AbstractA method is described for the determination of 22 tetrachlorodibenzo‐p‐dioxin isomers at the low part‐per‐trillion (ppt) level. High resolution, narrow bore, open tubular columns (OTCs) with 100 μm i.d. can achieve better separations than presently used 320 or 250 μm i.d. columns in about half the total time. Relative retention times and response factors for all 22 TCDD isomers are presented for the electron capture detection and for the molecular ion mass of individual isomers under electron impact ionization with selected ion monitor