Extraction and analysis of intact glucosinolates—A validated pressurized liquid extraction/liquid chromatography–mass spectrometry protocol for Isatis tinctoria, and qualitative analysis of other cruciferous plants

Glucosinolates have attracted significant interest due to the chemopreventive properties of some of their transformation products. Numerous protocols for the extraction and analysis of glucosinolates have been published, but limited effort has been devoted to optimize and validate crucial extraction parameters and sample preparation steps. We carried out a systematic optimization and validation of a quantitative assay for the direct analysis of intact glucosinolates in Isatis tinctoria leaves (woad, Brassicaceae). Various parameters such as solvent composition, particle size, temperature, and number of required extraction steps were optimized using pressurized liquid extraction (PLE). We observed thermal degradation of glucosinolates at temperatures above 50 °C, and loss of >60% within 10 min at 100 °C, but no enzymatic degradation in the leaf samples at ambient temperature. Excellent peak shape and resolution was obtained by reversed-phase chromatography on a Phenomenex Aqua column using 10 mM ammonium formate as ion-pair reagent. Detection was carried out by electrospray ionisation mass spectrometry in the negative ion mode. Analysis of cruciferous vegetables and spices such as broccoli (Brassica oleracea L. var. italica), garden cress (Lepidium sativum L.) and black mustard (Sinapis nigra L.) demonstrated the general applicability of the method.