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Solid-State Carbon-13 Nuclear Magnetic Resonance of Humic Acids at High Magnetic Field Strengths

^a Department of Chemistry, The Ohio State University, 100 W. 18th Ave., Columbus, OH 43210
^b The Campus Chemical Instrumentation Center, The Ohio State University, 176 W. 19th Ave., Columbus, OH 43210

* Corresponding author (hatcher{at}chemistry.ohio-state.edu)

Received for publication April 2, 2001. Use of solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy has become commonplace in studies of humic substances in soils and sediments, but when modern high-field spectrometers are employed, care must be taken to ensure that the data obtained accurately reflect the chemical composition of these complex materials in environmental systems. In an effort to evaluate the quality of solid-state ¹³C NMR spectra obtained with modern high-field spectrometers, we conducted a series of experiments to examine spectra of various humic acids taken under a variety of conditions. We evaluate conditions for obtaining semiquantitative cross polarization magic angle spinning (CPMAS) ¹³C NMR spectra of humic acids at high magnetic field and spinning frequency. We examine the cross polarization (CP) dynamics under both traditional and ramp CP conditions on Cedar Creek humic acid. Fitted equilibrium intensities from these CP dynamic studies compare to within 3.4% of the intensities determined from a Bloch decay spectrum of the same sample. With a 1-ms contact time, ramp CP and traditional CP spectra were acquired on this sample and were found to compare to within 5.4% of the Bloch decay spectrum; however, the signal-to-noise ratio per hour of data acquisition was found to double under ramp CP conditions. These results demonstrate the power of applying modern solid-state NMR techniques at high magnetic field strengths. With these techniques, high-quality, semiquantitative spectra can be quickly produced, allowing the application of solid-state NMR techniques to more environmentally relevant samples, especially those where the quantity is limited.

Abbreviations: BD, Bloch decay • CP, cross polarization • CPMAS, cross polarization magic angle spinning • CSA, chemical shift anisotropy • CT, contact time • CW, continuous wave • HA, humic acid • I₀, thermal equilibrium magnetization • MAS, magic angle spinning • NMR, nuclear magnetic resonance • rf, radio frequency • rms, root mean square • SN, signal-to-noise ratio • SN/h, signal-to-noise ratio per hour • SOM, soil organic matter • T₁, spin lattice relaxation time • TPPM, two pulse phase modulated

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