Applications of Nuclear Magnetic Resonance Spectroscopy to the Study of the Structure of Soil Organic Matter

Abstract

Recent developments in the application of nuclear magnetic resonance (n.m.r.) spectroscopy to soil science are reviewed. Progress in 'H. 13C and CP-I3C n.m.r. spectroscopy of humic substances is reported. Methods of determining the fraction of aromatic carbon in soil organic matter extracts are discussed, and methods by which structural group analysis can be carried out on in siru organic matter of whole soils are reviewed. IntroductionNUCLEAR magnetic resonance spectroscopy (n.m.r.) has been an invaluable technique in elucidating the structure of pure and simple mixtures of low molecular weight (approximately less than 2000) organic compounds, but has had limited applicability to the study of complex mixtures.Until about 1970 n.m.r. spectroscopy was an insensitive technique, and spectra from samples of concentration less than 50 mg were determined with difficulty. In complex mixtures such as soil organic matter extracts, the concentrations of individual components are small, and consequently the spectra of organic matter extracts were poor. Over the last ten years, however, Fourier transform n.m.r. techniques have greatly improved spectral quality, and hitherto unobtainable spectra of complex mixtures have been recorded.This paper reviews the literature on the applications of n.m.r. spectroscopy to the study of the structure of soil organic matter, and suggests lines of investigation whereby research should be profitable. Initially I outline some basic n.m.r. principles, then review research to elucidate the structure of extracts of soil organic matter using 'H and I3C n.m.r. techniques and discuss attempts to quantify 13C n.m.r. data. Finally I look at current solid state work which holds promise as a technique for structural group analysis of the in siru organic matter of whole soils.

Mean residence times and characteristics of humic substances extracted from a Taiwan soil

“…The freeze-dried humic fractions were subjected to solid state 13 C NMR analysis on a Bruker DSX 400 with cross polarization and magic-angle spinning (CPMAS), proton decoupling with contact time 2.00 ms, recycle time 5.00 s, spectral width 20000.0 Hz, spinning speed 3500 Hz, number of scans 6000, spectrometer frequency 50.330 MHz, and acquisition time 51.25 ms. According to the method recommended by Wilson (1981), Wershaw (1985), and Stearman et al (1989), the range of chemical shift of 0-190 ppm of the solid state CPMAS 13 C NMR spectra were divided into the ranges of 0-50, 50-90, 90-110, 110-140, 140-160, and 160-190 ppm. The areas covered by the spectra in these ranges were used to estimate the distribution percentages of characteristic carbons in the various samples.…”

Effect of Humic Substance Photodegradation on Bacterial Growth and Respiration in Lake Water

“…Quantitative 13 C NMR data were obtained from 100 mg of HS samples dissolved in 1 ml H 2 O-D 2 O (1:1), adjusted to pH 7 in 10-mm tubes, and analyzed on a Varian model 300 spectrometer at 75.429 MHz using inverse gated decoupling with an 8-s delay (3). Quantification of the 13 C NMR spectra was done by integrating the area under the bands (64). Elemental analyses of the humic samples were carried out by Huffman Laboratories (Golden, CO) according to the method of Huffman and Stuber (31).…”

Europium-binding abilities of dissolved humic substances isolated from deep groundwater in Horonobe area, Hokkaido, Japan

“…The inverse-gated decoupling technique was applied for the measurement, under the following conditions: pulse width 458; acquisition time 0.839 s. A total repetition time of 2.5 s was applied to permit relaxation of all the spins, and 4000-20,000 scans were accumulated. Chemical shift assignments were made using the data reported by Wilson [30] and Fujitake and Kawahigashi [31]. The carbon contents were evaluated as the percent of peak area of individual carbons to the total area in the 13 C NMR spectra.…”