Comparative study on the analytical performance of different detectors for the liquid chromatographic analysis of tobramycin

Pulsed electrochemical detection (PED) with a triple step potential–time waveform has been used successfully for the direct detection of several non-chromophoric aminoglycosides at alkaline pH following ion-pair liquid chromatography (LC). This direct detection method has helped to overcome the various flaws observed in the analysis of aminoglycosides using time consuming derivatization procedures. However, this technique also suffers from some disadvantages. Hence, improvements of PED as well as alternative detection techniques would be welcome for the detection of drug molecules lacking UV absorbing chromophores. In this study a comparison was made between the analytical performance of different brands of PED detectors and two different evaporative light scattering detectors (ELSD) from the same manufacturer. Different PED waveforms (triple, quadruple and six potential–time waveforms) were also examined for the detection of tobramycin, a representative example of the group of non-chromophoric aminoglycosides. As starting point, the LC–PED method of the European Pharmacopoeia (Ph. Eur.) was taken. As the Ph. Eur. method prescribes the use of a non-volatile mobile phase, an alternative one had to be used in combination with ELSD. The analytical performance was compared with regards to sensitivity, linearity and long-term stability.