DEVIL: Duke Environmental Stable Isotope Laboratory

Nicholas School of the Environment, Division of Earth and Ocean Sciences

 

  

Solid samples | Water samples

Preparing Tin Capsules For 13C and/or 15N Isotopic Analyses Using Microbalance

Note: Please read these instructions carefully - improperly prepared samples cannot be run. The requirements for samples being analyzed for %C/%N only are less exacting (the most important thing is that they not leak) - consult Will Cook (cwcook@duke.edu) for more information.
For preparing other types of samples, please consult first with lab director Jon Karr (jkarr@duke.edu, 919-660-7418). Thanks!

  1. Use a 96-well plastic culture tray and download the tray template. Fill out the template as you load the tray, recording sample ID and weight in mg for each tray position. Please send duplicates of several (up to 6) representative samples in each tray. Send us a copy of this template with the samples. Also, remember to fill out and email back the sample submission form. This expedites our setting up the analysis run, as we can copy and paste information. Please rename the completed submission file in a way that identifies the sender and the samples - include the last name of the submitter plus a brief description of samples, e.g. "karr FACEsoilJan03.xls".
  2. Your samples must be completely dry, homogenized, and finely milled or mortared. Sharp edged material such as large wood splinters may split the sample capsule.
  3. Clean the work area. If there is dirt on the weighing table, brush it off, and you may want to clean it further with a damp wipe and some alcohol. Lay a large kimwipe or two out on the clean dry surface.
  4. Clean utensils thoroughly using Kimwipes and methanol, then allow to dry briefly. Lay the utensils on clean Kimwipes and cover them with the same. Note: avoid skin or eye contact with methanol. Use gloves and eye protection. The cleaner your materials are, the lower the blank will be. Everything must be absolutely dry or you will not be able to get a stable weight with the microbalance. Methanol evaporates faster than other alcohols. Don't use acetone here because residuals could possibly interfere with the mass 44 signal.
  5. Make sure the weighing pan is clean (use methanol on a kimwipe if unsure) and the balance is leveled. Make sure the glass doors are closed when taring or weighing. Use gloves to avoid contaminating samples or utensils. Tare the tin capsule, handling only with clean foreceps, and only set the sample on clean surfaces.
  6. Using a clean spatula, transfer the sample material into the capsule while it is resting in the hole in the steel sample-folding block. (The folding block is optional - careful use of foreceps and a narrow flat spatula will work, too.) It helps to spread the opening of the capsule somewhat. Avoid spillage, and remove any that occurs. Use a small brush on the balance.
  7. Weigh the sample, and continue transferring until sample weight is adequate but below 100 mg - roughly 3-5 µmol of N, but no more than 12 µmol N, and 10-200 µmol of C. If C:N ratio requires more than 200 µmol C, consult with lab director. Samples should be no larger than needed to meet the minimum µmol requirements for both N and C. In general, samples will range from less than a milligram to a few tens of milligrams. You must have at least a rough idea of %C and %N to do this. If not, you can expect to have to repeat some analyses.
    • Use the smallest available capsule size that will safely contain the sample with no leakage (e.g. 9x5 mm, 4x6 mm, or 3.5x5 mm).
    • Do not make samples any larger than needed to meet minimum size requirements when this involves tens of milligrams of sample (e.g. soils or low N plant parts).
    • Maximum sample weight is 100 mg. If the sample gets stuck in the tray well, it will likely get stuck in our autosampler, causing serious problems.
    • Here's how to quickly figure out target weights in mg from percentages: 4.2 divided by %N will give you the target weight in mg for a sample to contain 3 µmol N; 7 divided by %N = 5 µmol N. 12 divided by %C = 10 µmol C and 240 divided by %C = 200 µmol C. For example, a soil sample with 2% C should be weighed out to between 6-120 mg, preferably closer to 6. A typical leaf sample with 2% N and a C/N ratio of 10-20 should weigh between 2.1 and 3.5 mg for N analysis. However, a sample of pine leaves with 0.7%N and 50%C would require a consultation with the lab director.
    • Low %C samples to be analyzed for 13C only can contain as little as 1 to 5 µmol C, but any samples with less than 5 µmol C should be identified since they require running without helium dilution.
    These steps will reduce the ash buildup in the furnace and expedite your analyses.
  8. To seal the capsule, crimp the top of the capsule near the top with a pair of straight foreceps and fold over snugly, as if folding the top of a paper lunch bag. Use the edge of the forceps and another foreceps or flat spatula to gently compact the capsule into a small, tight cube or ball. There should be no stray edges or loose sides hanging out. Be very careful not to puncture the capsule. If this happens, you must tear it apart with foreceps and start over. When the capsule is done, pick it up and drop it lightly on the block a couple times to make sure it doesn't leak. Leaking samples, flattened samples (rather than cube-shaped), or capsules with stray or loose edges will not be run since they can jam the autosampler, cross-contaminate samples, and ruin the analyses.
  9. Record the final weight after the capsule is sealed. Record the weight in milligrams to the nearest microgram (3 decimal places). Place capsule in sample tray and record the weight on the tray template. Clean all utensils lightly with Kimwipes and methanol after completing each sample, air dry briefly.
  10. Secure the lid of the culture tray with rubber bands or laboratory label tape and label the tray when done. Avoid using Scotch tape, since the adhesive can cause problems. To prevent leakage during shipping, it is advisable to use the smallest size tray wells available and to pad the interior with Kimwipes.

Be sure to use capsules designed for elemental analysis. Our supplier is Costech (1-800-524-7219). Other suppliers include CE Elantech (1-888-232-4676) and Elementar Americas/Elemental Microanalysis (1-800-659-9885). Capsules are generally about $0.10 each depending on capsule size and how many you order.

For samples of unusual size or composition, please consult with the lab director.


Solid samples | Water samples

Preparing Water Samples for 18O and/or 2H Analyses

Water should be free of particulates. Marine or terrestrial fresh waters can be filtered upon collection using an inline 0.47 micron filter with a peristaltic pump, or with a large plastic syringe pushing through a syringe filter holder with pre-combusted 0.7 micron glass fiber filters. DO NOT USE VACUUM FILTRATION as this can cause isotopic fractionation. SAMPLES SHOULD NOT BE ACIDIFIED OR POISONED, as the TC/EA analyses may be affected. Filtration and refrigeration should be sufficient. Cryogenically or azeotropically distilled waters extracted from soils or plants will not require filtration, but there must be no residual toluene, ethanol or other reagent. Samples should be stored and shipped in vials small enough that there is no airspace. WE RECOMMEND USING BOROSILICATE SCREW-CAP VIALS, 12MM X 32MM, WITH SILICONE/PTFE SEPTA. These are available from VWR, Fisher, and others. Clearly label each vial with indelible ink and ship in Styrofoam containers such that the vials can't rattle or break. For extra leakage security, you can wrap the outside of the vial in parafilm around the lid area. For very tiny samples, vials with conical interiors and volume of 0.3 ml are available. The analysis involves only a few microliters, but storage and shipping of tiny samples can be problematic without the correct vials.

cwcook@duke.edu