Response of halogenated compounds in ion mobility spectrometry depending on their structural features

Ion mobility spectrometry (IMS) with handheld and transportable devices permits the sensitive detection of chlorinated compounds which are important in environmental monitoring. The ion mobility spectra in negative measuring modus mostly show one product ion peak (H₂O)_nCl⁻] due to dissociative electron attachments. In this paper, we investigated relevant chlorinated compounds (R-Cl) where R represents allyl-, benzyl-, phenyl-, alkyl- and vinyl-groups. These groups cause differences in the R-Cl bond strength and differences in the cleavage of chlorine can therefore be expected. All chlorinated substances investigated provide the same product ion peak at 2.75 cm² Vs⁻¹ independent on the different C-Cl bond strength. However, distinct influences of structural features on the peak intensities of the (H₂O)_nCl⁻ product ion peak were established. Generally, increasing sensitivities were obtained in the order chlorobenzenes < vinyl- < allyl- < alkyl compounds < benzylchlorides. Sensitivities and limits of detection (LODs) of aromatic compounds depend on the nature and position of second substituent. Electron-withdrawing substituents (chlorine, fluorine, nitrile) enhance sensitivity while electron-repelling substituents decrease it. A dependence of sensitivity on the chain length or ring size can be observed for alkyl compounds. Additional influences of intramolecular interactions on the sensitivity were found for di-halogenated compounds. Therefore, the quantification of negative product ion peaks of chlorinated compounds requires a consideration of structural features of analytes.