Several newspapers claim that U.S. Secretary of State Kerry said that there is proof of sarin gas used in the recent Syrian incident. These claims are false.
- McClatchy headlines: Kerry says it was sarin gas in Syria; Congress questions US stake
- The Washington Post: Sarin gas used in Syria attack, Kerry says
- The BBC: John Kerry: US ‘has evidence of Syrian sarin use’
All these headline claims are false. From the McClatchy report this is what Kerry said:
“We have learned through samples that were provided to the United States and that have now been tested from first responders in east Damascus (that) hair samples and blood samples have tested positive for signatures of sarin,” Kerry said on NBC’s Meet the Press.
It is important to differentiate between sarin and mere “signatures of sarin”. (UPDATE) The “signature” does not say much about what chemical exposure happened. The U.S. Army book Medical Aspects of Chemical Warfareexplicitly says that concluding on a chemical agent exposure from “signatures” in bio-samples is false (Chapter 22(pdf)):
Assay of Parent Compoundsanalyzing for parent nerve agents from biomedical matrices, such as blood or urine, is not a viable diagnostic technique for retrospective detection of exposure.
(/update) Sarin, a fluid at room temperature, evaporates and decomposes rapidly. A few hours after sarin exposure it is nearly impossible to find pure sarin samples on or within a human body. Sarin molecules react with other molecules and fall apart. What might be found in a biological sample of someone who was exposed to sarin are therefore only decomposition products of sarin. But the same decomposition products can also occur from exposition to other chemical substances. Especially exposure to typical farming insecticides, chemically organophosphates, is likely to create the same decomposition products that sarin exposure does.
A number of reports have claimed to have proven the use of sarin through tests on hair, clothing, blood, tissue, and urine samples.At least one study shows that the presence of a nerve agent could be deduced by examining post-mortem blood samples for presence or lack of acetylcholinesterase, up to a week after death. A person who has died from Sarin exposure would have little or no acetylcholinesterase present. It should be noted that this would only indicate the presence of a nerve agent and would not specifically indicate Sarin versus any other nerve agent (or even organophosphate pesticide intoxication) nor would it conclusively indicate nerve agent as a cause of death, as other factors may have killed the victim, such as conventional trauma.
One of the decomposition products of Sarin in the human body is methylphosphonic acid. A study shows that this substance is detectable in urine by use of mass spectrometry. This particular substance is not specific to Sarin.
Is it possible other substances could produce false positives for sarin?
Yes. Generally, the more sophisticated and expensive the detection technique, the less scope for false positives. The false positives depend entirely on the detection method. IMS is often fooled by chemicals of the same molecular weight as Sarin. Organophosphate-based pesticides are very similar chemicals to nerve agent chemical weapons, so they may pose a false positive.
The hair and blood samples the U.S. tested came from the insurgents in Syria through an insecure custody chain of evidence. They did not test positive for sarin but showed decomposition products that may have come from exposure to sarin or may have come from exposure to insecticides or some other class of chemical substances.
It is also very important to keep in mind that even proven evidence of exposure to sarin, or any insecticide, does not say anything at all about how such an exposure might have happened and who might have been responsible for it.
The indications that the insurgents in Syria might have been responsible are at least as strong as the indications of government use. The insurgents, who do want the U.S. to intervene on their side, also have a very strong motive to create such an incident.
It is irresponsible that headline writers and journalists fail to explain these contexts and claim “sarin usage” while the evidence thereof is inconclusive and while not even the U.S. government made such a indefensible claim at all.