I don't think the burden of proof is on the victims of a tyrannical regime.

Update:

• Chemical irritant canisters (like what is used to saturate an area with CS gas) can burn as hot as 800°C, so taking one for evidence is not easy to do.
• The burden of proof should be on the government to prove what they are using and that anyone under their command is following the correct procedures. If if anyone is found to be in violation, they should be investigated at the least. That is not what the regime is doing, they are simply denying without proof, going as far as to say they aren't even using chemical munitions.