We did some study about using different O-rings to store CO2 for isotopic analysis. Our paper was published in Ghemical Geology, 86 1991, 259-261, by Revesz and Coplen: "Caution on the use of Viton or FETFE O-rings in carbon dioxide sample containers for delta O-18 analysis." Kinga Revesz > Kinga Revesz chemist Water Resources Division U.S.Geological Survey 431 National Center Reston, VA. 22092 T: 703-648-5865 Fax: 703 648-5274 [log in to unmask]