SEPARATION SPEED, RETENTION, AND DISPERSION IN ASYMMETRICAL FLOW FIELD-FLOW FRACTIONATION AS FUNCTIONS OF CHANNEL DIMENSIONS AND FLOW-RATES

The theory for the asymmetrical flow field-flow fractionation channel was evaluated experimentally. The assumption of a constant crossflow along the channel was satisfactory. The zone-broadening theory was extended to take into account the longitudinal diffusion and the width of the starting sample zone. In the present apparatus the longitudinal diffusion was negligible but may have to be considered for lower outlet flow rates. The axial flow velocity gradient has the advantage that the contribution by the width of the starting sample zone to the final zone width decreases. For rectangular channels with a crossflow rate to outlet flow rate ratio of 10, a 1.5-cm-wide starting zone contributes to the plate height by 0.001 cm only. The effect of channel length was investigated. Although theory predicts that the channels can be as short as 10 cm, a 0.01-0.04-cm contribution to the plate height not accounted for by theory suggests that it is better to use longer channels. An upper limit in length if found around 60 cm. The effect of crossflow rate, outlet flow rate, and channel thickness on speed, efficiency, and dilution was demonstrated.