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RESEARCH PRODUCT

Effect of RF Field Inhomogeneity and Sample Restriction on Spectral Resolution of CP/MAS-13C NMR Spectra of Natural Organic Matter

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It is well known that the induced B1 magnetic field in an NMR coil is inhomogeneously distributed. However, this issue has so far received little attention in the field of environmental NMR. As this research field often aims at quanti- tative results as well as relaxation phenomena, the repercussions of such inhomogeneity on peak integrals and relaxation times need to be taken into account. The objective of the present study was to test standard recording conditions on different sample positions in an NMR coil in order to determine the effect of the RF field inhomogeneity on the spectrum of a molecularly complex humic material and on some standard molecules of known structure and conformation. To this end, we measured the peak integral and signal half-height width of constant sample amounts at different heights in the rotor. In addition, the effect of sample posi- tion in the rotor on T1H and T2C relaxation times was determined. We showed that the response profiles of different chemical groups are not necessarily comparable to each other and that spectra of natural organic matter can change when confined to different regions of the coil. Furthermore, the relaxation measurements revealed that T1H and T2C relaxation times are position-dependent. Finally, the application of sample res- triction to the homogeneous region appeared very promising for enhancing the resolution of spectra of complex mixtures.