Adjusting the retention factor to improve the resolution between one pair of solutes may lead to unacceptably long retention times for other solutes. For example, suppose we need to analyze a four-component mixture with baseline resolution and with a run-time of less than 20 min. Our initial choice of conditions gives the chromatogram shown here in (a). Although we successfully separate components 3 and 4 within 15 min, we fail to separate components 1 and 2. Adjusting the conditions to improve the resolution for the first two components provides a good separation of all four components, but the run-time is now too long (b). The problem of finding a single set of acceptable operating conditions is known as the general elution problem.
One solution to the general elution problem is to make incremental adjustments to the separation conditions over time. Thus, we choose our initial chromatographic conditions to optimize the resolution for early eluting solutes. As the separation progresses, we adjust the chromatographic conditions to favor the separation of later eluting solutes (c). In gas chromatography this is accomplished by controlling the column’s temperature; for liquid chromatography the same effect is obtained by increasing the solvent’s eluting strength.