Quantitation in the Solid-State <sup>13</sup>C NMR Analysis of Soil and Organic Soil Fractions

Keeler, Camille; Maciel, Gary E.

doi:10.1021/ac020679k

Cited by 100 publications

(87 citation statements)

References 45 publications

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“…The earliest application demonstrated the applicability of the technique (Hatcher et al, 1980 ;Barron and Wilson, 1981 ;Wilson, 1981 ;Wilson et al, 1981a -c ;Preston and Ripmeester, 1982 ). More recently, solid -state NMR techniques have been used extensively to gain insight into the structural components of NOM in whole soils and sediments (Wilson, 1987 ;Knicker et al, 1993 ;K ö gel -Knabner and Ziegler, 1993 ;Preston, 1996 ;K ö gel -Knabner, 1997K ö gel -Knabner, , 2000Preston et al, 1997 ;Mahieu et al, 1999 ;Nierop et al, 1999 ;Knicker, 2000aKnicker, , 2004Spaccini et al, 2000 ;Hatcher et al, 2001 ;Keeler and Maciel, 2003 ;Smernik and Baldock, 2005 ;Xing et al, 2005 ;Dickens et al, 2006 ;Keeler et al, 2006 ). Solid -state NMR studies mostly examine the distribution of carbon ( 13 C nuclei) within a whole soil or sediment; however, there have also been applications of 15 N solid -state NMR techniques (Knicker et al, 1993 ;Guggenberger et al, 1994 ;Knicker, 2000aKnicker, , 2004Smernik and Baldock, 2005 ).…”

Section: Whole Soils and Sedimentsmentioning

confidence: 99%

“…Analysis of whole soils and sediments is often additionally confounded by the presence of paramagnetic species, such as iron, in the mineral component of the sample. In 2003 Keeler performed an excellent study examining the infl uence of paramagnetic species on the 13 C NMR of whole soils; any reader intending to attempt NMR studies on whole soils or sediments is strongly encouraged to refer to this article (Keeler and Maciel, 2003 ). To overcome the infl uence of paramagnetic species, many researchers have proposed the use of hydrofl uoric acid (HF) to remove the mineral component through successive dissolution steps (Preston et al, 1989 ;Skjemstad et al, 1994b ;Schmidt et al, 1997a ;Gelinas et al, 2001 ;Gon ç alves et al, 2003 ).…”

Section: Introductionmentioning

confidence: 99%

“…Gelinas et al (2001) reported only small changes in the (C/N) a and (H/C) a ratios of recovered sedimentary NOM after treatment with HF and HCl and suggested that the molecular composition was not altered signifi cantly. Despite concerns of NOM structure changes during demineralization, pretreating samples using HF and/or HCl is considered to be the most advantageous approach because of increased spectral resolution and decreases in interferences from paramagnetic minerals (Skjemstad et al, 1994a ;Gelinas et al, 2001 ;Gon ç alves et al, 2003 ;Keeler and Maciel, 2003 ).…”

Section: Introductionmentioning

confidence: 99%

“…; Baldock and Smernik, 2002 ;Keeler and Maciel, 2003 ;Simpson and Hatcher, 2004 ;Smernik, 2005 ;Keeler et al, 2006 ). The emphasis of this research has been on comparisons between single pulse excitation methods that directly polarize 13 C nuclei (referred to as direct polarization, DP or Bloch decay) versus cross -polarization (referred to as CP) methods that polarize 13 C nuclei via nearby 1 H nuclei.…”

mentioning

confidence: 99%

“…At higher MAS spinning speeds, the CP effi ciency can be compromised and can be overcome by applying the ramped amplitude (RAMP) CP pulse program (Cook andLangford, 1998, 1999 ), which matches the Hartman -Hahn condition necessary for effi cient CP. Studies comparing DP and CP methods have collectively demonstrated that DP methods are more quantitative and that CP methods may underestimate carbons that are not in the vicinity of protons (Conte et al, 1997(Conte et al, , 2004Mao et al, 2000 ;Baldock and Smernik, 2002 ;Keeler and Maciel, 2003 ;Simpson and Hatcher, 2004 ). For example, Mao et al compared DP and CP techniques for humic materials from 14 different NOM samples of varying origins .…”

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confidence: 99%

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Section: Whole Soils and Sedimentsmentioning

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Section: Introductionmentioning

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Section: Introductionmentioning

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Solid-state NMR is widely used to characterize organic matter (OM), including litter inputs, peat, composts, and mineral soil, and to understand the fate of environmental contaminants. The combination of high-power decoupling, magic-angle spinning (MAS), and cross polarization (CP) to enhance sensitivity has greatly facilitated applications to 13 C, with lesser use of 15 N and 31 P. Focusing on 13 C, this article summarizes the development of the field and presents basic concepts of solid-state NMR, pulse sequences commonly used [direct polarization (DP), CP, dipolar dephasing (DD), and total sideband suppression (TOSS)], sample preparation and hydrofluoric acid pretreatment, general approaches to spectral acquisition and FID processing, removal of background signal, and data analysis including correction for spinning side bands (SSBs) and interpretation through a molecular mixing model of representative biopolymers. As CP NMR is inherently nonquantitative, and nuclei may be undectectable even with DP, techniques to improve quantitative reliability are discussed. More complex experiments can reveal spatially heterogeneous domains by generating subspectra of 13 C associated with protons with different relaxation times, and more recent developments allow spectral editing and two-dimensional (2-D) applications such as heteronuclear correlation (HETCOR). Applications of solid-state 15 N NMR are much more challenging, because of its low sensitivity and low natural abundance. Similarly, despite 100% natural abundance and high sensitivity, solid-state 31 P NMR is also limited by the small chemical shift range of phosphate minerals, and peak broadening and large higher order sidebands because of their close association with paramagnetics and large chemical shift anisotropy (CSA). However, studies with these nuclei can still provide much insight into OM cycling and the fate of fertilizers and contaminants in the environment. Introduction Principles of NMRThe basis of NMR spectroscopy is that only certain atomic nuclei have a nuclear spin I, and thus a magnetic moment μ, which interacts with a static magnetic field. These nuclei include 1 H, 13 C, 15 N, and 31 P (Table 1 1 ). In an external magnetic field B 0 , which is represented as a vector in the z direction, such nuclei split into 2I + 1 energy levels, with the splitting proportional to the field strength B 0 and γ , a constant for each nucleus. Nuclei with I = 1 / 2 , such as 1 H and 13 C, therefore, split into two populations. The very slight surplus of nuclei in the lower energy level, as determined by the Boltzmann equation, results at equilibrium in a macroscopic sample magnetization M 0 (Figure 1a). In fact, each nuclear spin is not statically aligned, but precessing around B 0 at the Larmor frequency ω, which corresponds to the energy gap ΔE:Rather than the laboratory (static) frame of reference, Figure 1 uses a reference frame rotating at the Larmor frequency, which simplifies the picture. In NMR, transitions between the energy levels are induced by a very bri...

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