The Reston Groundwater Dating Laboratory

Gas Sampling for Chlorofluorocarbons, Sulfur Hexafluoride and Other Gases

Gas Sampling Photos


Evacuated Flask Method

A flask is evacuated under high vacuum and sealed with a valve. The valve is opened at the sampling site and then closed. No pump or other equipment is needed. Sample volume is limited to the size of the flask at atmospheric pressure.

Pressurized Flask Method

Larger sample volumes can be obtained using this method over the evacuated flask method. A battery driven ultra pure pump is used to compress the air sample into the flask.

Glass Ampoule Method

The gas sample is sealed into a 100 cc glass ampoule. The air sample is pumped into a glass ampoule using an ultra-pure pump. Or the ampoule is evacuated in a special aparatus and then opened to the sample port. The neck of the ampoule is sealed with an oxygen-MAPP gas torch. This method has been used successfully in unsaturated zone studies where many samples are required.

Unsaturated Zone Sampling Points

Drive Point

This involves hammering a pipe into the ground to the desired depth and attaching a sampling port fitting. This procedure can be used for single depth unsaturated zone samples. Care must be taken to seal the pipe from the surface atmosphere.

Augered Holes

A sand pack is placed around the sampling port at the desired sampling depth. The hole is filled to the surface with natural backfill. Several sampling ports can be present at different levels in one hole.

Drilled Wells

Expanding cement is usually used to seal sampling ports at different levels. Several sampling ports can be present in one hole.

Air Sampling Procedure

1) Connect the two SS tubes to the pump as shown in Figure 1. Do not connect the sampling cylinder to the pump at this time. Sample upwind from parked cars or other sources of contamination.

2) Run the pump for 2 minutes to clean the pump and tubing.

3) Stop the pump by disconnecting the red wire from the battery. Attach the sampling cylinder to the pump as shown in Figure 1.

Bottle Diagram

Figure 1

4) Remove the cap from the cylinder and open valve A-2 to empty the tank of any contents. Open valve A-1 and connect the red wire to the battery.

5) Purge the sampling cylinder with air for 5 minutes.

6) Close valve A-2. Wait 3 minutes for the cylinder to pressurize.

7) Close valve A-1. Disconnect pump from the battery. Disconnect the sampling cylinders from the SS tubing. Replace end-caps on cylinders--make sure the caps are tightly secured.

8) Remove the SS tubing from the pump and cap openings to prevent contamination of the pump.

Bottle Diagram

Figure 2 Detail View