Vials: Amber screw cap (Aijiren PN ) Vial caps: Blue screw cap (Aijiren PN ) The five estrogen-related analytes were obtained from Sigma-Aldrich (St. Louis, MO) and dissolved in MeOH to a concen-tration of about 0.5 mg/mL. A composite sample was then made by combining 50 µL aliquots of each individual estrogen