Used primarily as a stable isotope-labeled internal standard in mass spectrometry and gas chromatography-mass spectrometry (GC-MS) analyses. Enables accurate quantification of o-toluidine and related aromatic amines in biological and environmental samples by minimizing matrix effects and instrument variability. Commonly applied in occupational health studies to monitor exposure to aromatic amines, especially in industries involving dyes, rubber, and pesticides. Its deuterated structure ensures distinct mass differentiation from non-labeled compounds, improving detection specificity and analytical precision.