True or apparent reversal of elution order during chiral high-performance liquid chromatography monitored by a polarimetric detector under different mobile phase conditions.

The use of all chromatographic modes (i.e. reversed-phase, normal-phase and polar organic modes) is becoming more and more systematic in chiral HPLC method development. When this method development is monitored by polarimetric detection, which is a widespread technique, one should not forget that the observed positive or negative rotation angle for a given absolute configuration depends on the solvent. As a cautionary illustration, a new example is reported for a racemate which is separated with the same elution order in ethanol or acetonitrile mobile phase on a CHIRALCEL OD-R column whereas the signs of polarimetric detection are reversed.