It is important to understand the various mechanisms that form the OPCW and what will happen in Douma.
The team heading to Douma is a "Fact Finding Mission" (FFM). This is an important point. 1/?
It is *not* within their mandate to assign responsibility, so don't expect a silver bullet from their report. 2/?
The last JIM found that the Syrian regime was responsible for Khan Sheikhoun, & multiple attacks using chlorine gas canisters dropped from helicopters. 3/5
Russia vetoed the extension of its mandate in Autumn 2017.
Therefore, there does not currently appear to be a mechanism at international level for assigning blame for these chemical attacks (Please keep me honest on this if I'm incorrect) 4/5
(Please, any UN/OPCW people, feel free to keep me honest if I've made a mistake)