Carbon-13 solid-state NMR of soil organic matter - using the technique effectively

Abstract

).Preston, C. M. 2001. Carbon-13 solid-state NMR of soil organic matter -using the technique effectively. Can. J. Soil Sci. 81: 255-270. Solid-state 13 C CPMAS NMR offers many options for characterizing carbon in soil organic matter (SOM). Its effectiveness, however, is often limited by a poor understanding of the techniques, and lack of hands-on access and training opportunities for students. Of nearly 250 modern NMR systems in Canada, approximately one is genuinely available for SOM studies, and there is poor communication between NMR operators and SOM users. While quantitative reliability can be addressed to some extent by multiple contact-time experiments or single-pulse (Bloch decay) spectra, it is also important to consider the effects of spectrometer background, spinning sidebands (especially with higher magnetic fields) and processing operations such as linebroadening, phasing and baseline correction. In many studies, more consideration needs to be given to instrument specifications, the type of information needed, and whether sample fractionation or pretreatment should be used. Structural information can be greatly enhanced by dipolar-dephasing and sideband suppression sequences. Sequences based on relaxation differences can reveal pools of carbon with different structures. Data analysis can be enhanced by principal component analysis, spectrum deconvolution and difference spectra. Studies of xenobiotics and C metabolism can be greatly aided by 13 C-labeling. However, a key limitation to SOM applications remains the gap in culture and expectations of the users.Key words: Soil organic matter, 13 C CPMAS NMR, Bloch decay, dipolar dephasing, spinning sidebands Preston, C. M. 2001. Analyse de la matière organique du sol par RMN au 13 C électronique -de l'importance de bien utiliser la technique. Can. J. Soil Sci. 81: 255-270. La RMN CPMAS 13 C électronique permet de caractériser le carbone présent dans la matière organique du sol (MOS) de diverses manières. Il arrive souvent cependant que cette technique perde de son efficacité parce qu'on la connaît mal et parce que les étudiants n'ont pas eu la chance de l'examiner ou de l'expérimenter. Sur les 250 systèmes modernes de RMN existant au Canada, un seul, à peu près, est vraiment réservé à l'étude de la MOS; de plus, la communication passe mal entre les opérateurs de systèmes RMN et les utilisateurs de MOS. Bien qu'on puisse remédier dans une certaine mesure au problème de la fiabilité quantitative en multipliant les expériences sur la durée de contact ou en recourant au spectre obtenu après une seule impulsion (décroissance de Bloch), il ne faut pas négliger les effets de l'arrière-plan du spectromètre, de la rotation des bandes latérales (surtout quand le champ magnétique est puissant) et des opérations de traitement comme l'élargissement des lignes, la mise en phase et la correction de la ligne de base. Dans de nombreux cas, on devrait accorder plus d'attention aux paramètres de l'instrument, au genre de données requises et à la nécessité de fractionner et...

Effects of charring on mass, organic carbon, and stable carbon isotope composition of wood

“…The total integrated area of CP spectra was 0.42-0.49 of that observed for the corresponding BD spectra (i.e., CP enhancement was < 0.5 compared to a theoretical maximum of 4). The discrepancy is even larger, as CP efficiency is reduced at high spinning speeds (Jakobsen et al, 1988), and the compensating ramped-amplitude technique (RAMP) was not available on our equipment (Preston, 2001). For three other chars derived from wood in wildfires, we found that CP efficiency at 8000 Hz was 0.57-0.73 of that at 4700 Hz (unpublished results).…”

“…Therefore, CPMAS spectra typically underrepresent amorphous long-chain CH 2 and the polycondensed structures of BC (Maroto-Valer et al, 1996;Skjemstad et al, 1999;Smernik and Oades, 2000;Freitas et al, 2001;Preston, 2001;Preston et al, 2002;Baldock and Smernik, 2002). To overcome this, we also obtained spectra using simple Bloch decay (BD) acquisition (without CP), with 720 scans and 100 s recycle time as determined to be sufficient for char samples by Smernik and Oades (2000) and Baldock and Smernik (2002).…”

“…For convenience, we refer to it as ''phenolic'' in this paper. Corrections were made for SSB by assuming equal intensities for the upfield and downfield sidebands (Preston, 2001). For softwood, the carboxyl peak was small, and a SSB correction was not necessary.…”

“…Therefore, the char spectra were only divided into alkyl and aromatic regions, with the boundary (typically around 80 ppm) determined by the minimum intensity ). The areas were corrected for the SSB of the aromatic signal, by assuming equal intensity for both sidebands (Preston, 2001). The spinning speed of 8000 Hz was chosen as a compromise between reduction of SSBs vs. the loss of CP efficiency with increasing spinning speeds (Jakobsen et al, 1988).…”

The Need of Analytical Techniques for In-Situ Study of Soil Organic Matter

“…The question invariably associated with soil organic matter (SOM) studies is whether to work with intact soil or to fractionate and if so, how. The early emphasis on humic and fulvic acids (HA, FA) and the often acrimonious debate on their relevance, was driven in part by the need to isolate those SOM forms which are both high in carbon and reasonably soluble and thus, most amenable to the chemical and spectroscopic techniques then available [7]. Instead, recently developed modern analytical techniques like Solid-state NMR offers great potential as a technique for identifying and quantifying the different forms of organic C in soil [8][9].…”