A Modified INA Method for the Analysis of Milk Thistle Using HPLC and Evaporative Light Scattering Detection

Introduction

One of the widely available nutraceutical products seen in supermarkets is a powdered form of Milk Thistle or silybum marianum, derived from the plant native to Mediterranean Europe. It has been used from ancient times as a liver tonic. The milky sap produced by Milk Thistle contains flavolignans, collectively known as the silymarin group. Growing consumer interest, combined with the low cost and growing availability of herbal supplements and remedies, has driven the need for standardization of these products. The Institute for Nutraceutical Advancement (INA) has developed a standardized method for the analysis of nutraceutical products, which involves an HPLC assay with UV detection. As per INA method 115.00, the marker constituents, silymarin group flavolignans, of Milk Thistle are analyzed using ultraviolet detection at 288nm. Unfortunately, as is common with any spectroscopic detection method, the phytochemicals vary in their absorbance response at 288nm according to their individual molar absorptivities. Over the broad range of phytochemicals comprising the sample, the wavelength of maximum absorbance (λ _max) may vary considerably from one constituent to the next. This can result in a chromatogram that shows the compounds of interest, but does not offer a true indication of their relative abundance. Achieving a recommended wavelength for detection is always a compromise. Shimadzu’s Evaporative Light Scattering Detector (ELSD-LT) offers an alternative to spectroscopic detectors. With it, analyte response more accurately reflects the relative abundance of sample constituents. The ELSD-LT is not a spectroscopic detector, but instead makes a light scattering measurement of analyte particles after they have been dried of mobile phase through evaporation. It excels at the analysis of chromophoric and non- chromophoric compounds alike as its response is independent of the spectral properties of both the analyte and solvents. The ELSD-LT is also blind to gradients, producing stable gradient baselines.

August 6, 2010 GMT