Hydrodynamic Chromatography

affected by their density. Particle size distributions (PSD) and their averages may be obtained. The separation takes place in packed (PHDC) or in open tubular capillary columns. Components are eluted in the order of decreasing size, as in size exclusion chromatography (SEC), a method which has many features in common with HDC. HDC appears as a complement, either to SEC, which is limited to small size and solute species, or to other fractionation processes based on the effect of an external field, e.g. field flow fractionation (FFF). They are all transport methods, but differ in order of elution and domain of separation. Resolution is examined below. Other techniques for size measurements are static ones and can be used to check the validity of the results from HDC. It is necessary to combine different measurement processes, according to the conditions and purposes required: to obtain a relative or absolute value, with or without separation, affecting or not the state of the sample, and depending on the size and resolution required.

In HDC the key value for a given particle of diameter Dp is called the separation factor RF. This is the ratio of the highest elution volume (Vm) to that measured for this particle (Vp):

This factor is greater than one, the inverse of the situation in thin-layer chromatography (TLC), where the solvent migration is more rapid than that of solute. By plotting the graph of known Dp versus RF a calibration curve is obtained. This allows interpolation of the diameter of an unknown sample (Figure 1A). Another presentation is similar to that of SEC, log Dp versus Vp (Figure 1B) or Dp versus (Vm — Vp).

Other terms, detailed below, have the same meaning as in general liquid chromatography. Despite interest in this method, there are only a few papers (around 200) in the literature, as compared to separation by other chromatographic principles. Those that do exist are related to theoretical as well as practical aspects. General presentation may be found in papers by Barth, Hamielec and Tijssen. Currently, effective applications and use are increasing.

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