High performance liquid chromatography-charged aerosol detection applying an inverse gradient for quantification of rhamnolipid biosurfactants

An inverse gradient was applied for the absolute quantification of rhamnolipid congeners to account for the detector⿿s dependency on the solvent composition. The CAD produces a uniform response not only for the analytes but also for structurally different (nonvolatile) compounds. It was demonstrated that n-dodecyl-β-d-maltoside or deoxycholic acid can be used as alternative standards. The method of HPLC-ultra violet (UV) detection after a derivatization of rhamnolipids and HAAs to their corresponding phenacyl esters confirmed the obtained results but required additional, laborious sample preparation steps. Sensitivity determined as limit of detection and limit of quantification for four mono-rhamnolipids was in the range of 0.3⿿1.0 and 1.2⿿2.0 μg/mL, respectively, for HPLC-CAD and 0.4 and 1.5 μg/mL, respectively, for HPLC-UV. Linearity for HPLC-CAD was at least 0.996 (R2) in the calibrated range of about 1⿿200 μg/mL. Hence, the here presented HPLC-CAD method allows absolute quantification of rhamnolipids and derivatives.