ng/mL in three runs daily for three consecutive days. Calibration curves were calculated by the nominal concentration of OSE and OCA calibration standards versus the peak area ratio of OSE and OCA to internal standard. Sensitivity of analytes was determined by calculating the signal-tonoise ratio of the lowest limit of quantitation of samples. The limits of detection and quantification were determined at S/N of 3 and 10 under the present chromatographic conditions. The limit of quantification for OSE and OCA was 1 ng/mL, according to the deviation of response for the linear range. The concentration of any sample higher than the calibration range was corrected by dilution with blank matrix containing internal standard. Accuracy and precision were evaluated by the nominal concentration or the observed concentration. Accuracy was expressed as: accuracy = 6100, while precision was expressed as: precision = 6100. The matrix effect and recovery for the quantification of OSE and OCA were evaluated by three sets of samples. For Set 1, stock get 946128-88-7 solutions of OSE, OCA and internal standard were prepared by 80% aqueous acetonitrile. These samples were then transferred into sample vials, and 2 mL of these samples were injected directly into the HPLC-MS/MS system. Set 2 comprised post-extraction fortified samples prepared by adding 10 mL standard solutions of the two analytes into 90 mL reconstituted extract solutions from three different lots of each blank biological matrix to obtain the concentrations of 2.5, 25 and 250 ng/mL of OSE and of OSE. After through mixing, samples of this set were transferred into sample vials, and 2 mL was injected into the HPLC-MS/MS system. Set 3 included pre-extraction fortified samples used for calibration. The samples were prepared by the same procedures as those for Set 2. The OSE and OCA were spiked with the blank biological matrices to obtain the concentrations of 2.5, 25 and 250 ng/mL. After SPE extraction and reconstitution, these samples were loaded into sample vials, and 2 mL of the sample solution was injected into the HPLC-MS/MS system. Recovery was evaluated by the peak area ratio of analytes to internal standard for extracted QC samples compared with the peak area ratio of analytes to internal standard, which was spiked after 0.5 M HClO4 treatment, followed by SPE extraction and multiplied by 100. Matrix effects were determined by comparing the peak area ratio of analytes to internal standard spiked post-SPE extraction with the peak area ratio of analytes to internal standard and multiplied by 100. Pharmacokinetic analysis and statistics Curves of various biological concentration data versus sampling time for OSE and OCA were constructed by their corrected pharmacokinetic curves and then processed by the WinNonlin Standard Edition Version 1.1. The pharmacokinetic parameters for plasma sample were calculated by a noncompartmental model. In addition, an extravascular input noncompartmental 
model was employed to obtain pharmacokinetic parameters for amniotic fluid, placenta, and fetus. All data are presented as mean 6 standard deviation. Student’s t test was used to evaluate differences and a value of P,0.05 was taken as statistically significant. Results and Discussion Method validation OSE, OCA and the internal standard were detected in the positive MRM mode and their retention times were approximately 2.5, 6.6 and 1.6 min, respectively. The mass spectra revealed peaks of OSE and OCA at m/z 313.2 and 285.2 co
