2.3. Selection of the internal standard
Ofloxacin was initially chosen as the internal standard. How-ever, when ciprofloxacin and ofloxacin were analysed togetherusing the conditions described above, they both had a similarretention time of around 3 min. The percentage of acetonitrile wasdecreased from 30% to 26%, 25%, 24% and 23% in an attempt tobetter resolve the peaks given by the two compounds. This didnot produce a considerable shift in the retention time of ofloxacin.The internal standard was subsequently changed to sulfadimidinesodium which has 2 pKa values of 2.65 ± 0.2 (pKa1) and 7.40 ± 0.2(pKa2) [12,13], good UV absorption at 277 nm and similar chemicalbehaviour under the extraction and chromatographic conditionsused in this study.