Margenot Andrew J, Parikh Sanjai J, Calderón Francisco J
Department of Crop Sciences, University of Illinois Urbana-Champaign;
Department of Land, Air and Water Resources, University of California Davis.
J Vis Exp. 2019 Jan 10(143). doi: 10.3791/57464.
Soil organic matter (SOM) underlies numerous soil processes and functions. Fourier transform infrared (FTIR) spectroscopy detects infrared-active 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) 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.
土壤有机质（SOM）是众多土壤过程和功能的基础。傅里叶变换红外（FTIR）光谱法可检测构成土壤有机成分的红外活性有机键。然而，土壤中相对较低的有机质含量（通常按质量计<5%）以及中红外（MIR）区域（4000 - 400 cm）中矿物和有机官能团的吸光度重叠，导致主要矿物吸光度产生大量干扰，对用于SOM表征的光谱解释构成挑战甚至阻碍。光谱减法是一种光谱的事后数学处理方法，通过数学方式去除矿物吸光度，可以减少矿物干扰并提高与有机官能团对应的光谱区域的分辨率。这需要一个富含矿物的参考光谱，对于给定的土壤样品，可以通过去除SOM凭经验获得该参考光谱。从土壤样品的原始（未处理）光谱中减去富含矿物的参考光谱，以产生代表SOM吸光度的光谱。常见的SOM去除方法包括高温燃烧（‘灰化’）和化学氧化。SOM去除方法的选择有两个考虑因素：（1）去除的SOM量，以及（2）矿物参考光谱中的吸光度伪像以及由此产生的减法光谱。为了避免对SOM有机官能团组成的光谱进行错误或有偏差的解释，这些潜在问题能够且应该被识别和量化。去除SOM后，将所得的富含矿物的样品用于收集矿物参考光谱。根据实验目标和样品特性，存在几种进行减法运算的策略，最显著的是减法因子的确定。所得的减法光谱需要根据上述方法进行仔细解释。对于许多含有大量矿物成分的土壤和其他环境样品，减法运算有很大潜力改善FTIR光谱对有机质组成的表征。
