Plasma samples were prepared by β-glucuronidase hydrolysis and protein precipitation with methanol in succession. The chromatographic separation was performed on a C18 column with the mobile phase of acetonitrile, methanol and water (containing 0.1% formic acid and 1% isopropanol) (40/10/50, v/v/v). The quantitation was performed on a triple quadrupole mass spectrometer operating through a multiple reaction monitoring (MRM) and negative ion mode. The precursor to product ion transitions monitored for S and quercetin (internal standard, IS) were m/z 285 → 137 and 301 → 121, respectively.
The calibration curve was linear (r = 0.999) over the concentration range of 4.0–513 ng/mL with a lower limit of quantitation of 4.0 ng/mL. The intra- and inter-day precision expressed by coefficient of variation (CV, %) was below 7.2%. The recovery was greater than 80%. The relative bioavailability of the test formulation to the reference was (108.4 ± 32.5)%.
The method developed in the present study was successfully validated and applied in a bioequivalence study of SG preparations in twenty-four healthy Chinese male volunteers.