Used as a labeled internal standard in analytical chemistry, particularly in mass spectrometry, to ensure accurate quantification of toluidine derivatives in complex mixtures. Its deuterated form provides a stable reference point, minimizing interference during analysis. Applied in environmental testing to detect and measure toluidine contamination in water and soil samples. Also utilized in pharmaceutical research for tracing and analyzing metabolites in drug development studies. Its isotopic labeling aids in distinguishing it from non-deuterated compounds, enhancing precision in experimental results.