With all due respect, you are missing my point. Both sides have chemical weapons, but only ISIS and the Assad regime have been documented as using them.  As I specified in my comment, ISIS isn't there.  This means this can't be blamed on ISIS. Since al-Nusra Front has no jet fighters,  it leaves either the Russians or the Syrians. Organizations like Amnesty International have confirmed that it was an air strike. 

 Just because you've never heard of air to surface missiles being used in chemical attacks doesn't mean it doesn't exist. Cold War Soviet Navy doctrine, for example, assumed a chemical warfare environment and all Russian tactical weapons, including air to surface missiles, can be configured to carry chemical weapons. The warhead is like any other once fitted to the airplane.

As chemical weapons expert, Col. Hamish de Bretton-Gordon, told BBC Radio 4 "If you blow up sarin, you destroy it.". This would preclude an airstrike on a chemical weapons facility.