Use of digitoxin and digoxin as internal standards in HPLC analysis of triterpene saponin-containing extracts

Saponins are widely distributed complex plant glycosides possessing a variety of structure-dependent bioactivities. Quantitation of individual saponins is difficult due to lack of available standards, mainly as a consequence of purification difficulties. Determination of total saponin content can be problematic, often relying on non-specific methods based on butanol solubility, haemolytic activity or formation of coloured derivatives.

To develop a general quantitative method based on the use of the readily available cardenolides, digitoxin (1) and digoxin (2), as internal standards in an HPLC-PAD-based analysis.

The cardenolides were run at a variety of concentrations to establish linearity and reproducibility of detector response and then evaluated as internal standards for quantitation of triterpene saponins in several plant-derived extracts by HPLC-PAD. Mixtures of saponins, largely freed from other extractables, were obtained by fractionation of total extracts on solid phase extraction columns (SPE) employing a water-methanol gradient and used for construction of calibration curves. Saponin identification and structural information was obtained via a single quadrupole mass detector using electrospray ionisation in negative ion mode (ESI(-)).

Saponin contents in six samples from five species were determined and compared with literature results and a gravimetric method based on butanol-water partitioning. Results were generally consistent with literature reports and superior to gravimetric butanol-water partitioning.

Digitoxin and digoxin are useful as internal standards in HPLC estimation of saponin content. Saponins from different species having similar structures and molecular weights afford similar calibration curves.