Molecular Basis of Migration

The molecular migration pathway in chromatography is a statistical pathway in which each molecule randomly executes the steps that constitute its particular migration path. If it were possible to outline the choices faced by a molecule (such as whether or not to adsorb in a given time interval or which streampath to follow around a particle), and the probability of each option, we would in theory be able to calculate the probability that the molecule would follow a particular path. Since all identical molecules are identically inclined, the probability of a certain path for a single molecule is equal to the fraction of all molecules taking that path. If, for instance, the probability is known for two paths, one displacing the molecule 20 mm and 

The two contrasting approaches, the macroscopic viewpoint which describes the bulk concentration behavior (last chapter) versus the microscopic viewpoint dealing with molecular statistics (this chapter), are not unique to chromatography. Both approaches offer their own special insights in the study of reaction rates, diffusion (Brownian motion), adsorption, entropy, and other physicochemical phenomena [2]. 

Longitudinal molecular diffusion. Solute molecules are engaged in ceaseless Brownian motion, which is responsible for diffusion. The component of this erratic motion along the column axis, superimposed on the downstream displacement caused by flow, is one source of zone broadening. 

Sorption-desorption. An important irregularity in molecular migration is caused by sorption and desorption. Each time a molecule is sorbed by the stationary phase, its downstream motion ceases. When desorbed, it proceeds again. The processes of sorption and desorption occur randomly, thus making this stop-and-go sequence an extremely erratic one. 

The molecular picture of chromatography, based on these underlying mechanisms, is one of a complex motion having numerous variations and discontinuities. The motion of the zone as a whole appears smooth, however, because the individual molecules cannot be perceived. Instead one observes (visually or instrumentally) only the bulk motion of solute, which is a smoothed average of the motion of a great number of erratically behaving molecules. 

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