FTIR Photoacoustic and ATR Spectroscopies of Soils with Aggregate Size Fractionation by Dry Sieving

[Image: see text] Granulometric fractionation as a source of additional information on organic-matter and inorganic matrix components of soils using FTIR–photoacoustic spectroscopy (FTIR–PAS) supported by attenuated–total reflection FTIR spectroscopy (ATR–FTIR) for a wide range of aggregate fractions (10–5000 μm) was used to compare the sensitivity, reproducibility, information contents, and representativity of fractionated samples. For chernozem and sod-podzolic soils and different agricultural-use chernozem samples, differences in the composition were found, manifested in normalized spectra of microaggregate fractions, with the range of 10–100 μm bearing the complete information. Most changes are observed in the soil organic matter range (1900–1340 cm(–1)), although these changes are slight, and in the soil-matrix region (550–300 cm(–1)). The latter region increases the intensity of bands corresponding to amorphous silica and clay minerals in fine fractions, while the intensity of bands attributed to quartz lattice vibrations decreases. FTIR–PAS spectra do not differ considerably at high interferometer modulation frequencies as the signal-penetration depth is comparable with particle sizes. The soil fractions below 20 μm result in the maximum sensitivity, reproducibility, and signal-to-noise ratio, showing no changes from coarser fractions by the information content and, thus, providing representative samples for analysis. The fractionation shows more differences in the sod-podzolic and chernozem soil fractions than the whole soil spectra. FTIR–PAS provides better sensitivity and reproducibility in the 4000–2000 cm(–1) region and ATR–FTIR in the 2000–100 cm(–1) region.