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Terahertz time-domain spectroscopy (THz-TDS) technique allows us to analyze plastic additives. A series of hindered amine light stabilizers (HALSs), including TMP, PMP, BB-PMP, Chimassorb 944 and 119, and a nitroxide free radical of 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy (HTEMPO) in the matrix of high-density polyethylene are characterized in the range of 0.2-2.6 THz. TMP, PMP, and BB-PMP exhibit individual well-resolved terahertz (THz) absorption spectra at room temperature. Furthermore, a linear relationship of the content of TMP versus the intensity of it first THz absorption peak are demonstrated with the detection limit of about 1.6% (w/w). In addition, PMP exhibit water-dependent characteristic THz absorption spectra. THz-TDS will afford a potential tool to investigate the stabilizing mechanism of HALS and monitor their evolution. [ABSTRACT FROM AUTHOR]