Chemical Forensics – Seeking Clues at the Molecular Level

By Michael Edmonds 19/03/2012

According to a recent report by the Center for a New American Security, a think tank specialising in US national security issues, “a generally skilled chemist can produce a chemical weapon, whereas a generally trained biologist is likely to have more difficulty propagating and conserving an unfamiliar pathogen.”

The threat of chemical (and biological) weapons has resulted in the development of new techniques that could prove suitable in detecting the presence and origin of various chemical weapons, providing evidence for the prosecution of those found preparing such weapons. Such techniques include:

Impurity profiling is a technique which involves carefully analysing a substance to see what other compounds are present in trace amounts. previously this method has been applied to the analysis of illicit drugs. During the drug process, various trace impurities are produced which may vary from batch to batch and supplier to supplier. By profiling the impurities, it is possible to determine whether different samples were produced by the same supplier. Also, where the starting materials include a natural product (e.g. cocaine produced from the coca plant), impurities may help identify geographically the source of the drug.

The “back yard” preparation of chemical weapons will produce impurities which will not only identify which procedure has been used to prepare the weapon, it may also identify specific sources. Thus the presence of even trace residues may provide investigators with clues about where chemical were obtained from. Researchers preparing sarin using commercial starting materials found that not only could impurity profiling help identify which company one of the precursor chemicals had been purchased from, it could also identify which chemical batch it came from.

Research looking at ricin, found that purification of ricin using four different methods (three of which were based on “anarchist literature”) showed that each method resulted in a different impurity profile.

Stable isotope ratio analysis can also be used to determine the source of a chemical weapon. For example, an analysis of 65 different samples of cyanide demonstrated that it was possible to differentiate 95% of the samples based on stable isotope ratios. Similar results have been found with various other toxic compounds which could be used as poisons. Isotope ratios can also be used to solve crimes in terms of isoscapes which I have written about elsewhere.

The comparison of diastereomeric ratios in compounds which contain chiral centres can also be used to identify the source. Researchers at Pacific Northwest National Laboratory have, for example, used nuclear magnetic resonance to show that the difference in diastereomeric ration between different samples of the rodenticide, brodifacoum, not only allowed them to trace it back to the manufacturer but also in some cases to the batch number.

As chemical techniques continue to advance we seem to be able to learn more and more from samples of less and less size. Criminals and terrorists beware – even trace amounts of molecules left behind at a crime scene could be your downfall.