Impact of remote protonation on 13C CPMAS NMR quantitation of charred and uncharred wood.

The impact of inefficient cross polarization (long TCH values), caused by long 13C-1H internuclear distances, on 13C CPMAS NMR spectra of charred and uncharred woods is determined by simultaneously fitting data from complementary variable spin lock and variable contact time experiments. As expected, the impact is minimal for uncharred woods, but is very significant for the charred woods. Quantification of the decrease in CPMAS signal intensity caused by both inefficient cross polarization and rapid T1rhoH relaxation is achieved using an advanced spin counting methodology, for which the term "spin accounting" is proposed.

Determination of T1rhoH relaxation rates in charred and uncharred wood and consequences for NMR quantitation.

The performance of three different techniques for determining proton rotating frame relaxation rates (T1rhoH) in charred and uncharred woods is compared. The variable contact time (VCT) experiment is shown to over-estimate T1rhoH. particularly for the charred samples, due to the presence of slowly cross-polarizing 13C nuclei.

Paramagnetic effects on solid state carbon-13 nuclear magnetic resonance spectra of soil organic matter.

The effects of paramagnetic species on solid state 13C nuclear magnetic resonance (NMR) spectra were quantified in a series of doping experiments. The degree of signal loss caused by paramagnetic metals was shown to depend not only on the quantity, but also on the nature of the paramagnetic species, as well as the intimacy of contact with the organic substrate and the type of NMR experiment. Two mechanisms of signal loss were distinguished--signal loss via loss of magnetic field homogeneity, which affects all 13C nuclei in a sample, and signal loss via interaction between electronic and nuclear spins, the effects of which were localized to the close environment of the paramagnetic species.

Background signal in solid state 13C NMR spectra of soil organic matter (SOM)-quantification and minimization.

13C background signal, obtained for an empty rotor, was shown by spin counting experiments to be equivalent to 1 mg of observable carbon for cross-polarization (CP) spectra and 69 mg of observable C for Bloch decay (BD) spectra. The BD background was mainly due to Kel-F in the stator. with minimal signal detected from the Kel-F end-caps.