... 2 ,3, 7,8-TCDD are 32 0, 32 2, and 257. Use either 37 Cl 4 -2 ,3, 7,8-TCDD or 13 C 12 -2 ,3, 7,8-TCDD as an internal standard. The m/z for thesetwo internal standards are 32 8 and 33 2, ... respectively.When using high resolution mass spectrometry (HRMS), the characteristicm/z for 2 ,3, 7,8-TCDD are 31 9.8965 and 32 1.8 936 . The m/z for the corresponding 37 C l4 - and 13C 12 -isomers in ... Chlorosub-stitution in positions 2, 3, 7, and 8 gives 2 ,3, 7,8-tetrachlorodibenzo- p -dioxin(2 ,3, 7,8-TCDD), CAS [1746-01-6], occurring in trace amount in chlorophenoxy-acid herbicide, 2,4,5-T. 2 ,3, 7,8-TCDD...