Samples from victims could fail to show evidence of sarin gas

It will be difficult – but not impossible – to detect the chemical breakdown products of any nerve agents that may have been used in the Damascus attack, but access to the victims could be critical in proving the illicit use of chemical weapons.

Sarin, the most likely nerve agent to be used in a chemical attack, is a small molecule that quickly breaks down within the human body and in the environment after it is released. However, a minuscule amount of breakdown product can persist in the victims’ blood for between 16 and 26 days.

The UN weapons inspectors will be seeking to take blood samples from living victims which they will take away for analysis in a relatively sophisticated setting, notably a laboratory with gas or liquid mass spectrometers that can detect the smallest amounts of specific chemicals.

Even so, Japanese scientists who analysed blood samples of sarin victims in the Matsumoto and Tokyo subway attacks of 1994 and 1995, failed to find breakdown products in some of the individuals who were known to be exposed to the lethal agent. So there is always a chance that the blood samples could give a negative result, even with genuine exposure to sarin.

Nevertheless, the UN scientists will be looking for the presence of sarin “adducts”, the compounds resulting from the binding of biological molecules to the smaller breakdown chemicals of the sarin molecule – notably isopropyl methylphosphonic acid (IMPA).

Experts believe that the presence of IMPA in the blood samples of victims will be incontrovertible evidence, if not proof, that sarin has been used. Such biochemical evidence will of course bolster any other chemical or physical evidence gleaned from the inspection of the site of the attack, along with the visual evidence from the many amateur videos taken of the victims within hours of the attack.

If the Syrian authorities were indeed playing for time in delaying its permission for the inspectors to visit the site and the victims – in the hope of the evidence being dissipated or destroyed – then they probably hadn’t bargained on the power of modern analytical chemistry to detect the smallest quantities of the unique breakdown products of the sarin nerve agent.

Tissue samples taken from the victims of previous attacks have already revealed the use of sarin according to French chemical weapons experts and Britain’s Defence Science and Technology Laboratory at Porton Down. However, these results did not indicate who had deployed the sarin and on what sort of scale.

The difference with the latest analysis is that UN inspectors are collecting and verifying the samples themselves, and they are in a much better position to judge the scale of the attack – and, possibly, who was responsible.