HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (12) Citing Articles via Google Scholar Google Scholar Articles by Smernik, R. J. Articles by Oades, J. M. Search for Related Content PubMed PubMed Citation Articles by Smernik, R. J. Articles by Oades, J. M. GeoRef GeoRef Citation Agricola Articles by Smernik, R. J. Articles by Oades, J. M. Related Collections Humic Substances Soil Analysis Soil Chemistry

Article
SYMPOSIUM PAPERS

Paramagnetic Effects on Solid State Carbon-13 Nuclear Magnetic Resonance Spectra of Soil Organic Matter

Department of Soil and Water, Waite Agricultural Research Institute, The University of Adelaide, Glen Osmond SA 5064, Australia

* Corresponding author (ronald.smernik{at}adelaide.edu.au)

Received for publication June 2, 2000.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The effects of paramagnetic species on solid state ¹³C nuclear magnetic resonance (NMR) spectra were quantified in a series of doping experiments. The degree of signal loss caused by paramagnetic metals was shown to depend not only on the quantity, but also on the nature of the paramagnetic species, as well as the intimacy of contact with the organic substrate and the type of NMR experiment. Two mechanisms of signal loss were distinguished—signal loss via loss of magnetic field homogeneity, which affects all ¹³C nuclei in a sample, and signal loss via interaction between electronic and nuclear spins, the effects of which were localized to the close environment of the paramagnetic species. Loss of field homogeneity is important for manganese species, but not for copper species, and is equally important for both cross polarization and Bloch decay experiments. The interaction between electronic and nuclear spins is highly dependent on the spin-lattice relaxation rate constant of the free electron (T₁e), as cations with very short T₁e values (e.g., Pr³⁺) cause less signal loss than cations with longer T₁e values (e.g., Cu²⁺, Mn²⁺). Cross polarization spectra are shown to be more susceptible than Bloch decay spectra to this mechanism of signal loss. Signal loss and increased relaxation rates brought about by paramagnetic species can be used to provide information on soil organic matter (SOM) heterogeneity in the submicron range. This is demonstrated for SOM doped with paramagnetic cations where selective signal loss and increased relaxation rates are used to determine the nature of cation exchange sites.

Abbreviations: CP, cross polarization • CPMAS, cross polarization with magic angle spinning • NMR, nuclear magnetic resonance • PSRE, proton spin relaxation editing • SOM, soil organic matter

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
THE PRESENCE of paramagnetic species is a major impediment to the use of solid state ¹³C NMR spectroscopy for the characterization of soil organic matter in whole soils and some soil fractions. For some soils, no NMR signal at all can be obtained. Even when a spectrum can be obtained, the question remains as to whether the relative intensities of the various resonances accurately reflect the distribution of functional groups present in the sample, given that some functional groups may be more affected than others by the presence of paramagnetic impurities.

Most studies on the effects of paramagnetic species on solid state ¹³C NMR spectroscopy of soil organic matter have concentrated on methods for improving the quality of spectra. Early attempts to reduce the influence of iron(III) minerals via reduction with dithionite met with mixed success (Oades et al., 1987; Arshad et al., 1988). The use of HF, which removes the majority of the mineral fraction, has proven more reliable (Preston et al., 1989; Skjemstad et al., 1994; Schmidt et al., 1997). However, in soils such as the mineral horizons of Spodosols, in which much of the organic matter is present in the form of organo–mineral complexes, HF treatment can result in the loss of carbon as water-soluble material once the clay minerals have been removed (Dai and Johnson, 1999).

In this paper, we present an overview of our recent work on the effects of paramagnetic species on solid state ¹³C NMR spectroscopy of soil organic matter (Smernik and Oades, 1999, 2000a–c; Smernik et al., 2000). We explore the various mechanisms through which paramagnetic materials affect solid state ¹³C NMR spectra, in particular with respect to the effects on quantitation. We also describe a number of experiments in which paramagnetic effects are actually turned to advantage, providing additional information on the structure of soil organic matter.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Samples were prepared as described in Smernik and Oades (1999)(2000a,c) and Smernik et al. (2000).

