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Structural identification by differential mass spectrometry as a criterion for selecting the best quantum chemical calculation of formation enthalpy for tetrachlorinated biphenyls

Authors


A. Covaci, Toxicological Center, University of Antwerp, Wilrijk, Belgium.

E-mail: adrian.covaci@ua.ac.be

Abstract

RATIONALE

The assignment of correct structures for isomers with similar mass spectra (e.g. polyhalogenated aromatic compounds) is not always successful when spectral libraries alone are employed or, even worse, when the compounds are not present in commercial spectral libraries.

METHODS

We present a computational method based on differential mass spectrometry (Diff-MS) for the validation of formation enthalpy (ΔfH) series calculated using quantum chemistry for the fragments produced in electron ionization (EI)-MS. The method simulates the chemical structure identification (CSI) of isomers with similar mass spectra using differential mass spectra and ΔfH series. The best ΔfH values were those from which the correct structures could be derived.

RESULTS

We have used six tetrachlorinated biphenyl isomers (TeCBs 44, 46, 52, 66, 74, 77). Their EI mass spectra were acquired at 70 eV and, for the principal ions, five series of ΔfH values were computed by the semi-empirical methods, AM1, MINDO3, MNDO, PM3, and RM1. The generation of differential mass spectra and the correlation with the ΔfH series for the calculation of probabilities from the list of structural assignments were carried out with the ordering algorithm (ORD) of the CSI-Diff-MS Data Analysis 3.1.1 program.

CONCLUSIONS

Intelligent software, used for structural elucidation based on MS and QCC, was employed to select the best values of the formation enthalpies of TeCBs. The advantages and disadvantages of the semi-empirical methods for the calculation of ΔfH values for different TeCB ions are critically presented. The best semi-empirical methods were RM1, AM1 and MINDO3, which can be used to calculate the ΔfH database necessary to identify TeCB isomers. This approach allowed the correct assignment of structures for isomers with very similar mass spectra and demonstrated the reliability of the correlation between differential mass spectra and the formation enthalpies of the fragment ions. Copyright © 2012 John Wiley & Sons, Ltd.

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