EthylenethioureaTandem Mass SpectrometrySpectrometry, Mass, Electrospray IonizationMass SpectrometryChromatography, LiquidChromatography, High Pressure LiquidGas Chromatography-Mass SpectrometryReproducibility of ResultsReference StandardsSolid Phase ExtractionLimit of DetectionCadmium RadioisotopesSpectrometry, Mass, Matrix-Assisted Laser Desorption-IonizationSensitivity and SpecificityCalibrationPeptidesMolecular StructureDrug StabilityProteomics