Bleach, or sodium hypochlorite (NaOCl) is optionally used during MiSeq post-run washes in order to eliminate run-to-run carryover of library template. I noticed a cloudiness in our 5% sodium hypochlorite. This stock solution was purchased several years ago and stored in the fridge as indicated on the label. The bottle had no expiration date. After some online searching it became obvious that bleach has a (very) limited shelf life, depending on the temperature and concentration. After several years our stock had decomposed to saltwater, and seemed to have some fungal growth! Fortunately our MiSeq seemed unaffected; a cursory check found no indications of run cross-contamination, and a later instrument annual maintenance found the capillaries clear and clean. Nevertheless, the moral of the story is: regularly buy fresh NaOCl for your post-run washes!
