The lab claims fixation will result in the dyes to leak with the cells. I think the idea is usually that fixation may perhaps problems mobile membranes. I did look for a paper which describes fixation making use of methanol or acetic acid, immediately after staining using a DCFDA derivative. https://jaidenfbsgt.buyoutblog.com/28061882/examine-this-report-on-2-fdck-hcl