Polypropylene 1.4 ml U-bottom tubes have internal threads that accept screw caps for secure closure in ultra low temperatures. The polypropylene screw caps (sold separately) have silicone O-rings that maintain their integrity in vapor phase liquid nitrogen. Certified RNase and DNase free and manufactured to Class 10,000 regulations. Working volume: 1.12 ml with screw cap or 1.19 ml with push cap.