Solid state ¹³C NMR spectra were obtained at a ¹³C frequency of 50.3 MHz on a Varian (Palo Alto, CA) Unity 200 spectrometer. Samples were packed in a 7-mm-diameter cylindrical zirconia rotor with Kel-F end-caps and spun at 5000 ± 100 Hz in a Doty Scientific (Columbia, SC) MAS probe. Free induction decays were acquired with a sweep width of 40 kHz; 1216 data points were collected over an acquisition time of 15 ms. All spectra were zero-filled to 8192 data points and processed with a 50-Hz Lorentzian line broadening and a 0.005-s Gaussian broadening. Chemical shifts were externally referenced to the methyl resonance of hexamethylbenzene at 17.36 ppm.

The ¹³C cross polarization with magic angle spinning (CPMAS) and Bloch decay NMR spectra were acquired using a standard pulse sequence (Wilson, 1987). A 1-ms contact time was used for CPMAS spectra. Spin counting calculations were performed using the method of Smernik and Oades (2000a)(b). Glycine (analytical reagent grade; Ajax Chemicals, Liverpool, NSW, Australia) was used as an external intensity standard. The T₁H and T₁H values were determined as described in Smernik and Oades (2000a). Proton spin relaxation editing (PSRE) was performed as described in Smernik et al. (2000).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Quantifying Paramagnetic Signal Loss and Increased Relaxation Rates
Paramagnetic materials have been shown to influence solid state ¹³C NMR spectra by causing (i) signal loss and broadening (Arshad et al., 1988; Munson and Haw, 1990; Oades et al., 1987; Preston et al., 1989; Randall et al., 1995; Schmidt et al., 1997; Skjemstad et al., 1994; Snape et al., 1989; Vassallo et al., 1987), (ii) increased relaxation rates (Brown, 1982; Ganapathy et al., 1981; Preston et al., 1984; Sullivan and Maciel, 1982), and/or (iii) changes in the chemical shift of resonances (Brough et al., 1993; Chacko et al., 1983; Ganapathy et al., 1986). Some of these paramagnetic effects are interrelated. For example, increased relaxation rates can bring about signal loss and signal broadening, although not all signal loss and broadening is due to increased relaxation rates. By doping organic samples with paramagnetic species, we have been able to quantify signal loss and increases in relaxation rates (Smernik and Oades, 1999; Smernik and Oades, 2000a, c). Through these studies we have shown that the type and size of the paramagnetic effect depends on a number of factors including (i) the amount of paramagnetic material, (ii) the type of paramagnetic material, (iii) the distribution of the paramagnetic material and the intimacy of contact between paramagnetic species and the organic material, and (iv) the type of NMR experiment. Signal broadening was also observed in some of the experiments, but the degree of broadening was not quantified. The final paramagnetic effect listed, changes in chemical shift, has been observed widely in solution NMR and is the basis for so-called lanthanide shift reagents. Lanthanide shifts have been observed in solid state ¹³C NMR spectra (Brough et al., 1993; Chacko et al., 1983; Ganapathy et al., 1986), but not for soil organic matter samples, and will not be discussed further here.

Signal Loss
There are two mechanisms via which paramagnetic materials cause signal loss in solid state ¹³C NMR experiments: (i) field inhomogeneity brought about by the bulk magnetic properties of the material (Aime et al., 1996; Alemany et al., 1984; Brough et al., 1993; Dereppe and Moreaux, 1987; Preston et al., 1989) and (ii) interaction between the nuclear and electronic spins (Bovey, 1988; Wilson, 1987). Nuclear magnetic resonance spectroscopy requires a strong, stable, and homogeneous applied magnetic field. Naturally, this field will be affected by the magnetic properties of paramagnetic and ferromagnetic minerals that occur in soil samples. The result is loss of spectral information through signal loss (decreased sensitivity) as well as signal broadening (decreased resolution). Nuclear spins are influenced by the spin of unpaired electrons in paramagnetic species, just as they are influenced by the spins of neighboring nuclei. The difference is that the magnetic moment of the electron is much greater than that of any nuclei, on account of the much smaller size of the electron, and hence the effects are greater. Electron–nuclear coupling generally results in rapid relaxation of the nuclear spins, which can cause signal loss and signal broadening.

