1. A. I. Popov, Humic Compounds: Properties, Structure, and Formation [in Russian], Izd. St. Petersburg Univ., St. Petersburg (2004).
2. G. V. Naumova, V. P. Strigutskii, N. A. Zhmakova, and T. F. Ovchinnikova, Khim. Tverd. Topl., No. 2, 3 – 13 (2001).
3. S. F. Shurkhai, O. V. Korzyuk, and L. F. Klunduk, Physics and Chemistry of Peat for Solving Ecological Problems [in Russian], Minsk (2002), pp. 227 – 229.
4. A. I. Agapov, N. P. Avvakumova, and E. K. Batalova, Vopr. Kurortol., No. 2, 33 – 35 (1999).
5. N. G. Bakhshieva and B. V. Ioffe, Electronic Spectra in Organic Chemistry [in Russian], Khimiya, Leningrad (1973).