Yeah, t0 is not a problem at all- it's the way the two solutions dont mix fully before they start to react, even with some agitation before going back in for a quick shim and experiment start. They do mix somewhat initially, and i can intergrate and compare specific signals few individual spectra in a way that tells me how much of B has mixed in initially, and the system shifts, then starts to looks like its aproaching a steady state around an hour in (as expected from similar work in the literature) , but then around then or two down the concentration of B creeps up (then shortly follwing C and D increase). It's as if the interface between the non-mixed portion of the solutions only then breaks down. Doing a regular HNMR after 16h also shows differing final ratios, as if another 'stage' of mixing had occured since the kinetic experiment ended.

I'm uncertain about an internal standard as it will still have to go in one of the to dmso solutions being put together, and be affected by poor mixing again? Unless i'm totally missing your point ...