The potential for signal loss through loss of magnetic field homogeneity brought about by paramagnetic impurities is illustrated in Table 1. In these samples, the organic material (cellulose) and the paramagnetic salts were mixed dry. There was minimal change in T₁H and T₁H relaxation rates of the cellulose, precluding any signal loss through decreased relaxation rates.

(i) The degree of signal loss is dependent on the concentration of paramagnetic species, since there was a greater degree of signal loss with the higher concentration of manganese(II) chloride.

(ii) The degree of signal loss is dependent on the type of paramagnetic species, since the presence of 9.8% copper resulted in negligible signal loss while the presence of 8.1% manganese resulted in the loss of more than half of the NMR signal of the cellulose.

(iii) The degree of signal loss is independent of the type of NMR experiment, since cross polarization (CP) and Bloch decay (BD) observabilities were virtually identical in each case.

The potential for signal loss through interaction between the nuclear spins (¹H and ¹³C) and the electronic spins of paramagnetic impurities is illustrated in Table 2. Solutions of metal chloride salts (Zn²⁺, Cu²⁺, and Pr³⁺) were reacted with organic substrates (chitin and pectin) to produce samples in which the paramagnetic cations were in close contact with the organic material (Smernik and Oades, 2000c). As expected, no signal loss was observed for the samples cation-exchanged with nonparamagnetic Zn²⁺. The concentrations of Cu²⁺ (1.7% and 11.9%) in these samples would not be expected to cause significant signal loss through loss of field homogeneity, based on the results in Table 1 above. The same is also true for the Pr³⁺–amended samples (Smernik and Oades, 2000a). Thus, all signal loss in these samples can be attributed to interaction between the nuclear and electronic spins.

(i) The degree of signal loss is dependent on the concentration of paramagnetic species, since there was a greater degree of CP signal loss with the higher concentration of Cu²⁺ and Pr³⁺ cations in the pectin samples compared with the corresponding chitin samples.

(ii) The degree of signal loss is dependent on the type of paramagnetic species, since CP signal loss was much greater for the Cu²⁺–amended samples than for the Pr³⁺–amended samples, despite the similar concentrations of cations.

(iii) The degree of signal loss is dependent on the type of NMR experiment, with much greater signal loss observed for the cross polarization (CP) spectra than for the Bloch decay (BD) spectra. This difference is due to the involvement of ¹H nuclei in the CP experiment. In a CP experiment, ¹H nuclei are irradiated with a radiofrequency pulse. Polarization is then transferred from the ¹H to the ¹³C population during the contact time. The ¹H relaxation during this time (T₁H relaxation) reduces the intensity in the resulting ¹³C CP spectrum. Since ¹H is an isotopically abundant nucleus, rapid relaxation of a ¹H nucleus close to a paramagnetic center can induce rapid relaxation of neighboring ¹H nuclei through the process of spin diffusion, hence spreading the signal loss effect. The Bloch decay experiment does not involve polarization transfer from the ¹H population, and spin diffusion is not an important process for the isotopically rare (1%) ¹³C nucleus.

We also determined the degree of signal loss brought about by cation exchange of a de-ashed soil with a number of nonparamagnetic and paramagnetic cations (Smernik and Oades, 1999). Table 3 confirms that cation exchange with nonparamagnetic cations (Na⁺, Ca²⁺, and Zn²⁺) did not affect observability. For the other paramagnetic cations, the degree of signal loss was related to electronic spin-lattice relaxation rates (T₁e) (Bovey, 1988). Species with very short T₁e values such as Pr³⁺, Eu³⁺, and Co²⁺ are not efficient relaxation agents and hence caused less signal loss than species with relatively long T₁e values such as Cu²⁺, Fe³⁺, and Mn²⁺, which have a large effect on T₁H. Note that the presence of as little as 1.66% copper in the exchange sites of the de-ashed soil organic matter resulted in the loss of 50% of the NMR signal. This contrasts with the lack of any signal loss when 9.8% copper was present as a salt in a physical mixture with cellulose (Table 1). Also of note is the loss of 77% of the NMR signal when just 1.02% manganese is present in cation exchange sites.

