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

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.