Normally, the Organisation for the Prohibition of Chemical Weapons (OPCW) carries out inspections when there is a suspicion of chemical weapons being used.

BBC: Syria chemical weapons claims: UK and US tread cautiously

For this off-site analysis, the Organisation for the Prohibition of Chemical Weapons (OPCW) - the implementing agency of the Chemical Weapons Convention - has established an international network of highly competent so-called "designated laboratories" that routinely undergo proficiency testing to ensure that their standard of analysis is up to the needs.

BBC: How to investigate chemical weapons allegations