Is Mid-Infrared Spectral Subtraction a Practical Approach for Accurate Study of Soil Organic Matter?.

As characterized as an economical, rapid and comment technique, the application of mid-infrared (mid-IR) spectroscopy has made a remarkable progress for the analysis of soils over the last two decades. Recently, Reeves III (2012, Geoderma 189:508-513) published a good paper in which he investigated the performance of ashing soil samples and then using spectral subtraction of diffuse reflectance Fourier transform mid-IR spectroscopy (DRIFTS) of original and ashed samples in highlighting the significance of the organic fraction. Due to the fact that many types of clay minerals undergo extreme spectral changes from ashing processes, he found that accurately interpreting the ash subtracted spectra (entire mid-infrared range from 4000 cm^-1 to 400 cm^-1) is nearly impossible. Then they suggested investigating other means to remove soil organic matter such as the use of hydrogen peroxide (H₂O₂) because it seems that this wet ashing method would not leave any additional inorganic residues to confound the spectra.  We extended this topic by examining two additional aspects which are crucial in spectral subtraction, including evaluations of (1) ashing processes on spectral changes of organic materials (corn starch, soybean powder, and humic acids) and (2) wet chemistry processes (H₂O₂, NaOCl, Na₂S₂O₈) on spectral changes of minerals (Wyoming Na-bentonite). We found that ashing corn starch to 500 °C has eliminated all materials so that no spectrum remains. However, ashing soybean powder and humic acids to the same temperature resulted in burn residuals left which are associated with strong absorbance peaks in the range of 1500 cm^-1 to 500 cm^-1. Wet ashing processes not only dramatically diminished absorbance intensity of spectra but also eradicated certain absorbance peaks. Accordingly, it is impractical to accurately interprete the ash subtracted spectra (mid-IR) of soil samples.