... Chlorosub-stitution in positions 2, 3, 7, and 8 gives 2,3 ,7, 8-tetrachlorodibenzo- p -dioxin(2,3 ,7, 8-TCDD), CAS [ 174 6-01-6], occurring in trace amount in chlorophenoxy-acid herbicide, 2,4,5-T. 2,3 ,7, 8-TCDD ... and 2 57. 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 328 and 332, respectively.When using ... characteristicm/z for 2,3 ,7, 8-TCDD are 319.8965 and 321.8936. The m/z for the corresponding 37 C l4 - and 13C 12 -isomers in HRMS are 3 27. 88 47 and 331.93 67, respectively.The quantitation...