Structure elucidation of four possible biogenic organohalogens using isotope exchange mass spectrometry

Abstract

The molecular structures of four unknown bioaccumulating halogenated compounds, C 10H 6N 2Br 3Cl 3, C 10H 6N 2Br 4Cl 2, C 10H 6N 2Br 5Cl, and C 10H 6N 2Br 6 were characterized using isotope exchange positive chemical ionization (IE–PCI) mass spectrometry (MS) and identified by comparison to synthesized standards. NH 3 and ND 3 were used as reagent gases for the IE–PCI–MS experiment. The shift in mass of the quasimolecular ion between the NH 3 and ND 3 PCI obtained spectra indicated the number of exchangeable hydrogens attached to the two nitrogen atoms in C 10H 6N 2Br 4Cl 2, and thus the type of amines present (primary, secondary, or tertiary). 19 compounds (13 amines of varying degree of substitution; six containing no nitrogen) were used as reference compounds and controls in the experiment to validate the IE–PCI technique. The results of the IE–PCI–MS indicated the presence of two tertiary amine functional groups. The molecular structures of the four hexahalogenated compounds were then proposed to be 1,1 ′-dimethyl-3,3 ′,4,-tribromo-4 ′,5,5 ′-trichloro-2,2 ′-bipyrrole, 1,1 ′-dimethyl-3,3 ′,4,4 ′-tetrabromo-5,5 ′-dichloro-2,2 ′-bipyrrole, 1,1 ′-dimethyl-3,3 ′,4,4 ′,5-pentabromo-5 ′-chloro-2,2 ′-bipyrrole, and 1,1 ′-dimethyl-3,3 ′,4,4 ′,5,5 ′-hexabromo-2,2 ′-bipyrrole and subsequently synthesized. Comparison of retention times and electron capture negative ionization (ECNI) full scans on various gas chromatography (GC) columns between the synthesized bipyrroles and the corresponding unknown compounds in biota indicated that three of the unknown compounds – possible marine natural products – were the proposed halogenated dimethyl bipyrroles. The placement of the halogen atoms on the fourth compound, C 10H 6N 2Br 3Cl 3 could not be unequivocally determined since the synthesized standard could not be fully characterized.