Used as a stable isotope-labeled internal standard in mass spectrometry for the quantification of cotinine in biological samples such as blood, urine, and saliva. Enables accurate measurement of tobacco exposure and nicotine metabolism in clinical and research settings. Its deuterated methyl group provides a distinct mass difference, allowing clear differentiation from endogenous cotinine during analytical testing. Commonly applied in pharmacokinetic studies, smoking cessation programs, and biomonitoring surveys.