Update: December, 2011
The 2006 N2O 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.
The reproducibility of N2O calibrations in the ambient range is ~0.22 ppb (95% confidence level). Reproducibility is estimated from repeated analysis of tertiary standards, including target tanks which are analyzed about once a month.
NOTE: We have recently adjusted N2O mole fractions determined between January 2006 and October 2011 because we have discovered that one of the five secondary standards used to calibrate the N2O instrument has been drifting. This secondary standard is drifting at a rate of -0.047 ppb per year. This induced an apparent scale drift of +0.024 ppb/yr. This drift not only impacted calibrations performed when this secondary standard was in use, but it also affected the transfer of the scale from one set of secondaries (used from Jan. 2006 to Jul. 2010) to a new set (in use since Jul. 2010). A new method of drift correction was implemented December 2011 and all data were reprocessed. In addition, curve fitting routines have been updated from “ordinary least squares” to “orthogonal distance regression”. These changes resulted in small (0.05-0.1 ppb) changes in assigned N2O values.
Because the scale is still based on the same 2006 primary standards, the scale has been renamed 2006A. Further, because the new curve fitting routines are applied on a monthly basis, it is not possible to determine a single formula to relate the 2006 and 2006A scales.
For updated N2O results, see the GML CCL Reference Gas Calibrations page.