Spin accounting and RESTORE - two new methods to improve quantitation in solid-state 13C NMR analysis of soil organic matter

Abstract

Rapid T1H relaxation and inefficient cross-polarization have long been known to affect quantitation in solid-state 13C cross-polarization (CP) NMR spectra of soil organic matter. We have developed two new techniques to overcome these problems. The first, spin accounting, enables accurate gauging of how quantitative a spectrum is likely to be. The result is expressed as the percentage of potential NMR signal that can be accounted for (Cobs). Spin accounting improves on the established spin counting technique by correcting for rapid T1H relaxation and inefficient cross-polarization. Spin accounting identifies three components: one that is well represented in CP spectra, one that is under-represented in CP spectra due to rapid T1H relaxation, and one that is under-represented in CP spectra due to inefficient cross-polarization. For a range of eight de-ashed soils, Cobs was in the range 83–106%, indicating that virtually all potential signal could be accounted for after correcting for rapid T1H relaxation and inefficient cross-polarization. The second new technique, RESTORE (REstoration of Spectra via TCH and T One Rho (T1H) Editing), generates subspectra for the three components identified in spin accounting. The sum of the three RESTORE subspectra is essentially a corrected CP spectrum. The RESTORE spectra of all eight soils more closely resembled the corresponding, and presumably quantitative, Bloch decay spectra than did the CP spectra. RESTORE identifies the types of structures underestimated by CP, and the cause of their underestimation. Rapid T1H relaxation most affected carbonyl and carbohydrate carbons, whereas inefficient cross-polarization most affected aromatic carbons.