Plate Theory and Extensions - Resolving Power of a Column > Page 62
Curves relating (n) and (k'A) for solute pairs having separation ratios of 1.02, 1.03, 1.05 and 1.07, calculated using equation (55) are shown in Figure 18. As the separation becomes more difficult (i.e., the separation ratio (aA/B) becomes smaller), the necessary efficiency (n) for resolution increases rapidly. This is due to the fact that as (aA/B) becomes smaller, the peaks become closer, consequently dispersion must be more constrained to reduce their width. To do this, the column must be made more efficient. Nevertheless, the dramatic increase in (n) when the capacity factor becomes small is not so obvious. This makes achieving short analysis times very difficult, as fast elution is achieved with small (k') values. However, the greater efficiencies needed at lower (k') values will require longer columns which will extend the analysis times. To resolve a solute pair with a separation ratio of 1.02, an efficiency of 360,000 theoretical would be required if the (k') value was 0.5. GC Capillary columns can provide such efficiencies but, in LC, such efficiencies would be extremely difficult and costly to produce. It follows that the phase system should be chosen so that the closest eluted solutes are not eluted at low (k') values. Less efficiency will be needed and, thus, shorter columns and consequently, shorter analysis times will be achieved. At (k') values that exceed 10, the required efficiency changes little as the capacity ratio increases. Thus, for fast analyses, the phase system provide a large separation ratio, but the first peak should elute at a (k') of 10 or more. The phase system should have high selectivity and retentive capacity so that minimum efficiency is required and the column can be as short as possible.