Preparation of uterine secretion metabolites
The collected samples were thawed on ice, and 20 μL of the sample was
extracted with 120 μL of pre-cooled 50% methanol, vortexed for 1 min,
and incubated at 25 °C for 10 min. The extraction mixture was then
stored overnight at −20 °C. After centrifugation at 4000 ×g for
20 min, the supernatants were transferred into new 96-well plates. The
samples were stored at −80 °C before liquid chromatography-mass
spectrometry (LC-MS) analysis. In addition, pooled quality control
samples were prepared by combining 10 μL of each extraction mixture.