The Effect of Stationary Phase Loading on the Performance of a Chromatographic System

The stationary phase loading of a column can affect a separation in two ways. As stated above, the greater the amount of stationary phase in a column, the more the solutes will be retained and, thus, the greater the separation. However, it must be pointed out that any change in stationary phase will affect the retention of all solutes proportionally and so will only improve the separation, if the peak widths remain unchanged. It will be seen later that under some circumstances the increase in stationary phase is accompanied by an increase in the film thickness that slows the rate of solute exchange between the phases. Slow exchange kinetics will increase peak dispersion (i.e. the peak width). It follows, that there will be a specific stationary phase loading that will provide the best compromise between peak separation and peak dispersion and, thus, provide the maximum resolution. The magnitude of the stationary phase loading can be quite critical for open tubular columns in GC. It follows, that the film thickness of the stationary phase cannot be increased indefinitely to move the peaks further and further apart as, eventually, the peaks will be broadened to a greater extent than they are separated.

The second reason for increasing the stationary phase load on a GC column (packed or open tube) is to permit a larger charge to be placed on the column. In trace analysis, a large sample may be necessary and under such circumstances the major component may overload the column to such an extent that a very broad asymmetric peak is formed that obscures the trace materials of interest. This asymmetric dispersion is due to the concentration of the major component in the stationary phase becoming so large that solute-solute interaction occurs causing the adsorption isotherm to become non-linear. The asymmetric dispersion by sample overload can be reduced by increasing the amount of stationary phase on the column, as a result, even with a larger charge, the sample concentration in the stationary phase is kept sufficiently low that the deleterious high concentrations that cause peak asymmetry are never reached.