The quantification of enantiomer mixtures is essential to determine the success of a synthesis, but HPLC separation of the peaks may not be sufficient to get accurate determinations.  It has been found (L Hinterman, J Org Chem, 2007, 72, 9790) that addition of water to the eluent during normal phase HPLC with a chiral stationary phase leads to better peak separation.  When using mixtures of alkanes (such as heptane) and alcohols (e.g. 2-propanol) it is sufficient to add 0.15%-03.% water.

In some examples peak separation was enhanced by small amounts of water, but on addition of more water peaks coallesced.  Even more water then gave a separation again, but the peaks were inverted.

It is clear that when performing HPLC the water content of the solvent is a critical factor which may, on the one hand lead to lack of reproducibility if not controlled, but on the other hand can be manipulated to advantage.