Cross polarization-single pulse/magic angle spinning (CPSP/MAS): A robust technique for routine soil analysis by solid-state NMR
Résumé
Soils are amongst the largest organic carbon reservoirs on earth containing approximately three times the carbon contained in all living systems and roughly double the amount present in the atmosphere. Soil organic matter is central to agriculture, carbon cycling, and contaminant sequestration, but due to its extreme heterogeneity it is challenging to study with most modern analytical approaches. As such 13C NMR spectroscopy has emerged as an indispensable technique for the characterization of soil organic matter in the solid-state. Single pulse (SP) 13C NMR approaches theoretically provide the highest level of quantitation for soil organic matter, however, due to its relative insensitivity, sample analysis can take a prohibitively long time. Consequently, for routine studies the more sensitive approach of cross-polarization under magic angle spinning conditions (CP/MAS) is more commonly utilized. In particular, 13C CP is extremely useful when low organic carbon content samples are compared and is normally used to reveal the nature, transformations and fate of organic matter in soils. Here, the performance of a novel NMR scheme, which so far has not yet been applied to soils samples, is investigated. The ramp-CPSP scheme adds a SP block to a ramp-CP scheme, taking advantage of both techniques without extending the experimental time. This method shows enhancements higher than 100% for key regions of the 13C spectra and also provides 13C profiles that are closer to quantitative when compared to the widely used ramp-CP scheme. In addition, a critical analysis of these enhancements is presented. Even under the worst case scenario, when the SP element adds little additional signal, the result still reflects the conventional ramp-CP experiment. As such the ramp-CPSP approach can be implemented in a routine fashion without drawback. The results shown here suggest replacing conventional ramp-CP with the ramp-CPSP sequence for routine soil analysis in the solid-state since it can save considerable experimental time and provide a more representative 13C spectrum of the soil in general.