Mass Spectrometric Identification of Multihydroxy Phenolic Compounds in Tibetan Herbal Medicines

A highly selective and accurate method based on derivatization with dansyl chloride coupled with liquid chromatography–mass spectrometry has been developed for identification of natural pharmacologically active phenolic compounds in extracts of Lomatogonium rotatum plants (Tibetan herbal medicine) obtained by solid-phase extraction. The number of hydroxyl groups on the dansylated phenols was estimated by LC–MS–MS analysis in positive-ion mode. Dansyl derivatization of the compounds introduced basic secondary nitrogen into the phenolic core structures and this was readily ionized when acidic HPLC mobile phases were used. MS fragmentation of the derivatives generated intense protonated molecular ions of m/z MH]⁺ (phenol aglycones were transformed into the corresponding free phenols by cleavage of an aglycone bond). Collision-induced dissociation of the protonated molecule generated characteristic product ions of m/z 234 and 171 corresponding to the protonated 5-(dimethylamino)naphthalene sulfoxide and 5-(dimethylamino)naphthalene moieties, respectively. Selected reaction monitoring based on the m/z MH]⁺ to 234 and 171 transitions was highly specific for these phenolic compounds. Characteristic ions with m/z values of MH – 234]⁺, MH – 2 × 234]⁺, and MH – 3 × 234]⁺ were of great importance for estimation of the presence of multihydroxyl groups on the phenolic backbone.