Improving infrared spectroscopy characterization of soil organic matter with spectral subtractions

Andrew J. Margenot, Sanjai J. Parikh, Francisco J. Calderón

Research output: Contribution to journalArticle

Abstract

Soil organic matter (SOM) underlies numerous soil processes and functions. Fourier transform infrared (FTIR) spectroscopy detects infraredactive organic bonds that constitute the organic component of soils. However, the relatively low organic matter content of soils (commonly < 5% by mass) and absorbance overlap of mineral and organic functional groups in the mid-infrared (MIR) region (4,000-400 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 mathematically removing mineral absorbances. This requires a mineral-enriched reference spectrum, which can be empirically obtained for a given soil sample by removing SOM. The mineral-enriched reference spectrum is subtracted from the original (untreated) spectrum of the soil sample to produce a spectrum representing SOM absorbances. 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 organic functional group composition of SOM. 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 organic matter composition.

Original languageEnglish (US)
Article numbere57464
JournalJournal of Visualized Experiments
Volume2019
Issue number143
DOIs
StatePublished - Jan 2019

Keywords

  • Diffuse reflectance infrared Fourier transform (DRIFT)
  • Environmental sciences
  • Fourier transform infrared (FTIR) spectroscopy
  • Functional group
  • Issue 143
  • Oxidation
  • Sodium hypochlorite (NaOCl)
  • Soil
  • Soil organic matter (SOM)
  • Spectral subtraction

ASJC Scopus subject areas

  • Neuroscience(all)
  • Chemical Engineering(all)
  • Biochemistry, Genetics and Molecular Biology(all)
  • Immunology and Microbiology(all)

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