2.8. HPLC analysis of triptolide
The amount of permeated triptolide was determined by HPLC using Agilent 1200 series consisting of a quaternary pump, autosampler, diode-array detector and workstation. The column was an ODS-2 HYPERSIL column (250 mm×4.6 mm, 5 μm). The elution was performed at 35 °C and the mobile phase was a methanol-water (45:55, v/v) mixture with a flow rate of 1.0 mL/min. The detection wavelength was monitored at 218 nm and the retention time was 7.4 min. The assay was linear (r2=0.9999) over a concentration range of 0.3–30 μg/mL. The detection limit of this method was 0.1 μg/mL. The RSD value for precision was below 0.5% and the percentage recoveries ranged 99.0–101.0%. No interference of the other formulation components was observed.