Near- versus Mid-Infrared Spectroscopy for On-Site Analysis of Soil C

Authors: Reeves Iii, James; Francis, Barry

Submitted to: Near Infrared Spectroscopy International Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 5/15/2009
Publication Date: N/A
Citation: N/A

Interpretive Summary: The Kyoto Protocol is an agreement among many of the world’s national to, among other things, reduce atmospheric carbon dioxide concentrations in order to reduce global warming. One potential method to do so is to sequester carbon in soils. This has had the effect of stimulating the establishment of national soil carbon inventories and is motivating the implementation of projects that promote carbon accumulation in terrestrial ecosystems (soils, trees, etc.). To be able to effectively monitor such projects, methods that are able to analyze vast number of samples within a reduced time period are needed. Currently standard methods of soil carbon analysis are either slow, require the use of toxic reagents or have accuracy problems. Spectroscopic techniques like near-infrared reflectance spectroscopy (NIRS) and diffuse reflectance Fourier transform spectroscopy (DRIFTS) which are based on the interaction of light radiation with samples, combined with chemometrics (mathematical methods for relating the spectral information to chemical composition), can simultaneously determine any number of variables, including different carbon forms. The objective of this work was to examine the potential for on-site analysis of soil C using NIRS or DRIFTS. While research has demonstrated that for the determination of soil C, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is often more accurate and produces more robust calibrations than near-infrared reflectance spectroscopy (NIRS) when analyzing ground, dry soils under laboratory conditions. However, DRIFTS is known to be more affected by moisture and sample preparation than NIRS even with the spectrometer sealed or purged with dry nitrogen gas to eliminate effects of moisture on the optics, and ambient carbon dioxide and moisture on the spectra. DRIFTS is also not considered to be feasible on samples containing high levels of moisture due to the strong water absorptions in the mid-infrared, although the presence of water is also known to degrade even near-infrared spectra and subsequent calibrations. While DRIFTS has been shown to be advantageous in the laboratory, if samples need to be ground and dried and instruments purged to obtain useable data, it may not be practical for on-site use. Studies were undertaken to determine the effect of ambient atmospheric conditions and soil state (ground, dried, etc.) on DRIFTS and NIRS (Fourier transform and scanning monochromator) calibrations for soil C. Results using a portable Fourier transform mid-infrared spectrometer (FTIR) over a wide range of ambient temperatures and humidity levels have demonstrated that purging of the FTIR is not necessary to obtain calibrations for inorganic or organic C in soils equal to those obtained in the laboratory under ideal conditions. Preliminary efforts have also demonstrated that drying of samples on-site occurs rapidly under even moderate temperatures (70's) and should not present a problem if dried samples are required.

Technical Abstract: Research has demonstrated that for the determination of soil C, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is often more accurate and produces more robust calibrations than near-infrared reflectance spectroscopy (NIRS) when analyzing ground, dry soils under laboratory conditions. However, DRIFTS is known to be more affected by moisture and sample preparation than NIRS even with the spectrometer sealed or purged with dry nitrogen gas to eliminate effects of moisture on the optics, and ambient carbon dioxide and moisture on the spectra. DRIFTS is also not considered to be feasible on samples containing high levels of moisture due to the strong water absorptions in the mid-infrared, although the presence of water is also known to degrade even near-infrared spectra and subsequent calibrations. While DRIFTS has been shown to be advantageous in the laboratory, if samples need to be ground and dried and instruments purged to obtain useable data, it may not be practical for on-site use. Studies were undertaken to determine the effect of ambient atmospheric conditions and soil state (ground, dried, etc.) on DRIFTS and NIRS (Fourier transform and scanning monochromator) calibrations for soil C. Results using a portable Fourier transform mid-infrared spectrometer (FTIR) over a wide range of ambient temperatures and humidity levels have demonstrated that purging of the FTIR is not necessary to obtain calibrations for inorganic or organic C in soils equal to those obtained in the laboratory under ideal conditions. Preliminary efforts have also demonstrated that drying of samples on-site occurs rapidly under even moderate temperatures (70's) and should not present a problem if dried samples are required.