ИСТИНА

There is a problem of standardization of humic substance (HS) preparations as the properties of the final product depend on the HS source and parameters of the technological process. IR spectroscopy is widely used to qualify the functional composition of HS. However, existing techniques (KBr pellets, paraffin dissolution, or films) have cumbersome sample preparation and low reproducibility of spectra. The aim of this work was the development of conditions for HS quantification in aqueous solutions and dry mixtures with minimal sample preparation. We used common commercial HS preparations "Aldrich", "Powhumus", "Sila Zhizni (Life Force)", and a collection of HS samples isolated from coal, peat, and soils. For commercial preparations, ATR-IR spectra of dry samples were recorded. The HSs under study have the same qualitative but different quantitative functional-group composition. HS "Aldrich" has a higher content of polysaccharides and aromatic radicals (1005–1035 and 910 cm–1, respectively). Coal HS mostly manifest intense bands of aliphatic groups, while bands corresponding to polysaccharides are less intense. For solid samples, the intensity of spectral bands is affected by the degree of dispersion of the sample and the contact between the ATR crystal and the sample, therefore, an internal standard is necessary. As the latter, we selected K3[Fe(CN)6], which has only one absorption band at 2115 cm–1, which does not overlap with characteristic bands of HS. Relative intensities of the bands 3690, 3620, 3330, 2922, 1560, 1380 and 1083 cm-1 are linearly dependent on HS content from 4 to 35% w/w. Aqueous solutions of HS in the range of 2–200 g/l show linear calibration dependences; the estimated limits of detection for the characteristic bands are 1 g/l for 1015, 1040, 1115, and 1388 cm–1, 6 g/l for 1570 cm–1, 11 g/l for 1388 cm–1, and 15 g/l for bands of 2854, 2924 and 3692 cm–1. Verification was made by the "added-found"method. The best sensitivity and accuracy are achieved when using calibration solutions prepared from the same test product. When determining HS using calibrations built for other HS preparations, the best results are obtained for bands 2924, 2852, 1560–1570, and 1380-1390 cm–1. Acknowledgement: The work was supported by RSF, grant no. 16-14-00167.