SF6
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 * X^2 + 9.3479e-1 * X + 0.21664 (where Y = 2006 scale, X = 2000 scale).
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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 reponse
(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 |
Assigned |
Residual |
|
|
|
(ppt) |
(ppt) |
(ppt) |
|
FA-1861 |
2000 |
2.41 |
2.39 |
-0.02 |
|
FA-1878 |
2000 |
2.91 |
2.89 |
-0.02 |
|
FA-1843 |
2000 |
4.73 |
4.76 |
0.03 |
|
FA-1850 |
2000 |
4.51 |
4.53 |
0.02 |
|
FA-1851 |
2000 |
4.32 |
4.35 |
0.03 |
|
FA-1856 |
2000 |
1.14 |
1.15 |
0.01 |
|
FA-1865 |
2000 |
4.12 |
4.13 |
0.01 |
|
FA-2205 |
2003 |
4.94 |
4.92 |
-0.02 |
|
FA-2207 |
2003 |
5.98 |
5.98 |
0.00 |
|
FA-2208 |
2003 |
6.97 |
7.01 |
0.03 |
|
FA-1940 |
2005 |
1.52 |
1.49 |
-0.03 |
|
FA-2139 |
2005 |
3.13 |
3.14 |
0.01 |
|
FA-2557 |
2005 |
3.86 |
3.85 |
-0.01 |
|
FA-2567 |
2005 |
5.99 |
6.02 |
0.03 |
|
FA-2569 |
2005 |
7.92 |
7.90 |
-0.02 |
|
FA-2585 |
2005 |
9.83 |
9.79 |
-0.04 |