NMR Method Can Chase Down Chemical Threats

In the race to trace chemical weapons back to perpetrators or sources of raw materials, any molecular signature from an impurity or a characteristic isotope ratio can give forensic chemists an edge. Soon, a promising addition could join that tool kit—the ability to distinguish different salts of the same organic compound via nuclear magnetic resonance spectroscopy. Different counterions, such as sulfate or nitrate, associated with a chemical of concern make it possible to tell otherwise identical samples apart. But detection methods such as ion chromatography quickly use up precious samples and aren’t always conclusive. Pacific Northwest National Laboratory chemist John R. Cort and summer intern Athena E. Metaxas used strychnine, a poison with a storied reputation, to show that NMR is a viable alternative (Magn. Reson. Chem. 2013, DOI: 10.1002/mrc.3945). The researchers obtained spectra of strychnine salts in various solvents and demonstrated that subtle changes to chemical shifts occur with various counter­ions. The effect is more pronounced in less polar solvents where the salt forms a tighter ion pair. Many compounds of interest to forensic chemists, including illicit drugs, exist as salts, so the method may be widely applicable if it can be perfected, Cort said.