Retention in field-flow fractionation with a moderate nonuniformity in the field force

In some field-flow fractionation (FFF) techniques, the basic analyte-field interaction parameter, λ, is not constant but varies within the channel cross section as a result of the nonuniformity of the force exerted by the field on the analyte. This is the case, for instance, in thermal FFF, because of the temperature dependence of the relevant physicochemical transport parameters. To account for this effect, a new FFF retention model is developed, allowing a linear variation of λ from the accumulation to the depletion wall, which is assumed to describe correctly moderate nonuniformity in λ in the vicinity of the accumulation wall. A methodology for sample characterization on the basis of this model is proposed. It associates λ(app), the apparent λ value derived from the retention ratio by means of the classical retention model, with a specific distance from the accumulation wall. An empirical relationship between that distance and λ(app) is derived. In the high retention limit, it is found that this specific distance is not equal, as sometimes intuitively believed, to the mean distance of the molecule or particle cloud to the accumulation wall but is approximately equal to twice this mean distance. The validity of the proposed approach is checked.