Aim To establish a fast, sensitive and accurate high‐performance liquid chromatography‐tandem mass spectrometry (HPLC‐MS/MS) method for determining the monosaccharide content of Qingzhuan Dark Tea polysaccharides in different years (2 years, 5 years and 11 years). Methods The optimised chromatographic conditions were achieved on a C18 column (5.0 μm, 250 mm × 4.6 mm inner diameter). The mobile phase flow rate was 0.9 mL/min and the column temperature was set to 27°C. The aqueous phase A (5 mM aqueous ammonium acetate) and organic phase B (acetonitrile) were used to elute the target analyses isocratically (0–60 min: 18% B). The mass spectrometer detector was equipped with an electron spray ionisation (ESI)source, and multiple reaction monitoring (MRM) mode was used for the determination of 1‐phenyl‐3‐methyl‐5‐pyrazolone (PMP) derived monosaccharides. Results We carried out a comprehensive methodological validation of PMP derived monosaccharides, including linearity, precision, stability and repeatability. Nine monosaccharides (rhamnose, mannose, ribose, glucose, galacturonic acid, xylose, galactose, fucose and arabinose) of Qingzhuan Dark Tea polysaccharides were identified, in which ribose and fucose were reported for the first time. The results showed the contents of these nine monosaccharides differed significantly among different years. Conclusions The validated method is reliable, accurate, repeatable and can be applied to quality assessment of these monosaccharides.

The effective selenylated polysaccharides (refer as TPS-Se) with an average molecular weight (M W ) of 3.109 × 104 Da was synthesized by the reduction of sodium selenite with ascorbic acid in the presence of Qingzhuan Dark Tea polysaccharides (TPS). The selenium (Se) content of TPS-Se was up to 37.05%. The differences between TPS and TPS-Se were compared in morphology, structure, antioxidant activity, and molecular weight using FT-IR, Raman spectroscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), SEM, AFM, and Integrated thermal gravimetric analysis. The results indicated that Se 4+ was reduced to Se 0 by ascorbic acid. Further, the Se 0 atoms aggregate into Se nuclei and bind to the C=O from TPS. Also, TPS-Se exhibited higher DPPH and ABTS radical-scavenging abilities than TPS. This difference may be attributed to the fact that Se has good antioxidant activity. The test results demonstrated that TPS-Se was prepared successfully by reducing sodium selenite with ascorbic acid, which exhibited a higher radical-scavenging ability. Finally, LD 50 value of 136.89 mg/kg, with a 95% confidence interval of 102.82~182.82 mg/kg, was determined by gavage administration for TPS-Se's.

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