This was known in the immediate aftermath of the "attack" actually. There were UN people on the ground who did not find any traces of sarin in the environment. Normally, trace amounts of sarin would be present for months.

This was completely and extremely cynically suppressed by the US media because it went against the narrative the media was pushing. You could find the OPCW report, but it was never reported on, and you'd need to know what is it you're looking for.

Last time I pointed this out publicly I was downvoted to hell. Think about it, what possible reason would Assad have to use the weapon which completely closes _all_ possible avenues to resolve the conflict?