A Modified Cross-Polarization Magic Angle Spinning ¹³C NMR Procedure for the Study of Humic Materials

Abstract

A new approach to the use of high-field instruments for cross-polarization (CP) magic angle spinning (MAS) 13C NMR for analysis of humic materials is described. This technique consists of using a high sample spinning rate and a ramp CP pulse sequence, which, among other advantages, addresses chemical shift anisotropy effects. The theoretical aspects of high spinning rates on line broadening for nonrigid solids are reviewed. Also, the ramp CP pulse sequence and its implications for humic materials are discussed. It is shown that the highest resolution and most informative spectra can be obtained via a high-field instrument if the sample is spun at a rate higher than 0.25 of the anisotropy of the chemical shift of the aromatic moieties and a ramp CP pulse sequence is used. This study raises concerns that previous methods may have underestimated the amount of aromatic carbons in humic materials. Because of the similarities between humic materials and coals, this work may also have implications in the CP-MAS 13C NMR approach to coals.

Silicon‐Assisted Direct Covalent Grafting on Metal Oxide Surfaces: Synthesis and Characterization of Carboxylate N,N′‐Ligands on TiO2

“…A ramped 1 H-pulse starting at 100 % power and decreasing until 50 % was used during contact time in order to circumvent Hartmann-Hahn mismatches. [53,54] The contact time was 2 ms. To obtain a good signal-to-noise ratio in 13 C CPMAS experiments, 2048 scans were accumulated using a delay of 2 s. The 13 C chemical shifts were referenced to tetramethylsilane and calibrated with glycine carbonyl signal, set at δ = 176.5 ppm. The thermogravimetric analysis was performed using a Micromeritics piece of equipment with a thermogravimetric analyzer TGA 2950 Hi-Re.…”

Chemical and spectroscopic characterization of marine dissolved organic matter isolated using coupled reverse osmosis–electrodialysis

“…The cross polarization magic angle spinning (CPMAS) technique (Schaefer and Stejskal, 1976) was applied with a spinning speed of 6.8 kHz and a pulse delay of 1 s. Spectra were obtained after accumulation of 40,000 to 80,000 scans. A ramped 1 H-pulse was used during the contact time of 1 ms to circumvent spin modulation during the Hartmann-Hahn contact (Cook et al, 1996;Peersen et al, 1993). Line broadenings between 10 and 100 Hz were applied.…”

Synthesis and characterization of novel, highly crystalline poly(vinyl alcohol) microspheres for chemoembolization therapy

“…The CP-MAS method with a linear amplitude variation of the 1 H pulse was used. [19][20][21] The strength of the 1 H decoupling field applied during the signal acquisition was 60 KHz. A contact time of 1 ms, a spinning frequency of 10 KHz, and recycle delays of 3 s were used.…”