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I am studying to analyze the IR spectra of soil samples. And I'm interested in the question - how to interpret the intervals in (1750..1820) and (3020 ... 2820) of the two samples on the graph?

The attached file - 1200..600 spectra of soil samples taken on Nikolet 6700 FT-IR Thermo Scientific in DRIFT mode and particular clippings of (1750..1820) and (3020 ... 2820) spectra.

Spectrogram in 1200..6000 nm

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UPDATE Scan method – FT-IR DRIFT 1 200 cm-1 .. 6 000 cm-1

  • Red line – paleosol untreated (only dry the samples and sieve it threw a 2 mm mesh)
  • Blue line – weathering sandy soil untreated (only dry the samples and sieve it threw a 2 mm mesh)
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  • $\begingroup$ You might be looking at humic and fulvic acid fractions of organic matter in the soil. Can you give us more details about the soil composition, even at the gross level (clayey, silty, sandy, what you are expecting to find if anything, how was the sample prepared/extracted)? $\endgroup$ – Todd Minehardt Nov 17 '16 at 16:04
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You are almost certainly looking at bands corresponding to functional groups in the humic acid fraction of your sample.

The band at 1750 - 1820 cm$^{-1}$ is likely the $\ce{C=O}$ stretching mode in $\ce{COOH}$ groups in humic acid, but this band is also used to differentiate the fulvic fraction from the humic fraction (see quoted text below). The band at 2820 - 3020 cm$^{-1}$ is likely aliphatic $\ce{C-H}$ stretching.

The composition of humic and fulvic acid fractions in soils varies quite a bit, and how the samples were prepared can significantly alter the composition of the fractions as well. Keep that in mind when interpreting your results. Your bands appear to be shifted from those in the reference I chose to cite, but that is to be expected depending on the composition of your soil and the organic matter contained therein (and can also indicate greater or lesser degrees of $\ce{H}$-bonding, resulting in red/blue shifts). The soil organic fractions referenced in the study below are sandy loams from Ibri, Oman, in an agricultural region. Your samples will differ in composition based on geography, geology, climate, and other factors.

Reference: Helal, Murad, and Helal. Characterization of different humic materials by various analytical techniques. Arabian Journal of Chemistry 4(1), 51–54 (2011) (link to free PDF from ScienceDirect):

The IR spectra of the three humic fractions are shown in Fig. 2. They have a diversity of bands more or less typical to those distinguishing the humic materials (Stevenson, 1994 and Aiken et al., 1985). Major absorption bands are in the regions of 3400–3300 cm$^{-1}$ ($\ce{H}$-bonded $\ce{OH}$ groups), 2940–2900 cm$^{−1}$ (aliphatic $\ce{C–H}$ stretching), 1750–1720 cm$^{−1}$ ($\ce{C=O}$ stretching of $\ce{COOH}$), 1620 cm$^{−1}$ (aromatic $\ce{C=C}$, $\ce{COO−}$, $\ce{H}$-bonded $\ce{C=O}$), 1280–1230 cm$^{1}$−1 ($\ce{C–O}$ stretching and $\ce{OH}$ deformation of $\ce{COOH}$) and 1040 cm$^{−1}$ ($\ce{C–O}$ stretching of polysaccharide or $\ce{Si–O}$ of silicate impurities). The spectra evidently show predominance of $\ce{OH}$, $\ce{COOH}$ and $\ce{COO−}$ groups which are the most characteristic features of soil humic materials. It is clear from the spectra that fulvic acid is characterized by stronger absorption near 1720 cm$^{−1}$ which implies the high carboxylate capacity, and that of humic acid is stronger than that of humin. This is exactly the case in the results of potentiometric titration (Helal, 2007). The spectrum of fulvic acid is also characterized by the absorption at 1400–1390 ($\ce{OH}$ deformation and $\ce{C–O}$ stretching of phenolic $\ce{OH}$). The results of IR spectra indicate that fulvic acid is more aliphatic, and humic acid and humin are more aromatic. It is obvious that the IR results are in good agreement with the other characterization findings.

Figure 2, from the above-referenced article. FA is fulvic acid and HA is humic acid.

Figure 2, from the above-referenced article

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    $\begingroup$ Thank you @Todd Minehard. This time I'm collecting a sample's history and will update this question in Monday, when my colleague from the lab will answer me. After updating the the question I beg to notify you, and I would be grateful for further clarification as this question requires more information on the soil properties and sample extraction/preparation method. I don't understand why in the region (2820 .. 3020), one sample has a lift with two small peaks, and the second has a slight rise with a negative skew. $\endgroup$ – Ilya Rusin Nov 18 '16 at 8:09
  • $\begingroup$ Today I've updated the question - Red line – paleosoil untreated (only dry the samples and sieve it threw a 2 mm mesh); Blue line – weathering sandy soil untreated (only dry the samples and sieve it threw a 2 mm mesh) $\endgroup$ – Ilya Rusin Nov 21 '16 at 9:47
  • $\begingroup$ @IlyaRusin - The two peaks in your paleosol are almost exact matches for what are identified as aliphatic $\ce{C-H}$ stretches. The weathered sample spectrum looks to me like it's a little shifted AND indicates that the aliphatic species that would otherwise contribute to those peaks are absent. Have a look at this publication, particularly Figure 1 and the first part of Results and Discussion. $\endgroup$ – Todd Minehardt Nov 21 '16 at 23:52

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