6.3 Sample Collection and Analysis

The primary source of CFCs in groundwater is the atmosphere, so care must be taken to eliminate contact between the sample and the atmosphere. Furthermore, CFCs from the atmosphere tend to sorb onto sampling equipment—especially items made of plastic—and desorption can artificially elevate CFC concentrations. Non‑porous materials such as metal and glass are recommended over plastic materials whenever possible. A simple and effective sampling method uses a 125 mL glass bottle with a narrow mouth as shown in Figure 26. The bottles and foil‑lined caps are placed inside a secondary beaker—approximately 2L in volume—and both the bottle, and the beaker are filled using a submersible pump with the discharge tube extending to near the bottom of the bottle as shown in Figure 26. The sample bottle and beaker are purged with 3 to 4L of water before removing the discharge tube. The foil‑lined cap is installed underwater to eliminate any contact with the atmosphere.

Figure 26 ‑ Sampling water for CFCs. a) Narrow‑mouth glass bottle with foil‑lined cap. b) Bottles should be filled by inserting a tube to the bottom, flushing more than 3L, and then installing the cap with no headspace. c) Alternatively, a metal container can be used instead of the beaker of (b).

The purge‑and‑trap method is typically used to analyze CFCs because of their relatively high solubility in water. CFC‑free carrier gas—typically [latex]N_{2}[/latex] or [latex]He[/latex]—is bubbled through the water inside a sparging chamber and then through a trap that retains CFCs but not the carrier gas or oxygen. Traps are typically filled with a porous polymer and are held at about – 30 °C. After sufficient stripping, the trap is warmed to release CFCs, which are then routed into a GC for analysis. Both packed and capillary columns have been used and typically CFC‑11, CFC‑12, and CFC‑113 can be analyzed in a single run. Because of the possibility of sample contamination during sampling, CFCs are often analyzed in triplicate or more.