Onset of sample concentration effects on retention in field‐flow fractionation

In field‐flow fractionation (FFF), when the sample amount is increased, a shift in retention occurs. Positive as well as negative deviations of the retention have been observed, depending on the type of sample and on the experimental conditions. Two concentration effects have been invoked to explain this retention shift: the viscosity effect (the velocity profile of the analyte suspension or solution is modified by the concentration dependence of the viscosity) and the mean distance effect (the mean distance of the analyte molecules or particles to the accumulation wall and the steady state cross‐sectional concentration profile of the analyte are affected by concentration‐dependent interactions between analyte molecules or particles). The general FFF retention equation has been solved in the case where the reciprocal of the viscosity and the FFF analyte‐field interaction parameter, λ, are linearly related to concentration. The two proportionality coefficients involved in these relationships are identified as the intrinsic viscosity and twice the second virial coefficient of the osmotic pressure, respectively. A general expression of the derivative of the analyte average relative velocity, R , with respect to the cross‐sectional average analyte concentration, 〈 c 〉, has been obtained as a function of these two coefficients, and of λ o , the limit of λ for infinite dilution. A survey of retention effects reported in FFF literature for various sample types and FFF methods is presented.