This study aims to compare photoacoustic (FTIR–PAS), diffuse reflectance (DRIFT), and attenuated total reflection (ATR) FTIR modalities in the wide wavenumber range from NIR (7500 cm⁻¹) to FIR (150 cm⁻¹) for the same silicate soil samples under the same conditions. The possibilities of non-destructive rapid qualitative analysis of soils by these modalities without comprehensive data treatment were compared. The assignment of more than 100 bands for the chernozem and sod-podzolic as common types of silicate types of soil was made. The following groups of bands of organic matter and inorganic matrix were reliably found in spectra of all or at least two modalities: 3690–3680 cm⁻¹ (hydrogen-bonded SiO–H…H₂O stretch, not ATR), 2930–2910 cm⁻¹ and 2860–2850 cm⁻¹ (methylene stretch), 1390–1380 cm⁻¹, (symmetric stretch carboxylate, DRIFT and FTIR–PAS); 2000–1990 cm⁻¹, 1885 cm⁻¹, and 1790–1783 cm⁻¹ (SiO₂ overtones, DRIFT and FTIR–PAS), 1163–1153 cm⁻¹, SiO₂ lattice (not FTIR–PAS), 1037 cm⁻¹ (Si–O or Al–O stretch), 796 cm⁻¹ (lattice symmetrical Si–O–Si stretch); 697 cm⁻¹, SiO₂; and 256 cm⁻¹ (not FTIR–PAS). Amide I, II, and III bands appear in DRIFT and FTIR–PAS spectra while not in ATR. Except for methylene and carboxylate groups, CH vibrations (3100–2900 cm⁻¹) are not seen in ATR. Bands at 1640–1630 cm⁻¹, 1620–1610 cm⁻¹, 1600–1598 cm⁻¹ (primary water bands and probably carboxylate) appear in the spectra of all three modalities but are unresolved and require data treatment. It is preferable to use all three modalities to characterize both soil organic matter and mineral composition. DRIFT provides the maximum number of bands in all three modalities and should be selected as a primary technique in the NIR and 4000–2000 cm⁻¹ regions for hydrogen-bonding bands, CH_X groups, and the silicate matrix. ATR–FTIR complements DRIFT and provides a good sensitivity for soil water and the matrix in 2000–400 cm⁻¹. FTIR–PAS in 4000–1500 cm⁻¹ reveals more bands than DRIFT and shows the highest sensitivity for absorption bands that do not appear in DRIFT or ATR-IR spectra. Thus, FTIR–PAS is expedient for supporting either DRIFT or ATR–FTIR. This modality comparison can be a basis for methodological support of IR spectroscopy of soils and similar organomineral complexes.

The capabilities of temperature-monitored IR spectroscopy for studying the organic matter and mineral composition of humic substances (HS) were tested. Temperature dependences of the mid-IR spectra of humic substances heated in the air in the range 25–215 °C (298–488 K, with a step of 2.5 °C)—for three commercially available samples isolated from brown coal (leonardite)—were performed. The characteristic bands were identified, and their changes in band maxima positions and intensities were compared. From the viewpoint of interpretation of HS components, the spectra were divided into regions of quartz lattice region (800–260 cm⁻¹), quartz overtone region (1270–800 cm⁻¹), humic substance organic matter region (1780–1270 cm⁻¹), quartz combination region (2800–1780 cm⁻¹), CH-speciation region (3100–2800 cm⁻¹), and hydrogen-speciation region (4000–3100 cm⁻¹) thus selected to contain the dominating type of bands. For the first time, a reversible change in the frequencies of the band maxima in IR spectra upon heating was observed, which can be interpreted as forming structures with a particular order in the studied humic substances in the dry state. For a single sample, both the band-shift scale and the functional dependence of the various bands on temperature differ significantly. The approach differentiates crystalline quartz bands, amorphous silica, and HSOM/surface groups experiencing a different temperature behavior of the band maxima and their intensities. Band-maximum temperature dependence can be considered more stable to changes in experimental conditions than band maxima at a single temperature, thus providing a more detailed HS structure analysis without HS decomposition or destruction.

The main examples of the application of thermal lens spectrometry and thermal lens microscopy as highly sensitive power-based thermooptical (photothermal) methods of molecular absorption spectroscopy...

Conditions for simple and sensitive FTIR quantitative determination of acrylonitrile, methyl acrylate, and itaconic acid in their copolymer films used as precursor products in carbon-fiber manufacturing are proposed. The measurement mode is IR-spectrophotometry (transmission) with internal spectrum normalization using the peak of acrylonitrile at 2242 cm⁻¹ as the main component and the determination of itaconic acid and methyl acrylate by their overlapping carbonyl peaks at 1740–1700 cm⁻¹. The calculations of comonomer concentrations were made using an external-standard method. The LOQs of itaconic acid and methyl acrylate are 0.5 and 2% w/w, respectively. The RSD of measurements is no higher than 7% rel. for methyl acrylate, 8% rel. for itaconic acid, and 1% rel. for acrylonitrile, which was shown by an example of analysis of PAN fiber samples with known concentrations of the components. The procedure was tested with several thermally unprocessed PAN precursor samples.