What is the matrix?  Typically if this is blood, the hemoglobin iron and air will keep everything oxidized.  Urine usually does not have sufficient enzyme activity to cause an issue in most cases.  However, free thiols either on proteins or glutathione could be responsible for the transformation (thiols in the presence of metals can cause reduction of sulfoxides while being oxidized to disulfides).  One possible solution if you feel it is enzyme mediated is to "denature" the sample (e.g., addition of organic solvent - many other possibilities exist - but will not necessarily remove glutathione).  Another possible solution is to take the sample through a rapid partial purification on SPE prior to  analysis.  If the extract is aqueous, add the extract directly to SPE, rinse with water and an aqueous/acetonitrile (ACN) mix that does not elute analyte(s), then elute with lowest percent of ACN in water that elutes all analytes of interest (e.g., 80% ACN/water).  Evaporate either to dryness or to water (volume can be estimated by weight) and analyze. 

Thanks Larry. It's human blood & plasma. Denaturing the sample or partial purification on SPE is not practical solution for me as I have to deal with numbers of sample intended for evaluation of bio-equivalency of two formulations. Further, I have to intact these blood or plasma samples for longer duration in deep freezer. Usage of DBS (Dried blood spot) was in my mind but many a time this technique is failed in practical use and also we are not having the require resources for the use of DBS.

I am looking for derivatizing the sulfoxide or to inhibit the responsible reductase enzymes or Glutathione which can be easy for me for handling of numbers of samples.

​If you didn't find good inhibitors, please see these reductase enzymes have generally a high affinity for oxidized amino acids like methionine sulfoxide and others...You can expose these aminoacids containing C-S fragments to H2O2, to obtain sulfoxides forms (that are competing molecules for the enzyme)...please assess carefully if your drug is a weak competing agent for these enzymes when compared with the sulfoxides forms and confirm before if the sulfoxides obtained are not artifact analytical for your method.

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