When the pH is increased, the DA-FLP resolution improves slightly (Fig. ), however, for the other impurities the separation with FLP worsens. The latter can be explained by considering the pH-induced changes in charge of the ON species. An eluent pH of 6.3 was used in further experiments as the separation is virtually identical to pH 7, but easier to prepare by adding equal molar concentrations of TEA and acetic acid.