FTIR spectral band shifts explained by OM–cation interactions

Background: The organic matter (OM) in soils interacts with polyvalent cations such as Ca²⁺ through hydroxyl (OH), carboxylic acid, ester, keto, aldehyde (summarized as C=O), and carboxylate (COO⁻) functional groups. Such interactions affect the bonding strength of the double bond between the C and the O atom in the functional groups, which is assumed to shift the wavenumber (WN) region of O–H (hydroxyl), C=O, COO⁻, and OMcₐₜ (i.e., C=O interacting with cations plus O–H groups) absorption band maxima in the Fourier transform infrared (FTIR) spectra. Such band shifts limit the evaluation of spectral information on OM in soil samples. Aims: The objective of this study was to analyze the extent of band shifts and the changes in absorption band intensities for relations with cation concentrations, and to estimate effects of band shifts on OM properties such as potential wettability of OM evaluated from FTIR absorption band ratios. Methods: Polygalacturonic acid (PGA) solutions were mixed with a CaCl₂ solution at different relations. The freeze‐dried mixtures were analyzed with FTIR spectroscopy in the mid‐infrared spectral range by using KBr‐technique. The FTIR spectra were interpreted with respect to O–H, C=O, COO⁻, and OMcₐₜ bands, the latter reflecting the formation of PGA‐Ca²⁺ complexes. Results: The FTIR spectra of the PGA–Ca mixtures compared to that of pure PGA indicate band shift effects by CaCl₂ addition on both, intensity and WN value of the OMcₐₜ, C=O and COO⁻ absorption band maxima. The COO⁻/C–O–C ratio increased with Ca²⁺ concentration while the C–H/C–O–C ratio decreased. Furthermore, the C=O and COO⁻ absorption band maxima were shifted towards lower WN values, while the OMcₐₜ absorption band was shifted towards higher WN values. The shift of OMcₐₜ band maxima was two times higher than that of the C=O band maximum and increased with Ca²⁺ concentration. Conclusion: Spectral band shifts depend on polyvalent cation concentration and limit automated interpretations of FTIR spectra without prior soil‐specific spectral corrections.