1,4-Dichlorobenzene-d4 is a deuterated isotope-labeled compound primarily used as an internal standard in analytical chemistry, particularly in techniques such as gas chromatography-mass spectrometry (GC-MS) and nuclear magnetic resonance (NMR) spectroscopy. The deuterium atoms enable accurate tracking and identification of chemical compounds in samples by providing distinct mass shifts or cleaner spectra with minimal hydrogen signal interference. It also serves in laboratory research for studying chemical reaction mechanisms and isotopic effects, acting as a reference to reduce analytical errors. Additionally, it can function as a tracer in environmental studies to monitor the movement and degradation of chlorinated aromatic compounds.