1

View this table: [in this window] [in a new window]   Table 4. The T1H and T1H relaxation rates of chitin and pectin 13C nuclear magnetic resonance (NMR) signal in samples doped with various cations (adapted from Smernik and Oades, 2000c).

(i) Cation amendment does not always result in increased relaxation rates (decreased T₁H and T₁H values). Amendment with nonparamagnetic Zn²⁺ resulted in increased T₁H and T₁H values for both chitin and pectin, which can be attributed to decreases in molecular mobility brought about by the chelating effects of the divalent cation. Divalent Cu²⁺ should also cause similar chelation effects and trivalent Pr³⁺ may cause even stronger chelation effects. Thus, the strength of any paramagnetic effect (to decrease T₁H and T₁H values) should be gauged against the Zn²⁺–amended samples, rather than against the unamended samples.

(ii) Paramagnetic decreases in T₁H and T₁H values are dependent on the concentration of paramagnetic cation, since much larger decreases were observed for the Cu²⁺–amended pectin (8.3% Cu) than for the Cu²⁺–amended chitin (1.7% Cu).

(iii) Pr³⁺ is much less efficient than Cu²⁺ at decreasing T₁H and T₁H values. This is because Cu²⁺ and Pr³⁺ have very different electronic spin-lattice relaxation rates (T₁e). Species with very short T₁e values such as Pr³⁺ are relatively inefficient at decreasing nuclear relaxation rates compared with species with relatively long T₁e values such as Cu²⁺.

(iv) T₁H values are more sensitive to paramagnetic species than are T₁H values. For example, amendment of pectin with Pr³⁺ resulted in a decrease in T₁H from 264 to 92 ms, whereas T₁H increased from 2.63 to 3.48 ms, indicating that the paramagnetic effect was not large enough to overcome the chelation effect.

Turning Paramagnetic Effects to Advantage
In this section we describe how we can turn signal loss and increased relaxation rates caused by paramagnetic species to advantage in the study of soil organic matter (SOM). This is based on the premise that some paramagnetic effects are localized and that SOM is heterogeneous. As we saw above, signal loss is brought about by two mechanisms. One of these mechanisms, loss of field homogeneity, affected ¹³C nuclei remote from the paramagnetic species and hence will not produce selective effects in a heterogeneous material. However, the other mechanism of signal loss was shown to operate only when there is intimate contact between the paramagnetic species and the organic substrate. Paramagnetic effects on relaxation rates were also shown to require such intimate contact. Thus in a heterogeneous sample, components with a higher concentration of paramagnetic centers will tend to have lower observabilities and more rapid relaxation rates. Selective increases in relaxation rates brought about by a nonuniform distribution of paramagnetic cations have been reported for a model sewage sludge (Pfeffer et al., 1984) and a soil humin (Preston and Newman, 1992).

Selective signal loss was observed when samples of SOM were doped with paramagnetic cations via cation exchange (Smernik and Oades, 1999). Figure 1 shows that cation amendment with nonparamagnetic Zn²⁺ did not significantly alter the distribution of ¹³C NMR resonances, whereas amendment with paramagnetic cations (Pr³⁺, Cu², and Mn²⁺) did produce observable changes. By subtracting spectra for the cation-amended samples from the spectrum for the unamended sample (Fig. 1 difference spectra), we can observe the portion of signal that is "lost" through paramagnetic effects. Signal loss in the Pr³⁺–amended sample is restricted to the immediate environment of the cation exchange sites, since Pr³⁺ has a very short T₁e value. The difference spectrum for the Pr³⁺–amended sample, which represents the 14% of signal lost on cation amendment (Table 3), contains strong carbonyl and O-alkyl peaks, suggesting that a significant number of cation exchange sites in this material are uronic acid structures. The CP spectra for the Cu²⁺– and Mn²⁺–amended samples are more affected by signal loss. These cations have relatively long T₁e values, and hence cause signal loss to more remote ¹³C nuclei by affecting T₁H relaxation rates. The distance over which T₁H relaxation rates are influenced by a paramagnetic center is determined by the extent of ¹H spin diffusion and is probably <3 nm in this case (Newman and Condron, 1995; Zumbulyadis, 1983). The remaining CP signal for the Cu²⁺– and Mn²⁺–amended samples (Fig. 1) represents structures least affected, and hence farthest from, cation exchange sites. These spectra contain proportionately more alkyl signal than the corresponding spectrum for the unamended sample.

