3H/3He Noble Gas Sampling
The Noble Gas Lab of Lamont-Doherty Earth Observatory has informed the Reston GW Dating Lab that many of the He samples that are being collected by USGS projects are not properly crimped. To properly seal the copper tube, the brackets on the top and bottom must actually make contact so that light can no longer be seen through the contact area. The portion of the bracket that actually crimps the copper tube will never close completely on either side of the tube due to the thickness of the copper. Short of breaking the bolts off, it is impossible to over tighten the flanges or pinch the ends off the copper tube because the crimp area is designed with the precise clearance for a proper seal. In the field, it has been noticed that some individuals lack the strength or the mechanical ability to tighten the nuts properly. In most cases the problem can be overcome by increasing the length of the wrench and by alternately tightening the nuts several turns at a time until the tube is completely sealed. If you suspect that a tube is not crimped properly, please do not send the sample to Lamont to be analyzed.
Proper collection of a water sample for 3H/3He and noble gases involves:
- Filling a special copper sample tube, in duplicate, that is used for noble gas analysis, helium and neon analyses, and determination of the 3He/4He isotope ratio (δ3He) of dissolved helium.
- Filling two 500cc bottles, which are used for tritium determination by helium ingrowth.
Safety-coated 500cc glass bottles with polycone seals are recommended for the tritium sample, though high-density polyethylene bottles with polycone seals may be substituted. If glass bottles are used, a headspace of several cc's should be left in the bottle to prevent breakage on warming due to expansion. The water samples for helium, neon, and tritium determinations are collected in special pinch-off copper tubes (3/8-inch diameter, 30-inch length, containing about 40 cc of water, and fitted with stainless steel pinch-off clamps at each end). These sample tubes are prepared and owned by Lamont-Doherty Earth Observatory who maintains a stock of them and ships directly to the project office .
Both the tritium and helium samples should be collected in duplicate. The duplicate water sample for tritium determination (duplicate 500cc bottle) should be retained at the Project Office, but both copper tubes should be submitted for each sample.
The copper tube can normally be flushed and filled within 5 minutes. Remove and discard the plastic caps that cover the ends of the copper tube, taking care not to scratch or bend or otherwise damage the ends of the copper tube. Damage to the ends of the copper tube may prevent proper attachment of the sample tube to the vacuum extraction line for sample preparation at the contract laboratory. The copper tube, which is fixed in an aluminum channel holding the stainless steel pinch-off clamps, is connected to a closed path from the well or pump. The connection to the well or pump can be of almost any material including plastic, rubber, or metal tubing, providing that all connections are airtight and will not come loose when back pressure is applied during closing of the pinch off clamps. Clear plastic tubing (Tygon) is preferred because one can visually observe whether air bubbles are present in the water line. It is recommended that connections be secured with stainless steel hose clamps, again being careful not to damage the end of the copper tube. The length of the path from the well or pump discharge should not exceed about 5 feet to minimize the possibility of gas separation from the water sample prior to sealing the copper tube.
Any trapped air or formation of gas bubbles in the copper tube helium water sample will produce erroneous results. Back pressure is normally applied to the discharge end of the copper tube during flushing. Figure 1 shows a diagram of the sample aparatus with plastic inlet tubing attached. The project office will need to obtain a small valve and suitable tubing to attach the valve to the discharge end of the copper tube. Both water flow and back pressure on the sample should be increased if gas bubble formation is observed in either clear plastic tubing. The valve should not be closed completely during filling because a steady flow of water must be maintained through the copper tube during sealing. An over-pressure of approximately 1 atmosphere, 14 psi, is normally sufficient to prevent gas bubble formation in the ground-water sample. However, in general, to prevent gas bubble formation, the back-pressure applied must exceed the internal pressure of the dissolved gases in the water sample.
