Plate Theory and Extensions - The Peak Capacity of a Chromatographic Column > Page 96


The curves show that the peak capacity increases with the column efficiency, as one would expect, but the major factor that influences peak capacity is the capacity ratio of the last peak. Thus, any aspect of the chromatographic system that might limit the value of (k') for the last peak will also limit the peak capacity. Davis and Giddings (28) have pointed out that the theoretical peak capacity is an exaggerated value of the true peak capacity. The individual (k') values for each solute in a realistic multi-component mixture will have a statistically irregular distribution. They very adroitly point out, that the solutes in real samples do not array themselves conveniently along the chromatogram four standard deviations apart to provide the maximum peak capacity. Nevertheless, the theoretical peak capacity values given by equation (77) can be used as a reasonable practical guide for comparing different columns although, in practice the theoretical values for peak capacity may never be realized.

Courtesy of the Journal of Chromatographic Science


Figure 25. Graph of Peak Capacity against Capacity Ratio

It is clear from Figure 25 that any property of the chromatographic system that limits the magnitude of (k') must also limit the peak capacity. One such property, that limits the magnitude of (k') will be the detector sensitivity. The later the elution (the higher the (k') value), the greater the peak dispersion and the smaller the peak height. At some limiting (k') value, the peak will be so small that it will be indiscernible from the noise.