The lab states fixation will bring about the dyes to leak in the cells. I do think the assumption is always that fixation may possibly harm mobile membranes. I did find a paper which describes fixation making use of methanol or acetic acid, just after staining by using a DCFDA https://2-fdckhcl35678.newsbloger.com/29916531/2-fdck-hcl-an-overview