1.Low level analysis of CFC-11, CFC-12,CFC-113, and SF6 By Purge-and-trap gas chromatography with electron capture detection.
1. Sample Introduction
Samples are introduced into a 200 mL sample loop with a custom built apparatus that uses nitrogen to push the sample out from the bottom of the bottle.
2. Purge-and-Trap
CFC are purged from the sample for 6 minutes with UHP N2 flowing at a rate of 150 mL/min. The stream of nitrogen containing the CFCs and SF6 is first passed over a trap containing magnesium perclorate (removes water vapor) and Ascarite (removes hydrogen sulfide, which can interfer with CFC-12). The dry, hydrogen sulfide free gas stream is then passed over a Porapak N/Carboxen 1000 trap held at -20oC which quantitatively removes the CFCs and SF6 from the N2 purge gas stream. After the 4 minute purge the main trap is isolated and electrically heated to 180o C to release the CFCs and SF6 from the trapping material. Purging efficiency is checked by isolating a water sample in the purge chamber after it has been purged once and purging it a second time. Purging efficiencies are generally higher than 90%.
3. Cryofocusing
Because of the relatively large amount of gas used to purge a sample, the CFCs spread out on the main trap as they are being purged from the water sample. If the CFCs were injected into the gas chromatograph (GC) directly from the main trap the resulting peaks would broad, diffuse and difficult to accurately quantify. Therefore the CFCs are transferred from the hot main trap to a smaller volume cryofocusing trap packed with Porapak N and held at -15oC. The main to cryofocusing trap transfer is accomplished with UHP He flowing at 13 mL/min for 1 minute. This results in the CFCs being trapped on the cryofocusing trap in a nice tight plug.
4. Gas Chromatography
After the CFCs have been transferred to the cryofocusing trap, the trap is flashed heated electrically to 180oC and the CFCs and SF6 are transferred to the gas chromatographic column with UHP He flowing at 7.5 mL/min. The following chromatographic conditions are used. Column: 30 m x 0.32 mm GasPro capilliary column. Carrier Gas: He flowing at 7.5 mL/min, with the flow rate controlled using a mass flow controller. Column temperature is initially 120oC then ramped at 10oC/min to180oC and held for 1 min. As the CFCs and SF6 elute from the column they are detected using an electron capture detector (ECD). The limit of detection for this method is 0.005 picomoles/Kg for CFC-11, CFC-12 and CFC-113 and 0.050 femtamoles/Kg for SF6.
5. Standards and Blanks
Gas phase standards are prepared at the University of Washington and are referenced to the SIO absolute calibration scale. The approximate concentration of CFC-11, CFC-12, CFC-113 and SF6 in these standards is 611, 458, 250, and 1 picomoles of the respective CFC per mole of N2 (parts-per-trillion). One standard containing all four compounds is used to construct a calibration curve by injecting different volumes of the standard. A fixed volume sample loop is loaded with the standard and the loaded sample loop is purged-and-trapped as described above. Various combinations of 5 different volume sample loops are used to construct a calibration curve consisting of at least 20 points. A calibration curve is run with every job. The ECD response can change with time and atmospheric pressure. In order to account for detector drift with time and atmospheric pressure, a single standard volume is injected after every eight unknowns.
Standards containing such low CFC and SF6 concentrations are not available from NIST or commercially therefore the standards prepared at the University of Washington are calibrated against standards obtained from the Scripps Institution of Oceanography and National Oceanographic (SIO). SIO is the currently accepted absolute calibration scale used in monitoring background atmospheric levels of CFCs and SF6.
System blanks are determined by loading the water sample loop with UHP N2, and then purging-and-trapping the N2 as described above. A blank is run after every eight unknowns. Blanks generally contain undetectable amounts of CFCs and SF6.
6. Piston Flow Age Determination
CFC and SF6 ages are calculated assuming piston flow. The measured concentration, estimates of the recharge temperature and elevation along with the solubility relationship for each compound are used to calculate an equivalent atmospheric concentration. The equivalent atmospheric concentration for each compound is matched to the atmospheric history to determine the year of recharge. Ages are accurate to within 2 years.
