Holding-time distributions of the Gaussian type☆

Abstract Holding-time distributions are of great importance in a considerable number of separation and conversion processes. In the majority of cases it is desired that the distribution be as narrow as possible and there are accordingly many examples where a peak signal at the entrance gives rise to a comparatively narrow band in the effluent, which has then Gaussian shape. Several mechanisms have been proposed to account for such a phenomenon. In the present study, these mechanisms are analysed on a statistical basis in terms of their basic variables. It is also shown that various mechanisms may be combined by adding variances, diffusion coefficients or heights corresponding to a theoretical plate. It is obviously not possible to identify the mechanism by observing a single Gaussian elution curve. Neither is this possible by observing a series of such curves for systems or columns of different lengths, in all theories the width of the Gaussian curve increasing in proportion to the square root of the length of the system. In the experiments, variation of flow rate, particle size, and diffusion coefficients is necessary in order to reach conclusions regarding the mechanism or mechanisms responsible for the elution curve.