Dispersion in Chromatography Columns - The Random Walk Model > Page 7
When a stream of mobile phase carrying a solute impinges upon a particle, the stream divides and flows around the particle. Part of the divided stream then joins other split streams from neighboring particles, impinges on another and divides again. When a sample is placed on the column at the center of the packing, initially it is in a condition of non-radial equilibrium, but as a result of this process the sample spreads across the column during passage through the column and eventually achieves radial equilibrium (the concentration of solute is constant across a cross section of the column. Early work in liquid chromatography, used relatively low inlet pressures and, thus, samples could be injected directly onto the column with a syringe through an appropriate septum device as in gas chromatography. This method of injection often resulted in radial equilibrium never being achieved by the solutes before they were eluted. The introduction of the sample valve, however, aids in establishing radial equilibrium early in the separation but unless some special spreading device is employed at the front of the column, it will not necessarily occur at the point of injection. The stream splitting process is depicted in figure (2).
Figure 2. The Mechanism of Radial Dispersion
If a particular molecule is considered passing round a particle, it will suffer a lateral movement that can be seen to be given by,