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
Materials:
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.
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