Simultaneous determination of N-oxides and free bases of pyrrolizidine alkaloids by cation-exchange solid-phase extraction and ion-pair high-performance liquid chromatography

Cation-exchange solid-phase extraction using LiChrolut SCX (Merck, Darmstadt) cartridges filled with polymeric strong cation-exchanger enabled efficient isolation of both N-oxides and free bases of pyrrolizidine alkaloids (PAs). The recoveries were about 80% for retrorsine-N-oxide, 90% for retrorsine and 100% for senkirkine and were assessed both by TLC–densitometry and ion-pair high-performance liquid chromatography (HPIPC) on Hypersil BDS C8 stationary phase and hexane-1-sulfonic acid as ion-pairing agent. The applied HPIPC gradient procedure was suitable for separation of PAs with various types of structures (N-oxides, free bases, otonecine-PAs). The method limits of detection and quantitation, respectively, ranged from 0.06 ng/μl (senecionine) and 0.2 ng/μl (senkirkine) to 0.1 and 0.35 ng/μl for retrorsine-N-oxide. For each component calibrated by linear regression method, correlation coefficients were higher than 0.9995 (six-point calibration from 4 to 100 μg/ml). The elaborated procedure was used in searching for PAs in plant derived samples from Symphytum sp. (comfrey), Petasites hybridus and Petasites albus (butterbur), Tussilago farfara (coltsfoot), Emilia coccinea (tassel flower) and Doronicum columnae (leopard’s bane). For the last three samples macrocyclic PAs (senecionine, senecionine-N-oxide, senkirkine) have been detected for the first time. Details of precision of the analyses are also included.