| Cross polarization (CP) magic angle spinning (MAS) 13C-NMR spectroscopy is a solid state
NMR technique widely used to study chemical composition of organic materials
with low or no solubility in the common deuterated solvents used to run liquid
state NMR experiments. Based on the magnetization transfer from abundant nuclei
(with spin of 1 -2) having a high gyromagnetic ratio (γ), such as protons, to the
less abundant 13C nuclei with low γ values, 13C-CPMAS NMR spectroscopy is
often applied in environmental chemistry to obtain quantitative information on
the chemical composition of natural organic matter (NOM) (Conte et al., 2004),
although its quantitative assessment is still matter of heavy debates. Many authors
(Baldock et al., 1997; Conte et al., 1997, 2002; Dria et al., 2002; Kiem et al., 2000;
Kögel-Knabner, 2000; Preston, 2001), reported that the application of appropriate instrument
setup as well as the use of special pulse sequences and correct spectra elaboration
may provide signal intensities that are directly proportional to the amount of nuclei
creating a NMR signal. However, many other papers dealt with the quantitative
unsuitability of 13C-CPMAS NMR spectroscopy. Among those, Mao et al. (2000),
Smernik and Oades (2000 a,b), and Preston (2001) reported that cross-polarized NMR
techniques may fail in a complete excitation of the 13C nuclei. In fact, the amount of
observable carbons via 13C-CPMAS NMR spectroscopy appeared, in many cases, lower
than that measured by a direct observation of the 13C nuclei. As a consequence,
cross-polarized NMR techniques may provide spectra where signal distribution may not be
representative of the quantitative distribution of the different natural organic matter
components.
Cross-polarization is obtained after application of an initial 90Ë x pulse on protons and a
further spin lock pulse (along the y axis) having a fixed length (contact time) for both nuclei
(1H and 13C) once the Hartmann-Hahn condition is matched. The Hartmann-Hahn condition
can be expressed as γHB1H = γCB1C, where γH and γC are the gyromagnetic ratios of
protons and carbons, whereas B1H and B1C are the 1H and 13C radio-frequency (r.f.) fields
applied to the nuclei. The Hartmann-Hahn condition is affected by the H-C dipolar
interaction strength (Stejskal & Memory, 1994). All the factors affecting dipolar interactions
may mismatch the Hartmann-Hahn condition and prevent a quantitative representation
of the NOM chemical composition (Conte et al., 2004). It has been reported that
under low speed MAS conditions, broad matching profiles are centered around the
Hartmann-Hahn condition....... With increasing spinning speed the Hartmann-Hahn
matching profiles break down in a series of narrow matching bands separated by the
rotor frequency (Stejskal & Memory, 1994). In order to account for the instability
of the Hartmann-Hahn condition at higher rotor spin rates (>10 kHz), variable
amplitude cross-polarization techniques (RAMP-CP) have been developed (Metz et al.,
1996).
So far, to our knowledge, the prevailing way used to obtain quantitative 13C-CPMAS
NMR results was to optimize the 1H and 13C spin lock r.f. fields on simple standard systems
such as glycine and to use those r.f. field values to run experiments on unknown organic
samples.
The aim of the present study was to experimentally evidence that the stability of the
Hartmann-Hahn condition was different for different samples with a known structure.
Moreover, Hartmann-Hahn profiles of four different humic acids (HAs) were also
provided in order to show that the 1H/13C r.f. spin lock field strength must also be
tested on the HAs prior to a quantitative evaluation of their 13C-CPMAS NMR
spectra.
Baldock, J.A., Oades, J.M., Nelson, P.N., Skene, T.M., Golchin, A. & Clarke, P., 1997.
Assessing the extent of decomposition of natural organic materials using solid-state C-13
NMR spectroscopy. Australian Journal of Soil Research, 35, 1061-1083.
Conte, P., Piccolo, A., van Lagen, B., Buurman, P. & de Jager, P.A., 1997. Quantitative
Aspects of Solid-State 13C-NMR Spectra of Humic Substances from Soils of Volcanic
Systems. Geoderma, 80, 327-338.
Conte, P., Piccolo, A., van Lagen, B., Buurman, P. & Hemminga, M.A., 2002. Elemental
quantitation of natural organic matter by CPMAS C-13 NMR spectroscopy. Solid State
Nuclear Magnetic Resonance, 21, 158-170.
Conte, P., Spaccini, R. & Piccolo, A., 2004. State of the art of CPMAS C-13-NMR
spectroscopy applied to natural organic matter. Progress in Nuclear Magnetic Resonance
Spectroscopy, 44, 215-223.
Dria, K.J., Sachleben, J.R. & Hatcher, P.G., 2002. Solid-state carbon-13 nuclear magnetic
resonance of humic acids at high magnetic field strengths. Journal of Environmental Quality,
31, 393-401.
Kiem, R., Knicker, H., Korschens, M. & Kogel-Knabner, I., 2000. Refractory organic
carbon in C-depleted arable soils, as studied by C-13 NMR spectroscopy and carbohydrate
analysis. Organic Geochemistry, 31, 655-668.
Kögel-Knabner, I., 2000. Analytical approaches for characterizing soil organic matter.
Organic Geochemistry, 31, 609-625.
Mao, J.D., Hu, W.G., Schmidt-Rohr, K., Davies, G., Ghabbour, E.A. & Xing, B., 2000.
Quantitative characterization of humic substances by solid-state carbon-13 nuclear magnetic
resonance. Soil Science Society of America Journal, 64, 873-884.
Metz, G., Ziliox, M. & Smith, S.O., 1996. Towards quantitative CP-MAS NMR. Solid
State Nuclear Magnetic Resonance, 7, 155-160.
Preston, C.M., 2001. Carbon-13 solid-state NMR of soil organic matter - using the
technique effectively. Canadian Journal of Soil Science, 81, 255-270.
Smernik, R.J. & Oades, J.M., 2000a. The use of spin counting for determining
quantitation in solid state C-13 NMR spectra of natural organic matter 1. Model systems and
the effects of paramagnetic impurities. Geoderma, 96, 101-129.
Smernik, R.J. & Oades, J.M., 2000b. The use of spin counting for determining
quantitation in solid state C-13 NMR spectra of natural organic matter 2. HF-treated soil
fractions. Geoderma, 96, 159-171.
Stejskal, E.O. & Memory, J.D., 1994. High Resolution NMR in the Solid State.
Fundamentals of CP/MAS. Oxford University Press, New York. |