I have had a problem, along with others in my lab, in which we snap freeze tissue samples in liquid nitrogen. When the tubes are removed before placing in a -80 degree C freezer, some tube caps burst off. . I have never had this problem before until recently. My guess is that some caps may not have a strong enough seal which is causing a small amount of N2 to enter the tube and then evaporate as the tube warms (causing it to expand and burst). But again I have never had a problem with this before. Does any one have suggestions to prevent this from occurring.