View larger version (20K): [in this window] [in a new window]   Fig. 1. Carbon-13 cross polarization with magic angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectra of unamended and cation-amended de-ashed soil organic matter (SOM). Difference spectra were generated by subtracting the cation-amended spectra from the unamended spectrum (adapted from Smernik and Oades, 1999).

View larger version (12K): [in this window] [in a new window]   Fig. 2. Inversion-recovery spectra of a typical ultraviolet (UV) photo-oxidized soil.

View larger version (17K): [in this window] [in a new window]   Fig. 3. Generation of proton spin relaxation editing (PSRE) subspectra from the linear combination of the fully relaxed and a partially relaxed inversion-recovery spectrum.

View larger version (18K): [in this window] [in a new window]   Fig. 4. Proton spin relaxation editing (PSRE) subspectra for Cu2+–amended de-ashed soil organic matter (SOM) (Smernik and Oades, 1999).

Figure 5 shows that the short recycle delay Bloch decay spectrum for the unamended sample is very different from the CP spectrum of the same sample (Fig. 1). This difference is due to differences in T₁C values for the various resonances. In this sample, the alkyl resonance is less affected by saturation than are other resonances, indicating that (at least some) of the alkyl carbon is characterized by a shorter T₁C value. Amendment with nonparamagnetic Zn²⁺ did not cause any significant change to the spectrum. Amendment with Pr³⁺ caused some signal loss in the carbonyl region. This lost signal can be ascribed to the carboxylate cation binding sites. It should be noted that the degree of signal loss is less than for the CP spectrum of the sample (Fig. 1), probably because for the CP spectrum, spin diffusion through the ¹H population increases the sphere of influence. Amendment with Cu²⁺ had quite a different effect (Fig. 5). Signal intensity increased in most spectral regions other than the alkyl region. This increase in signal intensity can be ascribed to decreases in T₁C that result in less signal saturation. The most intense resonances in the difference spectrum are those due carbonyl and O-alkyl carbons, confirming the importance of uronic acids as binding sites. Amendment with Mn²⁺ produced a broad spectrum with increased intensity in the carbonyl, aromatic, and O-alkyl regions, but a decrease in the intensity of the alkyl region. Increases in signal intensity can be ascribed to decreases in T₁C, as observed for the Cu²⁺–amended sample. Signal loss is most likely due to loss of field homogeneity, as seen for the manganese salt doped cellulose samples (see above).

View larger version (22K): [in this window] [in a new window]   Fig. 5. Short recycle delay 13C Bloch decay nuclear magnetic resonance (NMR) spectra of unamended and cation-amended de-ashed soil organic matter (SOM). Difference spectra were generated by subtracting the unamended spectrum from the cation-amended spectra (adapted from Smernik and Oades, 1999).

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

This study confirms that iron, being the most abundant paramagnetic metal in most soils, is the one most likely to be a problem for NMR spectroscopy. However, this study also shows that manganese can have large detrimental effects, even at low concentrations, and that copper, due to its affinity for organic matter, can also cause serious problems in CP spectra at low concentrations. The form of the paramagnetic species is also important; discrete particles of iron minerals will undoubtedly cause signal loss, but this signal loss will be nonselective and hence will not affect the relative sizes of NMR resonances. If, however, the iron is present in organo–mineral complexes, or occupies cation exchange sites, the NMR signal of carbons in close contact will be selectively affected.

Although the effects of paramagnetic species are often detrimental to the study of SOM using solid state ¹³C NMR, they can actually be turned to advantage. Selective paramagnetically induced signal loss and increased relaxation rates can be used to probe aspects of the structure of SOM, especially those related to heterogeneity on the submicron scale.

