The lab suggests fixation will lead to the dyes to leak with the cells. I feel the assumption is always that fixation might injury mobile membranes. I did locate a paper which describes fixation applying methanol or acetic acid, following staining with a DCFDA derivative. DCFDA would be the reagent https://finnnssql.fitnell.com/68742505/everything-about-2-fdck-hcl