Gas Sampling for Chlorofluorocarbons and Sulfur Hexafluoride
Evacuated Flask Method
A flask is evacuated under high vacuum and sealed. The valve is opened and then closed to sample air. The advantage of this technique is that no pump or other equipment is needed. Disadvantages include the fact that flasks are expensive ($500-600 each), only a small amount of gas is sampled, samples are often found contaminated, sample loops have to be evacuated, and the pressure of the gas in the loops should be measured. NOAA used this method before 1977.
Pressurized Flask Method
Flasks are filled with ultra-pure nitrogen or evacuated. This requires an ultra-pure pump to compress the air. The advantages are that a large volume of gas is collected, several analyses can be performed, samples are less often contaminated, and samples are easier to analyze. Disadvantages include the fact that flasks are expensive ($500-600 each), and a pump ($450 to purchase one new) is required to collect the samples. NOAA has used this method since 1977.
Glass Ampoule Method
The gas sample is sealed in a 100 cc glass ampoule. The procedure was developed by Busenberg and Plummer, 1992. The air sample is pumped into a glass ampoule using an ultra-pure pump. The neck of the ampoule is sealed with an oxygen-MAPP gas torch. The advantage to this method is that the samples are inexpensive ($18 each), ampoules can be used up to 5 times (ampoules can be restemmed for about $ 3), many samples can be collected and archived, and CFCs are stable in the borosilicate glass ampoules. Disadvantages include the small amount of gas sampled, the fact that sample loops have to be evacuated, the pressure of the gas in the loops should be measured, and a pump and other equipment is required (including oxygen, MAPP gas, and a torch). This method has been successfully used at INEEL, Idaho and at the Nuclear Test Site in Nevada. Over 100 samples have been collected in one trip.
Unsaturated Zone Sampling Points
This involves hammering a pipe into the ground to the desired depth. This procedure does not work if rocks are present. Unsatisfactory samples are often obtained, and this is not a recommended sampling procedure.
A gravel pack is placed around the sampling port. The hole is filled with natural backfill. Several sampling ports can be present in one hole. This is the recommended method.
Expanding cement is usually used. Several sampling ports can be present at one site. Excellent results are obtained from these wells.
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, and 6-7 feet above the ground!)
2) Run the pump for 5 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.
4) The cylinders are filled with compressed ultra-pure nitrogen. Remove the cap from the cylinder and open valve A-2 to empty the tank. Open valve A-1 and connect the red wire to the battery.
5) Purge the sampling cylinder with air for 10 minutes.
6) Close valve A-2. Wait 3 minutes
7) Close valve A-1. Disconnect pump from the battery. Disconnect the sampling cylinders from the ~2 foot 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.
Figure 2 Detail View