The usual approach to mathematically modelling the zero length column (ZLC) has been to treat the column as being physically short enough for the carrier phase to be considered well mixed. This work presents an evaluation of this assumption for a sorbent bed of arbitrary length using a systematic comparison of the standard model to a plug flow model. The case of finite dispersion was modelled and shown to be intermediate to the two extremes. Using this comparison, a criterion was developed by which the two cases can be considered identical for practical purposes. It was shown that the axial flow pattern in the sorbent bed may be disregarded when the dimensionless parameter L > 15, assuming that interstitial fluid holdup is negligible. The usual ZLC analysis may then be applied for extracting diffusion and adsorption parameters from the desorption curve.