It's a 0.2mL tube with 25uL inside, so the headspace is about 175uL. There was definite condensation on the inside of the cap- I considered swirling the sample but prefer not to for a few reasons. When I tested the samples I found the concentration of my analyte increased faster during storage in the fridge as opposed to at RT. That seemed odd because I expected more evaporation in the tubes stored at RT. Does that make sense? Just trying to rationalise why analyte concentration seemed to deteriorate (increase) faster in tubes stored in the fridge, not at RT.