The WMO CO Mole Fraction Scale consists of a set of 17 CO reference mixtures, ranging from 25 to 1003 ppb (parts per billion by mole fraction) in zero natural air and are contained in 5.9-L, high-pressure aluminum cylinders. Carbon monoxide was measured using gas chromatography with a mercuric oxide detector. The concentration range of the standards is sufficient to cover that of the background troposphere and also that found in remote locations affected by anthropogenic activities.
The low concentration standards were prepared by gravimetric methods using one of three high concentration standards as the parent. Two of the parents were prepared by gravimetric methods starting from high-purity (99.97%) CO to have concentrations of about 250 ppm (parts per million) CO. A total of 14 atmospheric level primary standards were prepared from these two parents. The third parent was a NIST SRM (National Institute of Standards and Technology, Standard Reference Material) having 9.7 ppm CO, from which three standards were prepared.
Novelli, P. C., J. E. Collins, Jr., R. C. Myers, G. W. Sachse, and H. E. Scheel (1994), Reevaluation of the NOAA/ESRL carbon monoxide reference scale and comparisons with CO reference gasses at NASA-Langley and the Fraunhofer Institut, J. Geophys. Res., 99, No. D6, 12, 833-12, 839.
Novelli, P. C., J. W. Elkins and L. P. Steele (1991), The Development and Evaluation of a Gravimetric Reference Scale For Measurements of Atmospheric Carbon Monoxide, Journal of Geophysical Research-Atmospheres, 96(D7), 13109-13121, JD01108.