Does anyone know how to prepare the soil sample before analysis solid-state 13C-nuclear magnetic resonance(NMR)? I want to measure the chemical composition of organic matter. But the pretreatment of soil in literature differed so much that I do not know which one I should use.
Hi, Yuan.
Two main problems you may encounter when trying to obtain a good NMR spectrum: (i) the content in organic matter may be so low that the signal is very weak, and (ii) the soil has a high content in iron, which interferes the signal.
For both problems, the usual solution is a pre-treatment with HF. Hydrofluoric acid will destroy a substantial part of the mineral component, thus allowing a concentration of the OM; in addition it also solubilizes most, if not all iron forms. Thus both problems are solved.
Search for the pioneer work of Smernik & Oades for a detailed study of this problem (year 2000-2001).
The inconvenient of the HF treatment, is that also a part of the organic matter is solubilized, and since the treatment involves discarding the HF solution, this solubilized organic matter is lost. The solubilized organic matter may be not representative (as to its composition) of the overall organic matter of the sample: it may be enriched in labile, easily solubilized compounds (carbohydrates, aino acids, etc). Thus a distortion in the characteristics of the organic matter is to be expected.
Thus, you must reach a compromise between the need of removing iron and the need of avoiding big losses of organic matter in the treatment. Highly exhaustive HF treatments should be avoided, in my view.
A procedure could be as follows: put the ground soil sample (say, 2 g?) in a centrifugue teflon tube, and add 10 ml of 10% HF. Submit overnight to slight (not energic) end-over-end shaking. Next day, centrifugue the tube, and decant (discard) the liquid phase (HF plus silicates, iron, etc). Repeat the treatment twice. Finally, submit the sample to a washing with water (also, agitation, centrifugation, decantation, etc). Dry the residue. With the residue you should obtain a good NMR spectrum.
Some authors use a mixture of HF + HCl, instead of HF alone. Look for bibliography!
As to the problem of losses, shifts in the characteristics of OM due to the HF treatment, take a look to this paper:
Rumpel C. et al. (2006). Alteration of soil organic matter following treatment with hydrofluoric acid (HF). Organic Geochemistry 37: 1437-1451.
The amount of residue recovered should be enough to fill the glass tube for the sample, in the NMR spectrometer. Talk with your NMR technician in your research centre, to know how much final sample you will need for this.
No need to say, but I say it: be careful with HF. It is a dangerous chemical. Take advice from your directors as to the precautions you must take in the lab. The work under fume hood is compulsory.
If your soil is very rich in organic matter, the risk of losses may be high. Then, you could consider making a single cycle of extraction with HF. Or even none: and try to obtain the NMR spectrum directly. If you work with peat or compost, this could do. If you work with an OH horizon of a forest soil, perhaps, too. But in a A1 horizon, even if it is rich in organic matter, probably its iron content will be high enough to make the HF treatment necessary.
Also, I agree with Mr. Majumdar: take a look to several papers published in good journals (Soil Biology & Biochemistry, SSSAJ, Plant and Soil, Biology & fertility of Soils, Geoderma, European Journal of Soil Science...) and read carefully the 'Materials and Methods' section.
Good work!
Just use the methods used by papers published in the best journals in the field. As long as you can cite the extraction method it should be ok.
Iron oxides must be removed as well because they disturb the NMR. People normally use triple HF treatment for mineral soil. see DOI: 10.1111/j.1365-2389.1997.tb00552.x POM fractions may not need any treatment. For density fractionation I would recommend sodium polytungstate solutions, because it is inorganic.
Hi, Yuan.
Two main problems you may encounter when trying to obtain a good NMR spectrum: (i) the content in organic matter may be so low that the signal is very weak, and (ii) the soil has a high content in iron, which interferes the signal.
For both problems, the usual solution is a pre-treatment with HF. Hydrofluoric acid will destroy a substantial part of the mineral component, thus allowing a concentration of the OM; in addition it also solubilizes most, if not all iron forms. Thus both problems are solved.
Search for the pioneer work of Smernik & Oades for a detailed study of this problem (year 2000-2001).
The inconvenient of the HF treatment, is that also a part of the organic matter is solubilized, and since the treatment involves discarding the HF solution, this solubilized organic matter is lost. The solubilized organic matter may be not representative (as to its composition) of the overall organic matter of the sample: it may be enriched in labile, easily solubilized compounds (carbohydrates, aino acids, etc). Thus a distortion in the characteristics of the organic matter is to be expected.
Thus, you must reach a compromise between the need of removing iron and the need of avoiding big losses of organic matter in the treatment. Highly exhaustive HF treatments should be avoided, in my view.
A procedure could be as follows: put the ground soil sample (say, 2 g?) in a centrifugue teflon tube, and add 10 ml of 10% HF. Submit overnight to slight (not energic) end-over-end shaking. Next day, centrifugue the tube, and decant (discard) the liquid phase (HF plus silicates, iron, etc). Repeat the treatment twice. Finally, submit the sample to a washing with water (also, agitation, centrifugation, decantation, etc). Dry the residue. With the residue you should obtain a good NMR spectrum.
Some authors use a mixture of HF + HCl, instead of HF alone. Look for bibliography!
As to the problem of losses, shifts in the characteristics of OM due to the HF treatment, take a look to this paper:
Rumpel C. et al. (2006). Alteration of soil organic matter following treatment with hydrofluoric acid (HF). Organic Geochemistry 37: 1437-1451.
The amount of residue recovered should be enough to fill the glass tube for the sample, in the NMR spectrometer. Talk with your NMR technician in your research centre, to know how much final sample you will need for this.
No need to say, but I say it: be careful with HF. It is a dangerous chemical. Take advice from your directors as to the precautions you must take in the lab. The work under fume hood is compulsory.
If your soil is very rich in organic matter, the risk of losses may be high. Then, you could consider making a single cycle of extraction with HF. Or even none: and try to obtain the NMR spectrum directly. If you work with peat or compost, this could do. If you work with an OH horizon of a forest soil, perhaps, too. But in a A1 horizon, even if it is rich in organic matter, probably its iron content will be high enough to make the HF treatment necessary.
Also, I agree with Mr. Majumdar: take a look to several papers published in good journals (Soil Biology & Biochemistry, SSSAJ, Plant and Soil, Biology & fertility of Soils, Geoderma, European Journal of Soil Science...) and read carefully the 'Materials and Methods' section.
Good work!
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