Hydrodynamic relaxation using stopless flow injection in split inlet sedimentation field-flow fractionation.

In this paper relaxation effects in both the normal and steric operating modes of sedimentation field-flow fractionation are examined by using three different injection procedures: stop flow, stopless flow, and a new stopless flow procedure employing an inlet splitter. In the usual operation of field-flow fractionation (FFF), a stop procedure is used in which the channel flow is halted for an adequate period of time after injection for sample relaxation (in which the sample particles approach equillibrium near one wall) before the resumption of channel flow. If the channel flow is not stopped (stopless flow procedure), the elution profile is shifted and distorted due to the downstream migration of the particles during the relaxation process. To avoid peak distortion while retaining the advantages of the stopless flow procedure, a physical splitter at the channel inlet divides the entering flow stream into two substreams; sample is injected into only one of these. In this way a rapid (although not complete) hydrodynamic relaxation is realized. This stopless split flow injection procedure is compared to ordinary stop and stopless flow procedures using both submicrometer (normal FFF) and supramicrometer (steric FFF) polystyrene latex particles. It is found that much of the distortion normally accompanying stopless flow injection is eliminated by this new procedure. However, further optimization is needed to match the high resolution of the stop flow method.