Using Column Selectivity to Optimize Resolution
A second approach to improving resolution is to adjust alpha, α. In fact, when α is nearly 1, it usually is not possible to improve resolution by adjusting kB’ or N. Changes in α often have a more dramatic effect on resolution than kB’. For example, changing α from 1.1 to 1.5 improves resolution by 267%.
A change in α is possible if chromatographic conditions are altered in a man- ner that is more selective for one of the solutes. If a solute participates in a sec- ondary equilibrium reaction in either the stationary or mobile phase, then it may be possible to alter that phase in a way that selectively changes the solute’s capacity fac- tor. For example, Figure 12.13a shows how the pH of an aqueous mobile phase can be used to control the retention times, and thus the capacity factors, for two substi- tuted benzoic acids. The resulting change in α is shown in Figure 12.13b. In gas chromatography, adjustments in α are usually accomplished by changing the sta- tionary phase, whereas changing the composition of the mobile phase is used in liq- uid chromatography.