Diphenyl-d10 is a fully deuterated analog of diphenyl (biphenyl), used as a standard in nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS) for calibration and method development. Its d10 isotopic composition provides distinct signals without interfering with the analyte, enhancing accuracy in spectral analysis. It is also employed in research for isotopic labeling to track molecular pathways and reaction mechanisms in organic chemistry.