31 acidic model analytes were used in this study. Each was individually dissolved in methanol, methanol:water 1:1 v/v, or dimethyl sulfoxide, in a concentration of 1.0-5.6 mg mL -1 . From these stock solutions, a mixture of all at 5 µg mL -1 in water was prepared and stored in aliquots at -28 °C. This solution was used for spiking samples prior to extraction, at a level of 100 ng mL -1 .