I seeded cells into a single 96 well plate for an MTS assay at different time points. I want to read the first few rows at 16 hours, next few rows at 24 hrs and remaining rows at 48hrs. I was thinking I could just add MTS to the wells for that time point, read the absorbance and continue culturing the plate for the other time points. But then after seeding the cells I remembered that I need to take the lid of the plate to read and I face a contamination issue. Has anyone used the approach and how did they prevent contamination? Parafilm the wells that you want to keep covered? Or is there some other simple solution? Or opening the plate for such a brief period of time does not cause a problem because the cells will not be exposed that long nor will they be cultured much longer.