The problem of  "Negative interference" associated with liquid-liquid extraction-derivatization techniques.
D002849:Chromatography, Gas / Q000379:methods*; D003061:Codeine / Q000032:analysis; D003932:Heroin / Q000032:analysis*; D013058:Mass Spectrometry / Q000379:methods*; D009020:Morphine / Q000032:analysis; D009022:Morphine Derivatives / Q000032:analysis
D009022:Morphine Derivatives; D003932:Heroin; D009020:Morphine; D003061:Codeine