Procedures and Standards
CFC and SF6
LOW LEVEL ANALYSIS OF CFC-11, CFC-12 AND CFC-113 BY PURGE-AND-TRAP GAS CHROMATOGRAPHY WITH ELECTRON CAPTURE DETECTION.
Samples are introduced into a 30 mL sample loop with a custom built apparatus that uses nitrogen to push the sample out from the bottom of the bottle.
CFC are purged from the sample for 4 minutes with UHP N2 flowing at a rate of 150 mL/min. The stream of nitrogen containing the CFC 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 trap held at -10oC which quanitatively removes the CFC from the N2 purge gas stream. After the 4 minute purge the main trap is isolated and electrically heated to 140o C to release the CFC 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 99%.
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.
After the CFCs have been transferred to the cryofocusing trap, the trap is flashed heated electrically to 160oC and the CFC are transferred to the gas chromatographic column with UHP He flowing at 5 mL/min. The following chromatographic conditions are used. Column: 30 m x 0.32 mm GasPro capilliary column. Carrier Gas: He flowing at 5 mL/min, with the flow rate controlled using a mass flow controller. Column temperature: 90oC for 1 min, then 10oC/min to110oC, then 15oC/min to 170oC, hold at 170oC for 1 min. As the CFCs elute from the column they are detected using an electron capture detector. The limit of detection for this method is 0.010 picomoles/Kg for CFC-11, CFC-12 and CFC-113.
Standards and Blanks
Gas phase standards are prepared in our laboratory. The approximate concentration of CFC-11, CFC-12 and CFC-113 in these standards is 120, 270 and 80 picomoles of the respective CFC per mole of N2 (parts-per-trillion). One standard containing all three 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 10 points. A calibration curve is run at least once a week. In order to ensure that the detector response to the CFCs remains stable with time, a single standard volume is injected after every eight unknowns.
Standards containing such low CFC concentrations are not available from NIST, therefore the standards prepared in our laboratory are calibrated against standards obtained from the Scripps Institution of Oceanography and National Oceanographic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory. Groups at these two laboratories maintain the currently accepted absolute calibration scales used in monitoring background atmospheric levels of CFCs. These two absolute calibration scales agree to within 2% of each other.
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.
Periodically, usually about every six weeks, all measurements for the preceding time period are recomputed, applying statistical tests, and scrutinized for flaws in quality. This includes all measurements of unknowns, blanks, purging efficiencies, standards, etc. Only after this step is the result considered final. The results, which include CFC concentrations and derived recharge ages, are then reported in Data Releases, distinctive for each project or job.
Further Technical Information
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.
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, volume or weight, syringe or ampule number, 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 and efficiency corrections.
Using these procedures, every sample can be easily traced from the moment it arrives in the lab to the final result.