Quantitative Determination of Trace Lipopeptide Using HPLC through Double Carboxyls Labelling

Lipopeptides have potential applications in many industries. It is challenging to quantitatively determine low concentration of lipopeptide. In this paper a highly sensitive and quantitative method for determination of lipopeptide concentration is described. Fatty acids and surfactin were labelled with naphthalene ethylenediamine (NEDA). LC–MS and HPLC with fluorescence detection analyses shown that the surfactins with 2 free carboxyl groups were wholly labelled by 2 NEDA. Linear relationships were found between molar concentration of fatty acids and peak area of labelled fatty acids. Different fatty acids share the same quantitative regression equation and slope of equation due to the reason that each fatty acid was labelled by 1 NEDA. The relationship between surfactin molar concentration and peak area was obtained based on the concentration of the photoluminescent molecule. This method can detect concentrations as low as 3.1 nmol/L, which is less than the limits of the most sensitive method reported for the surfactins previously. The method can also be used to determine other lipopeptides with carboxyl groups.