Analysis of anthraquinones in Rubia tinctorum L. by liquid chromatography coupled with diode-array UV and mass spectrometric detection

A liquid chromatographic (LC) method for the separation of both anthraquinone glycosides and aglycones in extracts of Rubia tinctorum was improved. For on-line MS detection atmospheric pressure chemical ionisation as well as electrospray ionisation (ESI) were used. The glycosides were ionised in both positive and negative ionisation (NI) mode, the aglycones only in the NI mode. With ESI ammonia was added to the eluent post-column to deprotonate the compounds. The efficiency of mass detection of the hydroxyanthraquinone aglycones was found to depend on the pKa value of the component. LC–diode-array detection and LC–MS provide useful complementary information for the identification of anthraquinones in plant extracts, which was proven with the identification of munjistin and pseudopurpurin.