Solid state CPMAS 13C and 15N NMR spectroscopy in organic geochemistry and how spin dynamics can either aggravate or improve spectra interpretation

Abstract

Because the ongoing discussion about the reliability of solid state cross polarization magic angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectroscopy still leaves uncertainty with respect to its usefulness in organic geochemistry, the present work provides a brief introduction into the basic principles of this technique and gives some explanation about the potential source of non-quantitative results. In addition, relaxation data are supplied which are necessary for the correct adjustment of NMR acquisition parameters. High contents of paramagnetics or moisture indeed affect the CP dynamics, complicating quantification of solid state NMR data obtained with a standard protocol. Whereas the latter can be avoided by carefully drying the sample, the first is often circumvented by demineralization with hydrofluoric acid (HF). Although this can yield considerable organic matter loss, the given examples demonstrate that differences in the intensity distribution of the spectra before and after HF treatment are most likely due to a selective alteration of the relaxation kinetics of protons closely interacting with paramagnetics. It is further shown that single pulse excitation does not necessarily provide quantitative data, since some geopolymers (e.g. cellulose) relax extremely slowly. At high magnetic fields and low spinning speeds, spinning side bands can overlap with relevant signals and obscure the intensity distribution. At spinning speeds >6 kHz, the range of the correct Hartmann–Hahn match is reduced, resulting in a preferential intensity loss of weakly coupling carbons which can be avoided by the application of special pulse sequences. In principal, the acquisition of quantitative CPMAS NMR data from geochemical samples is possible, although this often requires an in depth analysis of the relaxation parameters. Further, the latter also represents a powerful tool for the identification of geochemical compounds by providing additional information about their physical status and their spatial relationships to each other.