Discovery of selective markers for chlorine gas exposure in concrete

Chlorine gas (Cl₂) is still being used as a chemical weapon in modern war zones. After an attack with this toxic chemical, verifying its use is challenging due to its reactivity and rapid evaporation. Additionally, only a limited number of selective markers relating to the use of chlorine gas are found. For forensic purposes it is necessary to discriminate Cl₂ from other chlorine containing chemicals. Therefore, the current study aims to use gas chromatography–mass spectrometry (GC–MS) and liquid chromatography-high resolution tandem mass spectrometry (LC-HRMS/MS) together with machine learning methods to discover selective markers for chlorine gas exposure. Four types of concrete originating from Europe or the Middle East were exposed to various levels of Cl₂ as well as to some commonly used chlorine containing chemicals. After extraction with various solvents, the samples were analyzed. Based on the tentative identification of 32 chlorinated markers with GC–MS and 146 markers with LC-HRMS/MS, principal component analysis (PCA) and linear discriminant analysis (LDA) models were constructed. A clear distinction between concrete samples exposed to bleach and chlorine gas was apparent. For forensic classification, a Bayesian likelihood ratio (LR) model was applied that showed limited rates of misleading evidence and maximum LRs of 0.038 to 93. The markers chloroacetone, dichloroacetone, tetrachlorophenol, and 5-chloro-8-quinolinol were found to be selective for chlorine gas exposure. Their identity was verified by GC–MS/MS and LC-MS/MS in comparison with commercially available reference standards. In conclusion, this study demonstrates the feasibility of chemical profiling in concrete to differentiate among various chlorinating agents.