FTIR spectral band shifts explained by OM–cation interactions

Abstract

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.

A LUCAS‐based mid‐infrared soil spectral library: Its usefulness for soil survey and precision agriculture#

“…Many studies investigated in detail the relationship between diffuse reflectance spectra and soil properties from underlying chemical perspective (e.g., Bornemann et al., 2010; Ellerbrock & Gerke, 2021). Nevertheless, practical application in lab routine analyses or in field studies is still subjected to methodological restrictions.…”

Calcium Phosphate/Hyaluronic Acid Composite Hydrogels for Local Antiosteoporotic Drug Delivery

“…In the research by R. H. Ellerbrock et al, the interaction between Ca 2+ and carboxyl groups of polygalacturonic acid (PGA) molecule was studied by FTIR, and it was revealed that the addition of Ca 2+ to PGA causes the formation of COO− groups, resulting in an increased intensity of COO− absorption and decreased intensity of COOH band. It was also observed that C=O and COO− absorption bands were shifted toward the lower wavenumbers ( Ellerbrock and Gerke, 2021 ). However, as sodium hyaluronate is used within this study, the COO- group is already formed in the HA molecule and the intensity change of C=O and COO− groups upon the addition of Ca 2+ are difficult to determine.…”

Assessing Factors Controlling Structural Changes of Humic Acids in Soils Amended with Organic Materials to Improve Soil Functionality

“…However, the concentration of aromatic compounds determined by FTIR was poorly correlated with the C/N ratio (R = −0.004), and thus there was a weak relationship between the NMR and FTIR (R = 0.018) aromatic fractions. This could have been due to band shifts in the FTIR spectra resulting from interaction of OM with polyvalent cations, which is suggested to shift the wavenumber region of functional groups [54,55].…”