ZONE BROADENING AND DILUTION IN RECTANGULAR AND TRAPEZOIDAL ASYMMETRICAL FLOW FIELD-FLOW FRACTIONATION CHANNELS

A new trapezoidal geometry of the asymmetrical flow field-flow fractionation (FFF) channel introduces an additional means to regulate the longitudinal flow velocity. The trapezoidal geometry, where the breadth decreases on going toward the channel outlet, makes it possible to level out the steep linear velocity gradients that can appear in the rectangular channels. Equations for the channel flow velocity gradient and the void time are derived. In addition, an equation for the nonequilibrium plate height is derived for asymmetrical channels. The equation suggests that the same efficiencies should be obtained in asymmetrical as in the original symmetrical channels under similar flow conditions. The relative peak dilution occuring in channels of different designs is evaluated. The results favor the trapezoidal versus the rectangular channel and asymmetrical versus symmetrical channels. The plate numbers calculated from the derived equations were compared with those found experimentally. The latter were found to deviate from the calculated numbers at high retention levels. Approximately 2600 plates have been reached.