LC-MS/MS quantitation of ribavirin in serum and identification of endogenous isobaric interferences

► Validation of an LC-MS/MS method for ribavirin and its isotopically labeled internal standard. ► A common MRM transition for ribavirin, 245 → 113, has endogenous isobaric interferences in serum. ► Isobaric interferences can be chromatographically separated from the ribavirin peak. ► Isobaric interferences are identified as uridine and cytosine.