Quantifying Effects of OM–Cation Interactions on Mid‐Infrared FTIR Absorption Spectra of Polygalacturonic Acid

ABSTRACT Background The composition of organic matter (OM) was characterized by Fourier transform infrared spectroscopy (FTIR) in terms of the relative number of functional groups. The presence of polyvalent cations in soil and effects of OM–cation interactions on FTIR absorption bands limit the interpretation of FTIR spectra of soil OM. Aim Quantify changes in intensity and wave number (WN) ranges for carboxyl groups in FTIR of OM–cation mixtures and identify their relations with cation concentration based on Lambert–Beer's law. Methods Freeze‐dried mixtures of polygalacturonic acid (PGA) as model substance and CaCl 2 (Ca–PGA) were analyzed with FTIR spectroscopy in mid‐infrared WN range using KBr technique. Lambert–Beer's extinction coefficient was determined for WNs in the range of the asymmetric COO − band (ν a COO − ). Results The intensity of the ν a COO − band maximum in FTIR spectra of Ca–PGA mixtures increased linearly with Ca content, and the band maximum shifted towards smaller WN. Regressions as well as relations of Lambert–Beer's extinction coefficient ratios could be useful to estimate the ν a COO − band intensity in FTIR of pure PGA from that of Ca–PGA mixtures. Discussion The regression‐based approach required a calibration with spectra of “pure” OM. The Lambert–Beer extinction coefficient ratios could be applied without prior calibration if spectra of the same OM–cation mixtures with different cation concentrations were available. Conclusions The empirical and extinction coefficient ratio‐based methods were both found useful for studying effects of OM–cation interactions on mid‐infrared FTIR absorption spectra to characterize OM composition.