Sulfur Hexafluoride (SF₆) WMO Scale

Update: September, 2014

The WMO/GAW X2006 SF₆ scale was updated to X2014 on 18 September 2014. The X2014 scale supercedes the X2006 scale.

The X2014 SF₆ scale is defined by 17 primary standards over the range 2-20 pmol mol^-1 (ppt), prepared by static gravimetric dilution in 5.9-L aluminum cylinders. Assigned values were determined by fitting a 2nd order polynomial to the response of each standard (relative to a 8.25 ppt natural air secondary standard) measured using an HP6890 GC with electron capture detection (GC-ECD). Analysis repeatability (precision) was typically less than 0.3% (one standard deviation).

Conversion from X2006 to X2014:

For SF₆ > 2 ppt:

The conversion has an uncertainty of ± 0.01 ppt.
Please use caution applying this equation to SF₆ > 10 ppt. Results will only be approximate.

Table 1: Comparison of X2006 to X2014 SF₆ scales over the range 1-12 pmol mol^-1 (ppt).

*Values above 10 ppt are included for illustration purposes, but are beyond the range of the X2006 scale.

Table 2: Primary Standards that define the X2014 SF₆ scale. Uncertainties associated with prepared mole fractions are approximately one standard deviation. Mole fractions assignments are based on GC-ECD analysis, using a 2nd order polynomial response function.

Figure 1: SF₆ primary standards analyzed by GC-ECD (color-coded by parent mixture).
Click on image to view full size figure.

Figure 2: SF₆ primary standard tree. Standards that define the X2014 scale are shown in yellow. Some standards that were used to define the previous X2006 scale were excluded from the X2014 scale because they are now at low pressure.
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