The lab suggests fixation will cause the dyes to leak with the cells. I believe the idea is the fact that fixation may well destruction mobile membranes. I did locate a paper which describes fixation applying methanol or acetic acid, following staining with a DCFDA derivative. DCFDA is the reagent https://2-fdck-hcl36801.bloggip.com/29400917/the-ultimate-guide-to-2-fdck-hcl