Dear Lidia, I recently purchased a CO std.gas tank from Messer Griesheim (not Linde) in Germany. The specified d18O value is +10%o. Michael Joachimski ---------------------------------------------- Dr. Michael Joachimski Institute of Geology and Mineralogy University of Erlangen Schlossgarten 5 91054 Erlangen/ Germany Tel.: +49-9131-852699 (office) Tel: +49-9131-859297 (lab) Fax: +49-9131-859295 [log in to unmask] http://www.geol.uni-erlangen.de/html/staff/joachims.html http://www.geol.uni-erlangen.de/html/Masspec/masspec.html ----------------------------------------------