Update: April, 2009
The NOAA-2006 SF6 scale was recently developed and supercedes the 2000 scale. Several standards that defined the 2000 scale were nearing the end of their useful lives and needed to be replaced.
The 2006 scale is based on 16 primary standards prepared by static gravimetric dilution from 99.99% pure SF6 in 5.9-L aluminum cylinders. All standards contain CO2 and N2O at near ambient levels. Assigned values were determined by fitting a line to the response of each standard (relative to a 5.45 ppt natural air secondary standard) measured using an HP6890 GC with electron capture detection. Analysis precision was typically less than 1% (one standard deviation).
The 2006 scale is 0.033 ppt higher than the 2000 scale at 4 ppt.
To update the 2000 scale to the 2006 scale use:
Y = 4.8546e-3 * X2 + 9.3479e-1 * X + 0.21664
(where Y = 2006 scale, X = 2000 scale).
|2000 Scale||2006 scale||Difference|
The main advantage of these new standards over the previous set is that they have enabled us to confirm that the ECD used for SF6 analysis has a linear response (0-10 ppt) and a zero intercept. A zero intercept was not evident with the previous set of standards.
The reproducibility of SF6 calibrations was evaluated by measuring five secondary standards (ranging ftom 4.5 to 7.8 ppt) every 2-3 weeks. Treating these standards as unknowns, the assigned mixing ratios have varied less than 0.04 ppt over the last 2 years. Therefore, we estimate the reproducibility of SF6 analysis to be 0.04 ppt.
Standards that define the NOAA-2006 SF6 scale
|Cylinder||Year||Prepared (ppt)||Assigned (ppt)||Residual (ppt)|