Figure 2 (A-D) shows photographs of the equipment used to take the noble gas and tritium sample. Figure 2A shows the copper tube in an aluminum channel with stainless steel pinch-off clamps, a 1/4-inch copper tube used to connect to the pump, and back-pressure valve. Figure 2B shows the parts of Figure 2A connected for sampling and a 500cc safety coated glass bottle with polycone seal for collection of a tritium sample. Figure 2C shows the back-pressure valve connected to the end of the copper tube with a section of clear plastic tubing for viewing bubbles. Figure 2D shows the ends of properly sealed copper tubes (note the centering of the tubes in the pinch-off clamps).
A socket wrench with 13-mm (note metric bolt) socket is used to turn the bolts that close the pinch-off clamps. Prior to turning the bolts, the entire line from the well through the copper tube should be tapped forcibly to dislodge any gas bubbles that may be in the line or copper tube. During the tapping process, the copper tube should be held at an approximate 45-degree angle with discharge pointing up, to assure that gas bubbles, if present, will be completely flushed. This tapping procedure normally requires about 1 minute to complete. Once satisfied that water flowing through the copper tube is free of any gas bubbles, the socket wrench is used to close the bolts on the pinch-off clamps, beginning with the bolts at the discharge end. Before turning the bolts, be sure to position the copper tube in the approximate center of the pinch-off clamp. There are two bolts on each clamp. Turn the bolts in successive order back and forth approximately four (4) times until firmly closed so that the blades of the pinch-off clamp close approximately evenly. The pinch-off clamps are machined to leave about a 1-mm space when the bolts are turned all the way down; however, care should be taken not to over tighten and break the bolts. After tightening the discharge end bolts, tighten the upstream bolts in the same manner, again centering the copper tube between the blade. When done, double check to be sure that all bolts are tight. The sample is then complete and the copper tube can be disconnected from the well or pump.
Remove the back-pressure valve from the discharge end of the copper tube. Precautions should be taken not to scratch or otherwise damage the ends of the copper tubes. If waters are corrosive, such as seawater or other saline or acidic waters, the ends of the copper tubes should be washed with dilute water to prevent corrosion, which might prevent obtaining a proper seal when extracting the noble gases. Care should be taken not to further bend the ends of the sealed copper tubes because they can easily break off. If the tubes were received with plastic caps protecting the ends, do not replace the caps after filling. Place the channels in the crate for storage and shipping with the open side facing up. Additional instructions on ground-water sampling for noble gases provided by Lamont-Doherty Earth Observatory appear at the end of this document.
Figure 2a. Copper Tube in Aluminum Channel
Figure 2b. Parts Connected for Sampling
Figure 2c. Back-Pressure Valve Connected
Figure 2d. Properly Sealed Copper Tubes and tritium bottles
USGS personnel will be responsible for proper collection of water samples, including (1) care in avoiding possible damage to ends of sample tubes which might preclude proper connection to laboratory high-vacuum extraction lines, and (2) exclusion of gas bubbles during sample collection. The contract lab will report samples that could not be extracted or analyzed because of improperly sealed sample containers and/or damaged sample containers. Improperly collected samples, such as samples containing gas bubbles, are usually not detected until the analysis of noble gases is made. Therefore, in most cases, the Project Office will be charged the full price of analysis for improperly collected samples, even though no age can be determined.
Submittal of samples to the contract laboratory
When the samples have been collected and are ready to be analyzed, the project office must download and complete the Sample Submission Form (an Excel spreadsheet). The completed submittal spreadsheet is returned by e-mail attachment to email@example.com. Sample ID tracking numbers will be assigned by the Reston GW Dating Laboratory and retuned to the project office via e-mail, and/or as stick on labels. It is the responsibility of the project office to properly identify each sample with the sample ID tracking number. The project office then ships the samples to Lamont-Doherty Earth Observatory and notifies the Reston Laboratory via e-mail to firstname.lastname@example.org with cc to email@example.com of date of shipment. Lamont-Doherty Earth Observatory will notify the Reston GW Dating Laboratory when samples are received and condition of samples upon receipt. The project office will be notified of any samples damaged in shipment. Note: NWQL ASR forms are not used to request analytical services for 3H/3He dating or noble gases. Use the submittal spreadsheet as described above.