The results, which include CFC concentrations, equivalent atmospheric concentrations and derived recharge ages, are then reported in Data Releases, distinctive for each project or job.
7. Further Technical Information
Bu, X., and M.J. Warner, Solubility of chlorofluorocarbon 113 in water and seawater, Deep-Sea Res. I, 42, 1151-1161, 1995.
Bullister, J.L., D.P. Wisegarver, and F.A. Menzia, The solubility of sulfur hexafluoride in water and seawater, Deep-Sea Res. I, 49, 175-187, 2002.
Happell, J.D., R.M. Price, Z. Top, and P.K. Swart, Evidence for the removal of CFC-11, CFC-12, and CFC-113 at the groundwater-surface water interface in the Everglades, J. Hydrology, 279, 94-105, 2003.
Prinn, R. G., R. F. Weiss, P. J. Fraser, P. G. Simmonds, D. M. Cunnold, F. N. Alyea, S. O'Doherty, P. Salameh, B. R. Miller, J. Huang, R. H. J. Wang, D. E. Hartley, C. Harth, L. P. Steele, G. Sturrock, P. M. Midgley, and A. McCulloch, A hisotry of chemically and radiatively important gases in air deduced from ALE/GAGE/AGAGE J. Geophys. Res., 105, 17,751-17,792, 2000.
Walker, S. J., R. F. Weiss and P. K. Salameh, Reconstructed histories of the annual mean atmospheric mole fractions for the halocarbons CFC-11, CFC-12, CFC-113 and carbon tetrachloride J. Geophys. Res., 105, 14,285-14,296, 2000.
Warner, M.J. and R.F. Weiss, Solubilities of chlorofluorocarbons 11 and 12 in water and seawater, Deep-Sea Res., 32, 1485-1497, 1985.
Happell J.D. and D.W.R. Wallace, Gravimetric preparation of gas phase standards containing volatile halogenated compounds for oceanographic applications, Deep Sea Res., 44, 1725-1738, 1997.
Happell J. D., D. W. R. Wallace, K. D. Wills, R.J. Wilke, and C.C. Neill, A purge-and-trap capillary column gas chromatographic method for the measurement of halocarbons in water and air Rep. BNL-63227, 19 pp., Brookhaven National Laboratory, Upton, NY, 1996.
Happell J.D., and D.W.R. Wallace, Removal of atmospheric CCl4 under bulk aerobic conditions in groundwater and soils, Environ. Sci. Technol., 32, 1244-1252, 1998.
Ekwurzel, B., P. Schlosser, W.M. Smethie, L.N. Plummer, E. Busenberg, R.L. Michel, R. Weppernig, and M. Shute, Dating of shallow groundwater: Comparison of the transient tracers 3H/ 3He, chlorofluorocarbons, and 85Kr, Water Resources Research, 30, 1693-1708, 1994.
2. Sample identification and flow of information
Water samples for CFC analysis are received and inventoried using the accompanying packing list or chain of custody supplied by the client. A computer worksheet listing sample name, salinity, temperature, sample collection date, and date of arrival into lab, as well as client information, is generated. At this time, each order is given a unique job number, and each sample decimal numbered within that job. For example, the job-sample number (CFC#), 123.05 indicates the fifth sample in the listing for job 123. The computer input is proofread, and the worksheet and labels are printed. An abbreviated copy of the worksheet listing is given to the administrative personnel to be filed with the client's records.
The preparation technician, to keep track of the progress of the samples, uses the worksheet. Preliminary results are recorded on this sheet as they become available through the computer. From the time the worksheet is printed, its CFC# refers to the sample. Labels are attached to each sample container. Once the sample is ready to be analyzed, the CFC# and all other sample information is entered into the computer that controls the gas chromatograph and collects the raw data. After the sample is analyzed, the computer controlling the gas chromatograph generates a database that includes all of the entered sample information along with the raw CFC peak areas. This database also contains the information needed to calculate calibration curves and blank, efficiency and time drift corrections.
Using these procedures, every sample can be easily traced from the moment it arrives in the lab to the final result.
