1 Bureau International des Poids et Mesures (BIPM), Pavillon de Breteuil, F-92312 Sèvres Cedex, France.
2 National Institute of Standards and Technology (NIST), 100 Bureau Drive, Gaithersburg, MD 20899-8393, USA (on secondment at the BIPM during the measurements reported here).
3 ICOS-Flask-and Calibration Laboratory (CAL-FCL). Max-Planck-Institute for Biogeochemistry. Kahlaische Str. 4. D-07745 Jena, Germany.