Naturally Occurring Glucosinolates in Plant Extracts of Rocket Salad (Eruca sativa L.) Identified by Liquid Chromatography Coupled with Negative Ion Electrospray Ionization and Quadrupole Ion-Trap Mass Spectrometry

A method for the comprehensive profiling of intact glucosinolates (GLSs), major and minor, occurring in leaves and seeds of rocket salad (Eruca sativa L.) is presented using optimized reversed-phase liquid chromatography (RP-LC) with electrospray ionization (ESI) ion trap mass spectrometry (ITMS). ESI-ITMS in the negative mode was confirmed to be very suitable to analyze these compounds in crude extracts. After extraction from the plant material with methanol/water (70:30 v/v) at 70 degrees C, the analytes of interest were separated on a C18 column using an eluent acidified with formic acid (0.1%) and modified with acetonitrile. All the GLSs found in leaves of rocket salad gave good signals corresponding to the deprotonated precursor ion, [M-H](-). Although the mass spectra also exhibited an analytically important non-covalent adduct ion at [2M-H](-), the structures of glucosinolates were confirmed by extensive sequential MS analysis, thereby substantially improving the identification of unknown compounds. The results obtained not only revealed in leaves of E. sativa at least twelve species of GLSs including seven aliphatic compounds (glucoraphanin with [M-H](-) at m/z ratio of 436, glucoerucin at m/z 420, 4-mercaptobutyl-GLS at m/z 406, progoitrin/epiprogoitrin at m/z 388, sinigrin at m/z 358,4-methylpentyl- and n-hexyl-GLS at m/z 402) and three indole glucosinolates (i.e., three N-heterocyclic compounds: 4-hydroxyglucobrassicin and 5-hydroxyglucobrassicin at m/z 463, and 4-methoxy-glucobrassicin at m/z 477), but also two structurally related compounds containing one intermolecular disulfide linkage (4-(beta-D-glucopyranosyldisulfanyl)butyl-GLS at m/z 600 and a dimeric 4-mercaptobutyl-GLS at m/z 811). This latter symmetric disulfide was previously considered as an artefact formed during extraction of GLSs from vegetative tissues. Glucosinolates were detected in the leaves with a wide range of contents (10-200 mu mol/g) and a great variation in the composition. Only three GLSs were identified in seeds of rocket salad, namely glucoraphanin, glucoerucin and 4-methoxyglucobrassicin. As expected, the most abundant GLS in seeds is glucoerucin. The feasibility of the strategy was also demonstrated using a rapeseed extract of certified reference material (BCR367R). The results indicated the usefulness of this method for a rapid, sensitive and comprehensive profiling of the GLS family naturally occurring in extracts of crude plant matter.