I am working for a drug for which, I cannot share name but having a Benzothiapine ring. Oxidative metabolite i.e. sulfoxide of drug is form in-vivo on dosing of the drug formulation. I am getting stuck where, this sulfoxide metabolite is readily converted to sulphide (back to drug substance) by reduction through reductase enzymes in blood and interfering to free-drug during detection in LCMS analysis. According to me, atmospheric oxygen and hemoglobin of blood play role for the conversion. Sulfoxide to sulphide (back to drug substance) conversion is not found after conversion of blood to plasma. Can anyone suggest the way out i.e. any reductase inhibitor that can work or derivetization of sulfoxide to prevent conversion to free-drug or any oxygen scavenger add in blood.