Polyelectrolyte effects in gel chromatography

When polyelectrolytes (i.e., lignosulfonates) are eluted from Sepfected by steric exclusion, polyelectrolyte expansion, ion exclusion, and ion inclusion effects. The last-mentioned effect arises from the interaction of charged sites in the high-MW macroions, which are more or less excluded from the gel phase with charged sites in more permeable macroions. The ion inclusion effect depends on the counterion in such a way that the fractionation of polyelectrolytes with a univalent counterion is affected to a greater extent than is the fractionation of polyelectrolytes with a divalent counterion. This is attributable to the existence of a higher density of free charges in the former case, since univalent counterions have a lesser tendency to form ion pairs with the charged sites of the macroion than have divalent counter-ions. The ion inclusion effect may have a favorable influence on the resolution of components of relatively equal molar volume, but of different charge densities. Polyelectrolyte effects are suppressed if the elution is carried oe polyelectrolyte behaves as a nonionic polymer. However, since the separation effected by differences in charge densities is eliminated (swamped out), resolution may deteriorate; nevertheless, the fractionation order is independent of the composition of the sample, the concentration of sample solution, and other fractionation variables. If one wishes to acquire a general picture of the composition of a certain polyelectrolyte, a solution of a simple electrolyte may be the most appropriate eluent. However, if polyelectrolyte components with small differences in hydrodynamic volume are to be separated, an eluent comprising a solution of a high-MW polyelectrolyte, completely excluded by the gel (e.g., dextran sulfate), can be used.