	ACKNOWLEDGMENTS

 
This work was funded by an Australian Research Council (ARC) grant.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

• Aime, S., I. Bertini, and C. Luchinat. 1996. Considerations on high resolution solid state NMR in paramagnetic molecules. Coord. Chem. Rev. 150:221–242.
• Alemany, L.B., D.M. Grant, R.J. Pugmire, and L.M. Stock. 1984. Solid state magnetic resonance of Illinois No. 6 coal and some reductive alkylation products. Fuel 63:513–521.
• Arshad, M.A., J.A. Ripmeester, and M. Schnitzer. 1988. Attempts to improve solid state ¹³C NMR spectra of whole mineral soils. Can. J. Soil Sci. 68:593–602.
• Bovey, F.A. 1988. Nuclear magnetic resonance spectroscopy. 2nd ed. Academic Press, San Diego, CA.
• Brough, A.R., C.P. Grey, and C.M. Dobson. 1993. Paramagnetic ions as structural probes in solid-state NMR: Distance measurements in crystalline lanthanide acetates. J. Am. Chem. Soc. 115:7318–7327.
• Brown, C.E. 1982. Effects of chemical modification and CoCl₂ addition on the ¹³C NMR spectra of carnosine as a solid powder. J. Am. Chem. Soc. 104:5608–5610.
• Chacko, V.P., S. Ganapathy, and R.G. Bryant. 1983. ¹³C CP-MAS NMR spectra of paramagnetic solids. J. Am. Chem. Soc. 105:5491– 5492.
• Dai, K.H., and C.E. Johnson. 1999. Applicability of solid-state ¹³C CP/MAS NMR analysis in Spodosols: Chemical removal of magnetic materials. Geoderma 93:289–310.
• Dereppe, J.-M., and C. Moreaux. 1987. A limitation of ¹³C CP-MAS NMR spectroscopy for the study of treated coals. Fuel 66:1008– 1009.
• Ganapathy, S., V.P. Chacko, R.G. Bryant, and M.C. Etter. 1986. Carbon CP-MASS NMR and X-ray crystal structure of paramagnetic lanthanide acetates. J. Am. Chem. Soc. 108:3159–3164.
• Ganapathy, S., A. Naito, and C.A. McDowell. 1981. Paramagnetic doping as an aid in obtaining high-resolution ¹³C NMR spectra of biomolecules in the solid state. J. Am. Chem. Soc. 103:6011–6015.
• Munson, E.J., and J.F. Haw. 1990. Effects of paramagnetic lanthanides on the study of carbonaceous deposits on zeolite catalysts by carbon-13 solid-state nuclear magnetic resonance spectroscopy. Anal. Chem. 62:2532–2536.
• Newman, R.H., and L.M. Condron. 1995. Separating subspectra from cross-polarisation magic-angle spinning NMR spectra by proton spin relaxation editing. Solid State NMR 140:1–8.
• Oades, J.M., A.M. Vassallo, A.G. Waters, and M.A. Wilson. 1987. Characterisation of organic matter in particle size and density fractions from a red-brown earth by solid-state ¹³C NMR. Aust. J. Soil Res. 25:71–82.
• Pfeffer, P.E., W.V. Gerasimowicz, and E.G. Piotrowski. 1984. Effect of paramagnetic iron on quantitation in carbon-13 cross polarisation magic angle spinning NMR spectroscopy of heterogeneous environmental matrices. Anal. Chem. 56:734–741.
• Preston, C.M., R.L. Dudley, C.A. Fyfe, and S.P. Mathur. 1984. Effects of variations in contact times and copper concentrations in a ¹³C CP/MAS NMR study of samples of four organic soils. Geoderma 33:245–253.
• Preston, C.M., and R.H. Newman. 1992. Demonstration of spatial heterogeneity in the organic matter of de-ashed humin samples by solid-state ¹³C CPMAS NMR. Can. J. Soil Sci. 72:13–19.
• Preston, C.M., M. Schnitzer, and J.A. Ripmeester. 1989. A spectroscopic and chemical investigation on the de-ashing of a humin. Soil Sci. Soc. Am. J. 53:1442–1447.[Abstract/Free Full Text]
