The latest tests from Porton Down will strengthen the consensus that the nerve agent sarin was used in the deadly attack in eastern Damascus in August.
Scientists at the lab found signs of sarin in soil taken from the site of the attack, and also on clothing from a person caught up in the atrocity that the US says killed more than 1,400 people.
The UK government will not say what signatures of sarin the scientists have found, but the details are important. The specific chemicals that tests reveal are often the only solid proof that a nerve agent has been used, and what kind of chemical was unleashed.
Tests on soil and clothing will rarely pick up sarin itself, because the agent breaks down swiftly when it meets water, which could be moisture in the air or sweat from the victim. More likely they will detect traces of sarin's breakdown products, but these must be interpreted with care.
Beyond finding intact sarin, the most convincing smoking gun for the nerve agent is a compound called isopropyl methylphosphonic acid (IMPA). This is the chemical that sarin degrades into first of all and it can come from nothing else. Among chemical weapons experts, a positive test for IMPA is generally regarded as proof of sarin.
But tests can struggle to pick up IMPA in samples much older than a week or so, because it too breaks down into other substances. One is a related compound, called methylphosphonic acid (MPA), but this is not solid proof for sarin: other agents also degrade into MPA, such as VX, soman and cylcosarin.
Another issue of prime concern is how reliable the samples are. The Porton Down scientists tested soil and clothing from an alleged victim of the attack, but the material comes from a second country. Without eyes on the samples from source to lab, it is impossible to rule out tampering. This is one of the key advantages of the UN inspection team, which gathered its own material and ensured that it was delivered to various labs around the world without being interfered with.
Porton Down's analysis will have taken around two days. As soon as the material arrived, scientists will have worked up control samples to check their analytical equipment worked properly. The clothing will then have been soaked in water to extract water-soluble chemicals, and then in an organic solvent to extract other substances. After calibrating their machines, they could then test vapours of the samples using a technique called GC-MS (for gas chromatography with mass spectrometry) or a more sensitive version called LC-MS, which uses a liquid sample instead of a gas.
Material gathered by the UN inspectors in Syria will take a couple more weeks or so to analyse for several reasons. For a start, the team collected far more samples, including soil, tissue, blood, urine, hair and wipes from munitions. These are sent to labs that belong to the OPCW (Organisation for the Prohibition of Chemical Weapons) network, but each sample arrives with two others that triples their workload.
The extra specimens help to ensure that the tests are valid. If the sample being analysed is a piece of clothing, the two extra samples will be a "blank", which is a clean piece of clothing, and a "spiked" fragment of clothing that has been intentionally laced with a small amount of a substance usually found after a chemical weapons attack. The labs are not told which is the authentic sample, and must detect the laced chemical and report a negative result for the blank.
That is just the start. If a laboratory finds signs of a chemical agent, it must follow up with a second test that uses different equipment. If that tests positive, the scientists must then synthesise the chemical the equipment has found and test that as a final proof. To make the procedure even more laborious, every sample must be sent to at least two laboratories for independent testing. If their results do not agree, a third laboratory performs a fresh analysis.