Description. Microtube, 1.5mL, Attached Screw Cap for Color Insert, with O-Ring, STERILE, PP, 500/Bag, 2 Bags/Case, 1000-Case. These disposable screwcap microcentrifuge tubes are available sterile or non-sterile. The sterile tubes include a cap with nitrile o-ring. The non-sterile tubes and caps are sold separately so that you have a choice of