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43597-AC2
Ultra-Trace Analytical Spectroscopy of Organic Compounds
The purpose of this project was investigation of Gas Chromatography- matrix isolation Fourier Transform Infrared spectroscopy-Mass Spectroscopy (GC-mi FTIR-MS) for the identification of individual analytes of petroleum and environmental interest. This technique couples conventional GC with both bench top MS and, simultaneously, with micro-FTIR. GC analytes are split ~30% to the MS and the remainder going to rotating cryogenic (10K) collection cell. The carrier gas is doped with ~ 2% Ar, which co-condenses with the analyte on the collector, resulting in a time resolved Ar matrix that can then be probed with an FTIR microscope. This enables investigators to acquire both conventional MS data and highly resolved vibrational spectra (mi FTIR) for individual chromatographic eluants. The combination of these data sets has the potential to enable investigators to accurately identify individual analytes, including differentiation of closely related isomeric species, without requiring isolation and purification of unknown compounds or access to purified reference materials.
The primary target compounds of interest in this investigation were various brominated flame retardants (BFR). BFRs are used to reduce flammability hazards in a wide range of products. At one time, these materials were presumed to be environmentally inert, due to their high molecular weights and low solubility. However, these compounds and their degradation products have now been detected worldwide in a variety of environmental and biological systems. Many of these compounds are structurally analogous to known polychloriniated compounds for which it has been shown that toxicological properties are congener- and stereoisomer-specific. It is generally believed that this will also be true for BFRs and their environmental and biological degradation products. However, absolute concentrations of BFRs and related derived products in most natural matricies are low, and the number of possible congeners is very large, making absolute structural identification of specific isomers challenging.
As a case study of this methodology, an investigation of synthetically prepared 1,2,3,4-tetrabromocyclohexane (1,2,3,4-TBCH ) was undertaken. This compound is one of the potential reductive dehalogenation products of hexabromocyclohexane (HBH) which been used as specialty BFR in some applications. There are six possible stereoisomers of this compound, which differ only in the relative orientation (axial or equatorial) of the bromine atoms relative to the plane of the cyclohexane ring. Two of these require at least one cis Br2 addition and do not occur. Hence, mixtures of this compound potentially consist of up to four epimeric products. Purified reference materials for these compounds are unavailable. GC-mi FTIR-MS-data were acquired for all components of this mixture. All compounds gave identical MS data consistent with 1,2,3,4-TBCH. However, due to differences in molecular symmetry, vibrational spectra (mi FTIR) are unique for each isomer. To achieve absolute identification for each isomer in the absence of data obtained from reference standards, the vibrational spectra all possible epimers were simulated by molecular modeling techniques. Comparison of model and empirical data allowed unambiguous assignment of individual epimeric structures.
Additional BFRs have also been investigated. The most widely used BFRs are brominated diphenyl ethers (PBDEs). There are 209 PBDE congeners and more than 800 congeners of the first hydroxylation product, which is believed to be an important intermediate in environmental and biological degradation of these materials. In most cases, isomeric products cannot be differentiated on the basis of MS alone. For this investigation, both PBDEs and a series of poly brominated phenols have been investigated. Mi FTIR data have been collected for most of the Br4-Br7 PBDEs and all of the available mono-, di- and tri-bromophenol isomers. Empirical structure-spectra correlations have been developed for the phenols and have been successfully applied to identification of isomeric PBDEs.
Impact on Students. The support received from PRF was used primarily for graduate student support. One PhD student was supported for two years and has now gone on to employment as an assistant professor at Vincennes University. Additionally, in the second year of support, the PI elected to forego summary salary support and instead used those funds to support additional graduate students over the summer. These students were assigned projects that provided them the opportunity to become familiar with this analytical technique (GC-mi FTIR-MS) and to apply this to a variety of additional systems of broad geological and environmental interest. One of these projects, an investigation of diterpene biomarkes in a Lower cretaceous fossil resin, has already resulted in a presentation and a submitted publication
