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University of California
Stable Isotope Laboratory
Earth and Planetary Sciences
1156 High Street Santa Cruz, CA 95064

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Methods of Inorganic Carbon removal from Soils and Sediments:

Sulfurous Acid Method: For Ocean Sediments

Hydrogen Chloride Fumigation Method: For Soils

Buffered Acetic Acid Method: For Lake Sediments

Sulfurous Acid Method: For Ocean Sediments

Following: Verado et al. Determination of Organic-Carbon and Nitrogen in Marine-Sediments using the Carlo-Erba-NA-1500 Analyzer, Deep-Sea Research Part a-Oceanographic Research Papers (1990) vol. 37 pp. 157-165

  • Small plastic weigh boats
  • Silver capsules 5mm x 9mm
  • 6% to 8% sulfurous acid
  • Sediment Samples
  • Eppendorf Micropipettes (10-100 µl capability)
  • Method:

    Step 1: Weigh sediment samples into silver boats

    Weigh out 5-20 mg of sediment sample per boat.
    Assuming sediments are ~0.1 to 1.0% organic carbon, this should yield at least ~5 to 20 µg organic carbon**
  • Each sample boat should be placed on a small plastic weighing boat with a flattened out silver causule underneath it. This Cup and Saucer method will ensure minimal sample loss during acidification/drying steps.
  • Ideally, one should create an Aluminum block as specified in Verardo et al. for the following acidification steps. For small sediment samples < 20mg, one can use the 96 well polystyrene tray to hold many samples. Be aware that bubbling and overflow are possible and that polystyrene has limited chemical resistance.
  • Step 2: In a ventilation hood, wet each sample with 30 µl MilliQ H2O
  • This will prevent a violent reaction between the CaCO3 and sulfurous acid (and subsequent loss of sample). If any sample is lost, discard boat and reweigh.
  • Step 3: Keep samples in hood and add 10 µl of sulfurous acid to each boat.
  • Observe the sample behavior/effervescence.
  • Again if sample is lost, discard and start over from step 1.
  • Step 4: Place acidified samples in a vented drying oven at 60°C for 15 minutes.
    Step 5: Once dry acidify again in hood, only this time with 30 µl of sulfurous acid and place back in 60°C drying oven.
    Step 6: Dry and repeat step 5 with 1x 50 µl and 3x 100 µl acid additions.
  • The sequential acid additions are critical to prevent sample loss.
  • Continue 100 µl acid additions until you reach 400 µl of total acid added, or until no effervescence is observed.

  • Hydrogen Chloride Fumigation Method: For Soils

    Step 1: Weigh soil samples into silver capsules (tin decomposes when exposed to acid) and arrange samples in a 96-well tray. Wet the samples by adding a small amount of water to each open silver capsule.

    Step 2: Place the 96-well tray in a desiccator containing a beaker of concentrated (12M) HCl and cover. Carbonates are acid digested, releasing CO2, over a period of 6 to 8 hours.

    Step 3: Dry the samples at 60°C and carefully crimp-seal the capsules. Beware that the capsules become brittle after drying and could cause a loss of material when crimping. Alternatively, place the whole capsule into a new silver capsule and crimp it closed.

    Note: Prepare standards with the same capsules as your samples (i.e., double wrap your standards in silver capsules, as you've done for your acidified samples). There is isotopic fractionation associated with different capsule materials: silver vs. tin.

    Buffered Acetic Acid Method: For Lake Sediments

  • Supplies and Solutions
  • 1) Sodium Acetate
    2) Glacial Acetic acid
    3) pH paper
    4) 1 (or 4) liter beaker, flask, or bottle.
    5) Polypropelyne centrifuge tubes (15 or 50 ml)
    Corning part # 430052, or 430290

    pH at 5: To make 1 liter of solution: Dissolve 82 g of sodium acetate in ~600 ml water. Add glacial acetic acid until tested pH is 5. Add water to make 1 liter. Test the pH using pH paper.

    pH to 4: To make a pH=4 solution takes quite a bit more acetic acid to reduce the pH in the buffer solution. To make 4 liters of solution: Add a bit less than 2 liters of water to 328 g of sodium acetate, dissolve it completely, then add about 1.5 liters of glacial acetic acid before testing pH. Then lower the pH as needed with more acid and top the bottle off with water.

    Note 1: For samples with labile nitrogen compounds using a higher pH may be preferable

    Note 2: At pH 4 dolomite dissolves really slowly, so it can take ~3 days if you have any in your sample.

  • Dissolution Procedure
  • 1) Add sample to centrifuge tube
    2) Add roughly 1/3 of centrifuge tube volume (5 ml to 15 ml - and depending on amount of sediment added) of buffered acetic acid solution to centrifuge tube
    3) Attach cap loosely
    4) Store in fume hood overnight (roughly 16 hours)
    5) Test by adding additional acetic acid solution to see whether reaction has gone to completion
    6) Add DI water until centrifuge tube is mostly full
    7) Tighten cap and shake or use a vortex mixer to suspend the sediment.
    8) Centrifuge down the sediment and decant off the supernatant fluid.
    9) Repeat steps 6-8 three-to-five times (i.e., enough rinse cycles to remove the acetic acid odor).
    10) Dry in oven or, preferably freeze dry the sediment to remove the excess water.

  • Note: When removing the sediments from the centrifuge tube, be careful not to scrape the polypropelyne tube material into your sample. Polypropelyne has a d13C of -20 permil.