Update: December, 2006
The NOAA-2006 N2O scale was recently developed. Several standards that defined the 2000 scale were nearing the end of their useful lives and needed to be replaced. Seven standards prepared in 2003 and 2005 have been added to the scale. Eleven standards from the 2000 scale were removed as they were getting low in pressure.
The 2006 scale is based on 13 primary standards prepared by static gravimetric dilution from 99.9% pure N2O in 5.9-L aluminum cylinders. All standards contain CO2 and SF6 at near ambient levels. Assigned values were determined by fitting a 2nd order polynomial to the response of each standard (relative to a 318 ppb natural air secondary standard) measured using an HP6890 GC with electron capture detection. Analysis precision was typically less than 0.05% (one standard deviation). The standard deviation of resuduals is 0.33 ppb
See: Hall et al., The NOAA nitrous oxide calibration scale for atmospheric observations, J. Gephys. Res., in press (2007).
The new scale is very similar to the 2000 scale, differing by only -0.19 ppb at 320 ppb N2O
The 2000 scale can be converted to the 2006 scale using the following polynomial:
Y = -2.2020510-7*X3 + 1.2070410-4*X2 + 0.98343*X
where Y is mole fraction (ppb) on the 2006 scale and X is mole fraction on the 2000 scale. The above relationship is valid over the range 260-370 ppb.
The reproducibility of N2O calibrations was evaluated by measuring a 313.4 ppb standard every 2-3 weeks for 2 years. The standard deviation of all measurements was 0.09 ppb and nearly all results were consistent within 0.2 ppb. Therefore, we estimate the reproducibility of N2O analysis to be 0.2 ppb or better.
Standards that define the NOAA-2006 N2O scale