The lab says fixation will bring about the dyes to leak from the cells. I feel the idea is the fact fixation may possibly problems cell membranes. I did look for a paper which describes fixation using methanol or acetic acid, after staining with a DCFDA by-product. DCFDA would be https://emiliolkgdu.ltfblog.com/28732131/2-fdck-hcl-an-overview