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 (Δ(f)H) 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 Δ(f)H series. The best Δ(f)H 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 Δ(f) H 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 Δ(f)H 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 Δ(f)H 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 Δ(f)H 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.