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Lethal molecule. A "ball-and-stick" model of the nerve agent sarin, C4H10FO2P. Carbon in black, hydrogen in white, fluorine in green, oxygen in red, and phosphorus in orange.

Ben Mills/Wikimedia

Backgrounder: Clues to a Chemical Weapons Attack in Syria

The United States is weighing a military strike against Syria after concluding that Syrian President Bashar al-Assad is responsible for an “undeniable” chemical attack that killed hundreds of people. This week, U.N. investigators are interviewing survivors and collecting samples in the area near Damascus where the 21 August attack allegedly occurred. The team includes experts from the World Health Organization and the Organisation for the Prohibition of Chemical Weapons (OPCW). Judging from videos and other accounts of the attack, chemical weapons specialists say that the investigators are likely to focus on suspicions that the attack involved the nerve agent sarin. It is an inhibitor of the enzyme acetylcholinesterase, which controls the body’s levels of the neurotransmitter acetylcholine. More background:

Q: Why do experts suspect that sarin was used, not another nerve agent or different chemical weapon such as mustard gas?

A: The fact that victims apparently had symptoms such as paralysis and convulsions, but didn’t suffer from skin blistering, points toward a nerve agent, not mustard gas, says Amesh Adalja, an expert on disaster medicine at the University of Pittsburgh Medical Center’s Center for Health Security in Baltimore, Maryland. Sarin is more likely than the deadlier nerve agent VX because that chemical is much less volatile and would leave contamination on equipment that could harm soldiers, according to chemist Carlos Fraga of Pacific Northwest National Laboratory in Richland, Washington. Some news accounts have also speculated that the attack involved a mixture of chemicals, which could complicate the analysis.

Q: What evidence will the U.N. investigators in Syria collect?

A: One thing to look for is debris left from the bombing, says Philip Coyle, a weapons expert and former White House national security official in the Obama administration who is now at the Center for Arms Control and Non-Proliferation in Washington, D.C. Sarin might be delivered in bomblets like those in this Wikipedia image, he says. Although each country may have its own way of packaging the agent, “people in the [Department of Defense] have a pretty good idea what Syrian weapons look like,” Coyle says.

When they interview survivors of the alleged attack, experts may test their blood for acetylcholinesterase using an assay (something like this) that is commonly used to screen for pesticide exposure. Even days after exposure, victims could still have abnormally low levels of the enzyme in their red blood cells and serum, Adalja says. But if survivors received atropine, an antidote to sarin, as has been reported, this may have restored their enzyme levels, so the test would not be useful.

Another key piece of evidence may be videos that show that victims’ pupils became very small when they were exposed, Adalja says. This is a telltale sign of exposure to high levels of acetylcholine, because it causes muscles to contract, including those that control the size of the iris.

Finding direct evidence that sarin was used will be difficult because the compound degrades quickly, particularly in warm weather, and does not persist in the body. But a breakdown product created when sarin is hydrolyzed (reacts with water), called isopropylmethylphosphonic acid (IPMA), has a much longer half-life and may still be present in soil or water samples, a swipe from a rocket, or even something like a paint chip. IPMA residues can linger for weeks or months, says chemist Armando Alcaraz of the U.S. Department of Energy’s Lawrence Livermore National Laboratory in California. The chemical is specific to sarin and its precursor chemicals. “If it was sarin, that’s what we would find,” Alcaraz says.

IPMA is also a metabolite of sarin in the body. It may be present in blood and urine samples from those who died in the bombing and from survivors; in the blood, it binds with acetylcholinesterase to form an adduct that may persist for 2 or 3 weeks.

Q: Where will the tests be done?

A: The U.N. inspectors may do initial tests in Syria if they have brought with them mobile instruments—gas chromatographs coupled to a mass spectrometer, Alcaraz says. Samples would then be sent to the OPCW center in The Hague in the Netherlands, which may distribute them to its network of 20 or so certified labs in the United States, Europe, and Asia for confirmation. (Livermore’s Alcaraz heads one of these labs.)

Q: How long will it take for the inspectors to complete their findings?

A: Alcaraz says if his lab receives samples, his team has up to 15 days to analyze it. It may take several weeks for a final report from the inspection team, Coyle says. But as he notes, the United States may not wait for the U.N. investigators’ report to launch a strike.