3.2. Selection of the internal standard
Other fluoroquinolones are often used as internal standards for ciprofloxacin, due to similarities in chromatographic behaviour [8], [19] and [20]. Initially, ofloxacin was chosen as the internal standard but both analytes eluted at similar retention times, making resolution poor and quantification difficult. Decreasing the amount of acetonitrile in the mobile phase should favour the interaction of ofloxacin to the hydrophobic stationary phase relative to the mobile phase and increase the retention time [21]. In this case this did not produce a considerable shift in the retention time of ofloxacin and the internal standard was subsequently changed to sulfadimidine sodium. The use of this internal standard for ciprofloxacin has not yet been documented in literature. This standard eluted well after ciprofloxacin and did not affect resolution