Sending the samples to the contract laboratory
Ship the sample tube(s) and water samples for tritium determination toLamont-Doherty Earth Observatory Dr. Peter Schlosser
61 Route 9W
Palisades, NY 10964 phone 845-365-8514
Precautions need to be taken to assure that samples shipped in winter months do not freeze during shipping. Place the aluminum channels in the crate with the open side facing up.
Billing by SV will be processed through the Branch of Regional Research, EB (contact: Melissa Schomody, AO; firstname.lastname@example.org).
Reporting of Results
Results will be reported to the project office as they are received from the contract laboratory. The Project Office should carefully review these results and follow with questions to the contract laboratory through Julian Wayland; email@example.com. Each report is mailed to the Project Office from the Reston GW Dating Laboratory, which retains a reference copy. A PDF electronic copy is also sent.
Questions about collection and submittal procedures, or information about sample status should be directed to Julian E. Wayland (firstname.lastname@example.org) 703-648-5847. All billing will be handled through Reston and all data will be delivered through Julian E. Wayland. Technical information on sample collection and interpretation of Tritium/Helium-3 age can be found at http://water.usgs.gov/lab/.
Additional Information from Lamont-Doherty Earth Observatory
WARNING. Watch out for gas bubbles in the plastic tubing! Make sure there is no contact of the water with air before you take the sample.
1. SAMPLER PREPARATION
Write the sample location, date, and time, on the aluminum channels with a waterproof marker. Mark the channels before you begin to draw water through the sample container. Whenever possible, take duplicate samples.
Both ends of the aluminum channel have clamps to hold a piece of copper tube. They may rattle loose during shipment. These clamps should be finger-tightened to hold the copper tube firmly on center in the aluminum channel (Figure 1).
Use a valve on the discharge end of the copper tube to apply back pressure and discourage degassing of the sample.
Open the pressure valve completely. Hold the copper sampler vertically (pressure valve UP) with one hand and the ratchet wrench with the other hand. Allow the water to run at least 1 minute through the system to flush the sample tube. Keep the pressure in the system as high as possible (it is safe up to 10 bars). Watch the tubing near the valve for bubbles. Bubbles do preferentially form near fittings. If bubbles are present, squeeze the tube or knock against the tube to get rid of the bubbles. You may also try to increase the pressure further by reducing the flow through the copper tube by partially closing the valve. During flushing, bang the side of the aluminum channel with the ratchet handle to remove trapped air bubbles from the copper tube. Watch again the tube between the valve and the sample container for bubbles. Make sure that the sample container was flushed at least 1 minute with bubble-free water. If you cannot avoid formation of bubbles, take the sample anyway, and mark the sample container accordingly.
Once flushing is complete and no air bubbles appear in the tubing, the copper tube may be closed off. Tighten the clamp which is closer to the pressure valve (top clamp, outflow) first. Tighten each screw a little at a time until the outer edges of the clamp touch. You have tightened the clamps sufficiently when you notice that it is getting much harder (almost impossible) to tighten the clamp further. Do not worry, it is not easy to shear the bolts. The clamp will bow somewhat around the copper, so that it maintains a constant pressure on the copper seal. Watch the plastic tubing for bubbles while tightening the top clamp. Tighten the bottom clamp and remove the plastic tubing. If you think there are bubbles inside the copper tube after it has been clamped, write "BUBBLES?" on the aluminum channel. Remove the pressure valve.
4. SAMPLER STORAGE
Shake the sample container a little to remove the excess water from the ends. If the water is very corrosive (high salt content), rinse the inside of the copper tube ends by spraying low salinity water into them and shaking out the excess.
Sampler should be returned to the box immediately after cleaning and then stored out of the weather with the open side of the channel facing up. The ends of the copper tube are very fragile after the tube is squeezed.
Points to Remember
- Avoid bubbles.
- Close clamp at outflow end first.
- Copper tube ends are very fragile after clamp is tight.
- If you think there are bubbles inside the copper tube after it has been clamped, write "BUBBLES?" on the aluminum channel.
- Make sure that the sample container is properly marked (date, time, location).
- Take duplicates whenever possible!