Rabeprazole-d4, a deuterated analog of rabeprazole, is used as an internal standard in mass spectrometry for the quantification of rabeprazole in biological samples. Its isotopic labeling enables precise differentiation from the parent compound. Helps in pharmacokinetic studies to understand drug metabolism and distribution. Essential in clinical research for ensuring accurate and precise measurements of rabeprazole levels in plasma or serum. Supports the development of analytical methods for therapeutic drug monitoring.