Fast LC-MS quantification of ptesculentoside, caudatoside, ptaquiloside and corresponding pterosins in bracken ferns

Ptaquiloside (PTA) is an illudane glycoside partly responsible for the carcinogenicity of bracken ferns (Pteridium sp.). The PTA analogues ptesculentoside (PTE) and caudatoside (CAU) have similar biochemical reactivity. However, both compounds are highly under-investigated due to the lack of analytical standards and appropriate methods. This study presents a robust method for preparation of analytical standards of PTE, CAU, PTA, the corresponding hydrolysis products: pterosins G, A and B, and an LC-MS based method for simultaneous quantification of the six compounds in bracken. The chromatographic separation of analytes takes 5 min. The observed linear range of quantification was 20-500 µg/L for PTA and pterosin B, and 10-250 µg/L for the remaining compounds (r > 0.999). The limits of detection were 0.08-0.26 µg/L for PTE, CAU and PTA and 0.01-0.03 µg/L for the pterosins, equivalent to 2.0-6.5 µg/g and 0.25-0.75 µg/g in dry weight, respectively. The method was applied on 18 samples of dried fern leaves from 6 continents. Results demonstrated high variation in concentrations of PTE, CAU and PTA with levels prior to hydrolysis up to 3,900, 2,200 and 2,100 µg/g respectively. This is the first analytical method for simultaneous and direct measurement of all six compounds. Its application demonstrated that bracken ferns contain significant amounts of PTE and CAU relative to PTA.