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Title: Improving infrared spectroscopy characterization of soil organic matter with spectral subtractions

Author
item MARGENOT, ANDREW - University Of Illinois
item PARIKH, SANJAI - University Of California, Davis
item Calderon, Francisco

Submitted to: Journal of Visualized Experiments
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/14/2018
Publication Date: 1/10/2019
Citation: Margenot, A., Parikh, S., Calderon, F.J. 2019. Improving infrared spectroscopy characterization of soil organic matter with spectral subtractions. Journal of Visualized Experiments. 143. https://www.jove.com/video/57464.
DOI: https://doi.org/10.3791/57464

Interpretive Summary: SOM underlies many soil functions and processes, but its characterization by FTIR spectroscopy is often challenged by mineral interferences. We describe a method to increase the utility of SOM analysis by FTIR spectroscopy by subtracting mineral interferences in soil spectra using empirically obtained mineral reference spectra.

Technical Abstract: Soil organic matter (SOM) underlies numerous soil processes and functions. Fourier transform infrared (FTIR) spectroscopy probes infrared-active organic bonds that constitute the OM component of soils. However, the relatively low OM content of soils (commonly <5% by mass) and absorbance overlap of mineral and organic functional groups in the mid-infrared (MIR) region (4000 – 400 cm-1) engenders substantial interference by dominant mineral absorbances, challenging or even preventing interpretation of spectra for SOM characterization. Spectral subtractions, a post-hoc mathematical treatment of spectra, can reduce mineral interference and enhance resolution of spectral regions corresponding to organic functional groups by removing overlapping mineral absorbances. A spectral subtraction entails the subtraction of a mineral reference spectrum from the original soil spectrum (mineral + SOM), resulting in an SOM-only (ideally) or SOM-enriched (typically) spectrum. This requires a mineral-enriched reference spectrum, which can be empirically obtained for a given soil sample by removing SOM. Common SOM removal methods include high-temperature combustion (‘ashing’) and chemical oxidation. Selection of the SOM removal method carries two considerations: (1) the amount of SOM removed, and (2) absorbance artifacts in the mineral reference spectrum and thus the resulting subtraction spectrum. These potential issues can, and should, be identified and quantified in order to avoid fallacious or biased interpretations of spectra for SOM organic functional group composition. Following SOM removal, the resulting mineral-enriched sample is used to collect a mineral reference spectrum. Several strategies exist to perform subtractions depending on the experimental goals and sample characteristics, most notably the determination of the subtraction factor. The resulting subtraction spectrum requires careful interpretation based on the aforementioned methodology. For many soil and other environmental samples containing substantial mineral components, subtractions have strong potential to improve FTIR spectroscopic characterization of OM composition.