed by these large intraparticle sp aces. The development of plate height expressions which represent disp ersion at interstitial velocities above 10 cm/min are discussed. Expla nations of the uncoupling of dispersion from eluent flow rate in conti nuous stationary phases, membranes, and gigaporous particles is shown to have their origins in the studies of distribution of particles and molecules in hydrodynamic chromatography columns, and to be adequately described by modifications of the van Deemter equation.