Quantitative determination of the sulfur-containing antioxidant ergothioneine by HPLC/ICP-QQQ-MS†
Interest in the sulfur-containing antioxidant ergothioneine calls for reliable analytical methods for its quantification. In this work, a method based on reversed-phase high performance liquid chromatography (RP-HPLC) coupled with elemental mass spectrometry detection in mass shift mode (inductively coupled plasma triple quadrupole mass spectrometry, ICP-QQQ-MS) using oxygen as the reaction gas was developed for the element-selective determination of ergothioneine in complex biological matrices. Application of an instrumental setup using a 6-port-valve and the introduction of a methanol gradient allowed the time-efficient analysis of samples containing strongly retained sulfur species besides ergothioneine without compromising ICPMS detection. In aqueous solution, limits of detection and quantification (LOD and LOQ) of the optimized method for m/z 32 → 48 (SO+) were 0.23 μg S per L and 0.80 μg S per L, respectively; measurements in a complex matrix (human hepatocyte carcinoma cells, HepG2) resulted in an LOD of 0.6 μg S per L and an LOQ of 2.3 μg S per L. Recoveries of ergothioneine from cell pellets spiked with the analyte before cell lysis (97 ± 3%) matched those obtained for cell culture medium spiked before syringe filtration (96 ± 9%) demonstrating that sample preparation did not impair the quantitative determination of ergothioneine. When HepG2 cells were exposed to ergothioneine via the culture medium, they showed low absorption; approximately 3% of the added ergothioneine was found in cell lysates, while most of it (≥85%) remained in the cell culture medium. The method is capable of separating ergothioneine from other biologically relevant sulfur-containing species and is expected to be of broad future use. Furthermore, the potential use for the simultaneous separation of selenium species, thereby extending the scope of possible applications, was demonstrated by applying it to water extracts of oyster mushrooms.