• Randall, E.W., N. Mahieu, D.S. Powlson, and B.T. Christensen. 1995. Fertilization effects on organic matter in physically fractionated soils as studied by ¹³C NMR: Results from two long-term field experiments. Eur. J. Soil Sci. 46:557–565.
• Schmidt, M.W.I., H. Knicker, P.G. Hatcher, and I. Kogel-Knabner. 1997. Improvement of ¹³C and ¹⁵N CPMAS NMR spectra of bulk soils, particle size fractions and organic material by treatment with 10% hydrofluoric acid. Eur. J. Soil Sci. 48:319–328.
• Skjemstad, J.O., P. Clarke, J.A. Taylor, J.M. Oades, and R.H. Newman. 1994. The removal of magnetic materials from surface soils. A solid state ¹³C CP/MAS NMR study. Aust. J. Soil Res. 32:1215–1229.
• Smernik, R.J., and J.M. Oades. 1999. Effects of added paramagnetic ions on the ¹³C NMR CP/MAS NMR spectrum of a de-ashed soil. Geoderma 89:219–248.
• Smernik, R.J., and J.M. Oades. 2000a. The use of spin counting for determining quantitation in solid state ¹³C NMR spectra of natural organic matter. 1. Model systems and the effects of paramagnetic impurities. Geoderma 96:101–129.
• Smernik, R.J., and J.M. Oades. 2000b. The use of spin counting for determining quantitation in solid state ¹³C NMR spectra of natural organic matter. 2. HF treated soil fractions. Geoderma 96:159–171.
• Smernik, R.J., and J.M. Oades. 2000c. Effects of paramagnetic cations on solid state ¹³C nuclear magnetic resonance (NMR) spectra of natural organic materials. Commun. Soil Sci. Plant Anal. 31:3011– 3026.
• Smernik, R.J., J.O. Skjemstad, and J.M. Oades. 2000. Virtual fractionation of charcoal from soil organic matter using solid state ¹³C NMR spectral editing. Aust. J. Soil Res. 38:665–684.
• Snape, C.E., D.E. Axelson, R.E. Botto, J.J. Delpuech, P. Tekely, B.C. Gerstein, M. Pruski, G.E. Maciel, and M.A. Wilson. 1989. Quantitative reliability of aromaticity and related measurements on coals by ¹³C NMR: A debate. Fuel. 68:547–560.
• Sullivan, M.J., and G.E. Maciel. 1982. Spin dynamics in the carbon-13 nuclear magnetic resonance spectrometric analysis of coal by cross polarization and magic-angle spinning. Anal. Chem. 54:1615– 1623.
• Vassallo, A.M., M.A. Wilson, P.J. Collin, J.M. Oades, A.G. Waters, and R.L. Malcolm. 1987. Structural analysis of geochemical samples by solid-state nuclear magnetic resonance spectrometry. Role of paramagnetic material. Anal. Chem. 59:558–562.
• Wilson, M.A. 1987. NMR techniques and applications in geochemistry and soil chemistry. Pergamon Press, Oxford.
• Zumbulyadis, N. 1983. Selective carbon excitation and the detection of spatial heterogeneity in cross-polarisation magic-angle-spinning NMR. J. Magn. Res. 53:486–494.

This article has been cited by other articles:

				S. Hunger, J. T. Sims, and D. L. Sparks How Accurate Is the Assessment of Phosphorus Pools in Poultry Litter by Sequential Extraction? J. Environ. Qual., January 1, 2005; 34(1): 382 - 389. [Abstract] [Full Text] [PDF]

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (12) Citing Articles via Google Scholar Google Scholar Articles by Smernik, R. J. Articles by Oades, J. M. Search for Related Content PubMed PubMed Citation Articles by Smernik, R. J. Articles by Oades, J. M. GeoRef GeoRef Citation Agricola Articles by Smernik, R. J. Articles by Oades, J. M. Related Collections Humic Substances Soil Analysis Soil Chemistry