As the world's chemical weapons watchdog prepares to send its "fact-finding" mission to Syria, there is a warning from a British expert that it might already be too late for them to undercover the proof that chlorine gas has been used as a weapon of war.
Hamish de Bretton-Gordon has just conducted tests on three sets of samples his own network within Syria taken from Hama and Idlib provinces.
He says they provide "incontrovertible evidence", that chlorine and ammonia have been deployed against civilians.
But the samples were taken a few days after the alleged attacks. Not the weeks that the OPCW has now waited.
Mr de Bretton-Gordon told ITV News the sites have been churned up since then and that the chemicals do not persist for long in the environment.