Used as a stable isotope-labeled internal standard in mass spectrometry-based bioanalytical studies to accurately quantify afatinib in biological samples such as plasma, serum, or tissue extracts. Its primary application lies in pharmacokinetic and metabolism studies during drug development and clinical research. The deuterium labeling ensures minimal mass overlap with the non-labeled compound, enabling high specificity and sensitivity in detection. It is essential for validating analytical methods and ensuring reliable measurement of drug concentrations in research settings.