Abstract
Position-specific isotope analysis (PSIA) by NMR spectroscopy is a technique that provides quantitative isotopic values for every site—a so-called isotopic fingerprint—of a compound of interest. The isotopic fingerprint can be used to link samples with a common origin or to attribute a synthetic chemical to its precursor source. Despite PSIA by NMR being a powerful tool in chemical forensics, it has not yet been applied on chemical warfare agents (CWAs). In this study, different batches of the CWA Soman were synthesized from three distinctive pinacolyl alcohols (PinOHs). Prior to NMR analysis, the Soman samples were hydrolyzed to the less toxic pinacolyl methylphosphonate (PMP), which is a common degradation product. The PinOHs and PMPs were applied to PSIA by 2H NMR experiments to measure the isotopic distribution of naturally abundant 2H within the pinacolyl moiety. By normalizing the 2H NMR peak areas, we show that the different PinOHs have unique intramolecular isotopic distributions. This normalization method makes the study independent of references and sample concentration. We also demonstrate, for the first time, that the isotopic fingerprint retrieved from PSIA by NMR remains stable during the production and degradation of the CWA. By comparing the intramolecular isotopic profiles of the precursor PinOH with the degradation product PMP, it is possible to attribute them to each other.
Highlights
Position-specific isotope analysis (PSIA) by NMR spectroscopy is a technique that provides quantitative isotopic values for every site a so-called isotopic fingerprint of a compound of interest
The chemical analysis of chemical warfare agents (CWAs) in environmental materials sampled at the scene was sufficient to provide evidence that a toxic chemical or its degradation products was present at the site
Since this study was exploratory and the pinacolyl moiety holds a sufficient number of parameters to make an intramolecular isotopic profile, we did not use any certified internal standard with a known 2H/1H ratio to get an absolute value for every position
Summary
Quantitative 2H NMR experiments were performed on samples of PMP in chloroform (CHCl3). The neat samples were prepared without any solvent, while the diluted samples were measured by volume, diluted in CHCl3 to achieve the same concentration of the stock solution as for the samples of PMP. As for PMP, 400 μL of the stock solution was transformed to a NMR tube and further diluted (CHCl3) to a total volume of 550 μL. The reference sample was replaced with the sample to be analyzed by the 2H NMR experiment, containing PinOH or PMP in CHCl3. This sample replacement procedure was applied to all 2H NMR experiments. The normalized 2H mean values of all replicates were calculated for every position of the pinacolyl moiety in the PinOHs and in the PMPs, respectively (Table S1)
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