Water samples for CFC and SF6 measurements were collected using a CTD/rosette sampling system. The CFC/SF6 sample was the first sample drawn from the rosette bottle. The samples were collected in 250 cc glass stoppered bottles through a PVC tube connected to the rosette bottle drain valve. Bubbles were cleared from the tube with water flowing and the tube was inserted to the bottom of the 250 cc glass bottle. The glass bottle was placed in a wide mouth plastic jar that extended above the opening of the glass bottle. The overflow water collected in the jar, filling and overflowing the jar and covering the glass bottle opening. The flow continued for 3 overflow volumes of the glass bottle (750 cc) and the glass stopper was then inserted underwater preventing air from being trapped in the sample. The bottle was then placed upside down in the jar, the jar was capped and the sample stored at 2°C in a refrigerator to prevent degassing. At the end of the cruise, the samples were packed in insulated boxes with cold packs to maintain the 2°C temperature and shipped by air freight to Lamont-Doherty Earth Observatory. The measurements were carried out at Lamont using a dual purge and trap system interfaced to a dual ECD (electron capture detector) HP6890 gas chromatograph. When a water sample is introduced into the system, it is split into two aliquots, a 20 cc aliquot for CFC measurement and a 180 cc aliquot for SF6 measurement. The aliquots are transferred to appropriately sized sparging chambers and stripped with ultra high purity nitrogen, which transports the extracted gases to cold traps (Unibeads-2s for CFCs, Carboxan-1000 for SF6) cooled to -80°C. The traps are then heated to 110°C for CFCs and 165°C for SF6 and flushed into the gas chromatograph where CFCs are separated with a Porasil-B pre-column and a Carbograph 1AC main column and SF6 is separated with a pre-column and main column of Molecular Sieve 5A. The gases are detected by the ECDs. The ECDs are calibrated by running gas standards with known concentrations of CFCs and SF6 prepared by Dr. John Bullister of NOAA/PMEL specifically for the CLIVAR Repeat Hydrography program. CFC concentrations are reported on the SIO98 calibration scale and SF6 concentrations on the SIO2005 calibration scale. Duplicate samples were collected and measured to provide a combined precision for sampling and measurement. The average differences from these duplicate measurements was the larger of 0.024 pmol/kg or 2.1% for CFC-11, 0.008 pmol/kg or 1.8% for CFC-12, 0.006 pmol/kg for CFC-113 and 0.046 fmol/kg or 4.3% for SF6.
