A Validated UHPLC-Tandem Mass Spectrometry Method for Quantitative Analysis of Flavonolignans in Milk Thistle (Silybum marianum) Extracts

Validated methods are needed for the analysis of natural product secondary metabolites. These methods are particularly important to translate in vitro observations to in vivo studies. Herein, a method is reported for the analysis of the key secondary metabolites, a series of flavonolignans and a flavonoid, from an extract prepared from the seeds of milk thistle [Silybum marianum (L.) Gaertn. (Asteraceae)]. This report represents the first UHPLC MS-MS method validated for quantitative analysis of these compounds. The method takes advantage of the excellent resolution achievable with UHPLC to provide a complete analysis in less than 7 min. The method is validated using both UV and MS detectors, making it applicable in laboratories with different types of analytical instrumentation available. Lower limits of quantitation achieved with this method range from 0.0400 µM to 0.160 µM with UV and from 0.0800 µM to 0.160 µM with MS. The new method is employed to evaluate variability in constituent composition in various commercial S. marianum extracts, and to show that storage of the milk thistle compounds in DMSO leads to degradation