I want to estimate the mass of organic and inorganic carbon in mostly carbonatic sand samples (90% Calcite /Dolomite). There is little evidences of clays in my samples, which add error to the standard LOI procedure. Also, I am expecting low OM (organic carbon) content from my sand samples as compared to mud flats or other type of more organic soils.
Current procedure:
However, I recently came across a paper which empirically demostrated that carbonatic sand and muds with low content of organic matter (below 5%) present high variance in LOI-estimated organic carbon (using T=550 degrees Celsius).
Thus, I think I should remove salt from my sand samples BEFORE the LOI procedure.
I was thinking to simply rinse my samples with fresh water, stir it, let all the particules settle, and change water a few times, then oven dry them and procede to LOI at the furnace.
Problem:
I am worried about interfering with the OM in the sample.
Does anyone have any hint on how I could make sure that salt does not have an influence on my LOI estimate of Organic Carbon?
If you do soak in water, you may want to centrifuge your samples back down in case you manage to suspend any fine particles. Also, be aware that some coarse organic material (especially dried plant material) may float to the surface, so you may have to be creative with decanting the supernatant.
Thx Donald A. Beebe . Organic matter will not dissolve into Dissoveld Organic Matter (DOM) once I soak my beach sand into fresh water?
The reason I am doing this analysis is because I also got the VNIR (350-950nm) spectral profile of these samples in the field.
Beside granulometry and mineralogy (XRD), I think it would be nice if I can model TOC from spectras, thus, I need TOC values.
Cheers!
If you are measuring TOC using fancy Xray stuff you should ground-truth it using DC furnace (with and without acid to differentiate organic from inorganic) and the old, tried and tested, Walkley-Black Dichromate reduction procedure. Extend the digestion time if you want to capture the more intractable organic fractions.
Nicolas Pucino Yes, some organic material will likely dissolve depending on soak time and temperature, and quality of OM. However, if you are not concerned with sub-ppt resolution, I cannot imagine that the loss would be significant. A simple trial with a few test specimens may reveal how much mass change is associated with the water extraction.
Very good explanations have been given. It appears to me that salt present in the samples may not have large influence on the variation of LOI value. Organic matter in soil can be determined from LOI at 550 degree centigrade, but in this case also, there are every possibilities for decomposition of carbonate minerals mainly associated with Calcium and Magnesium. Due to variations of these mineral content from place to place, LOI variations could be there and LOI thus, could not appeal for an accurate method for the determination of organic matter. Instead, organic carbon estimation by modified Walkly and Black method might be more accurate and determination of carbonate minerals could be done by acid titration.
Thx for your suggestions.
Donald A. Beebe : I will try to do the test with some subsamples that I have already washed to see if any significant difference is detectable using the LOI method.
Simon Leake , Phani Bhusan Ghosh : I think I will also send some subsamples to a lab in order to get Walkley-Black done and I might consider doing acid digestion for carbonates as well.
Thx everyone for your inputs!
Nick
From my experience dealing with siliciclastics verses carbonates you don't have to remove the salt from the sample in order to do LOI. There are many papers that use LOI as a proxy for OM content in carbonate sediments & environmentals (e.g. lagoonal marshes, salt marshes, carbonate shelves).
Jonathan Rogers : Thx for your input. I read the literature and I agree with you, nobody removes halite or "salt" from sediment before LOI.
However, I couldn't find any reputable paper describing LOI for carbonatic beach sand.
It might be useful to know that I am getting this LOI as an estimate only, because I use OM, carbonates %, grainsize, sorting, mineralogy and moisture to group insitu VNIR spectral signatures into endmembers for remote sensing applications.
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