Advice on Sampling
Advice on CFC and SF6 Sampling
SAMPLING OF ENVIRONMENTAL WATER FOR LOW LEVEL CFC/SF6 ANALYSIS
Methods Accepted of Drawing Water from Wells
Samples must be pumped from wells. Bailers will give unacceptable results because the groundwater sample will come in contact with air and will be contaminated by CFCs and SF6 that are in the air.
Most plastic materials will also contaminate a water sample because small amounts of plastizers will leach into the water sample. The easiest type of pump to use is a peristaltic pump. In this type of pump only the pump tubing comes in contact with the sample. We have tested many different types of peristaltic pump tubing and have found that Viton tubing is the only type of material that will not contaminate the sample. Commonly used pump tubing materials, such as silicone or Tygon will seriously contaminate the sample. If a peristaltic pump with Viton tubing is not available, the Tritium Laboratory will arrange a loan of one of our pumps (client pays for shipping of the pump). Other types of pumps that have been used to successfully sample for CFCs and SF6 are Bennett Pumps using copper tubing and Grunfos Rediflow 2 Pumps using polyethylene tubing. Also avoid the use of any type of lubricants, such as oils, greases, or sprays. Even small amounts of these materials will contaminate a sample. Insect repellent is another potential source of contamination.
Collection of Water for Low-Level CFC/SF6
Analysis Using Bottles and Foil Lined caps.
This method was developed by the USGS Reston Chlorofluorocarbon Laboratory. Please see http://water.usgs.gov/lab/chlorofluorocarbons/sampling/bottles/ for more information.
Source of bottles and caps
Bottles and caps will be provided by the Tritium laboratory upon request if the client pays for shipping.
If the client wishes to purchase their own bottles what is needed are 500 ml (16 oz) boston round clear glass with a cap size of 28-400. Bottles can be obtained from many different sources that supply glass containers. Frequently bottles will come with caps, however do not use the caps provided.
The caps must be lined with aluminum foil. This is the key to this method. The caps themselves can be plastic or metal, but either material must contain and aluminum foil liner. Do not use the cap if the foil liner is dented or scratched. Metal caps with aluminum foil liners can be obtained from SKS Bottle and Packaging, Inc. (Part number 6021-05, bag of 144 caps).
The bottles and caps should be THOROUGHLY rinsed with the water to be sampled. The bottles are filled and capped underwater in a beaker. Refrigeration-grade copper, polyethylene, polypropylene, viton or polyurethane pump tubing is required. Soft flexible tubing, such as tygon or silicone tubing will almost certainly contaminate the sample. The filling procedure is carried out within a two to four liter beaker. A plastic beaker or bucket is fine. Fill a minimum of 3 bottles per sample.
The procedure is shown below and is as follows (refer to the figure below):
- After the well has been purged, place the bottle in the beaker and then insert the end of the tubing from the pump all the way into the bottom of the bottle. Also place all the caps (2 or more) in the beaker.
- Fill the bottle as shown with water until it overflows.
- Continue to overflow the bottle until the beaker also overflows. Allow at least 2 liters of water to flow through the bottle and out of the beaker.
- Select a cap from the bottom of the beaker and tap it under water to dislodge air bubbles. Remove the pump tube from the bottle and cap the bottle TIGHTLY underwater without allowing the water in the bottle to come in contact with air. Flushing the bottle with more water is far better than with less water.
- Remove the bottle from the beaker and dry the bottle.
- Invert the bottle, tap it and check it for air bubbles. If there are bubbles, repeat the procedure from step 2 above. If it is necessary to refill the bottle, you must use a new cap.
- If there are no bubbles present, securely tape the cap to the bottle with electrical tape. Wrap the tape in a clockwise direction looking down at the bottle top. Do not forget to label each bottle with the well name, date, and time of sampling and the sequence number of each bottle as it was collected (i.e. 1, 2, etc).
USE ONLY THE BOTTLE CAPS DESCRIBED IN THE TEXT ABOVE
Because the entire contents of each sample must be used for the analysis, it is recommended that you take duplicate or triplicate samples from each source. This will provide backups in case one of the samples is lost during shipping or during the analytical procedure. We will typically analyze the duplicate or triplicate samples and provide the results to the submitter at no additional cost.