Hydrodynamic relaxation in flow field-flow fractionation using both split and frit inlets

Two means are described for achieving hydrodynamic relaxation and thus avoiding the stopflow injection procedure in field-flow fractionation (FFF): split flow injection and frit inlet injection. The advantages, disadvantages, and the theoretical basis of these procedures are discussed. Incremental band broadening due to the final relaxation step is examined theoretically and shown to be negligible when the flow rate of the sample inlet substream is small compared to the total channel flow rate. The optimization of the sample inlet flow rate is discussed. Experimental results for both injection procedures are reported for flow/steric (or hyperlayer) FFF applied to latex standards, confirming the expected trends. However, closer examination shows that the observed incremental band broadening associated with hydrodynamic relaxation is somewhat larger than the value predicted.