Environmental Engineering Reference
In-Depth Information
NMR spectra of solids are more affected by dipole-
dipole interactions as a result of the near-neighbor
magnetic dipoles than liquids. In
13
C NMR spectros-
copy of amorphous solid samples, static dipolar
interactions between
13
C and
1
H result in a large
amount of dipolar splitting (Skoog
et al.
1998). The
CSA and dipolar coupling are greatly or completely
reduced in solution by rapid, random molecular tum-
bling (Brownian motion), because in this case what
is observed is an average chemical shift (isotropic
chemical shift), which is given by the average of all
the possible orientations of the molecule in the
applied fi eld (McGregor 1997). In the solid state,
molecular movement is restricted to small oscilla-
tions around fi xed positions in the solid matrix and
the sample presents both CSA and dipolar coupling
phenomena, which are responsible for the broad
lines in the NMR spectrum.
The broadening of the line-shape observed in
NMR spectra of solid samples has posed challenges
for those wishing to study solid samples, such as
natural organic matter. Fortunately, broadening of
the NMR line-shape due to CSA, as well as, at some
level, due to dipole-dipole interactions can be allevi-
ated to a large extent by an experimental technique,
known as MAS (magic-angle-spinning). Both CSA
and dipolar coupling effects have a (1-3cos
2
polarization, which can produce high-resolution
13
C
spectra from solids (McGregor 1997; Skoog
et al.
1998).
3.4.4 Nuclear magnetic resonance applied
to sediments
The most used NMR method for the characterization
of sediments is solid-state NMR, by cross polariza-
tion-magic-angle spinning (CP-MAS)
13
C NMR
(Dickens
et al.
2006).
NMR analysis of the natural organic matter in
soils or sediment samples is highly complex owing
to the intricate, heterogeneous nature of natural par-
ticles, but especially because of the naturally occur-
ring paramagnetic centers, such as metal ions
(especially iron) and organic stable radicals within
the soil matrix. These paramagnetic centers are not
evenly distributed throughout the sample and can
lead to signal broadening
13
C NMR resonances,
decreasing the signal-to-noise ratio and causing para-
magnetic resonance shifts (Gélinas
et al.
2001). Until
now, the problem of signal loss due to stable organic
radicals remains poorly understood and does not
have a simple solution; in general, it is not addressed
(Cook 2004). On the other hand, for solid samples
such as soils, humin, or sediments, the removal of
the inorganic paramagnetic centers can be done by
repeated treatment with dilute HF through three or
four cycles (Cook 2004; Hedges & Oades 1997). For
liquid samples, such as humic or fulvic acids, the
metal ions can be removed using cation exchange
resin (Cook 2004).
Gélinas
et al.
(2001) studied marine sediments
using CP-MAS
13
C NMR analysis. Before the NMR
analysis, marine sediment samples were demineral-
ized through acid treatment developed by the authors,
which used HCl to dissolve carbonates and a mixture
of diluted HCl/HF to dissolve silicates. This deminer-
alization method removed minerals containing para-
magnetic elements that otherwise could interfere
with NMR analysis and allowed the authors to study
recently deposited marine sediments with low organic
carbon concentration, containing labile organic
matter, with minimal alteration of organic
structures.
Paramagnetic metal removal from marine sedi-
ments is not always necessary. Hedges & Oades
(1997) performed CP/MAS
13
C NMR analysis of
) term
in their mathematical description, which is zero if the
sample is rotated (about an axis making an angle
θ
θ
with
B
o
) at a high enough speed to force the magnetic
nuclei in the sample to experience the magic angle of
54.7°. Usually the very broad lines in the spectrum
are sharpened signifi cantly using MAS (McGregor
1997; Cook 2004; Sposito 2004).
Dipolar splitting in a
13
C spectrum can be removed
by irradiating the sample with a second radio fre-
quency corresponding to the peak of proton frequen-
cies recorded when the spectrum was being obtained.
This procedure, called dipolar decoupling uses a
series of pulses to average the dipolar interactions by
reorienting the spins. Dipolar coupling is used to
increase the sensitivity of less-sensitive nuclei using a
technique known as cross-polarization (CP), a com-
plicated pulsed technique that causes the resonance
frequencies of the
1
H and
13
C nuclei to become iden-
tical, and then promotes interactions between the
magnetic fi elds of the two nuclei. Currently, there are
instruments commercially available that incorporate
dipolar decoupling, magic angle spinning and cross
