Accuracy and Precision of CFC Analysis
The CFC concentrations are referenced to average air compositions measured at Niwot Ridge, Colorado Climate Monitoring and Diagnostics Laboratory (CMDL) of the National Oceanic and Atmospheric Administration (NOAA) (see http://www.cmdl.noaa.gov)). The standard deviation of repeated measurements is typically less than 2 percent of the reported value for CFC-11 and CFC-12, and about 3 percent for CFC-113, but increases as concentrations approach the detection limits of about 1 pg/kg for CFC-113 and about 0.5 pg/kg for CFC-11 and CFC-12. The analytical precision is about 50 percent at the detection limit and about 3 percent above 25 pg/kg. Eight standards and a blank are used for calibration. The ECD response is linear in the GW dating concentration ranges. The calibration ranges of the gas chromatograph are 0-1,200 pg/kg for CFC-11, 0-2,500 pg/kg for CFC-12, and 0-900 pg/kg for CFC-113. Reported concentrations beyond the above calibration limits should be considered estimates.
