4.4 Solid-state C-13 NMR

The stable isotope of carbon 13C has the interesting property that it has “nuclear spin”, a property which is analogous to the spin of electrons. Carbon-12 (12C), the most common isotope of carbon, lacks nuclear spin. Because of its nuclear spin, 13C atoms behave in a similar fashion to a tiny bar magnet whereas 12C atoms do not. In the absence of a magnetic field, the 13C atoms are randomly oriented, as are the 12C atoms. But when a magnetic field is applied, the 13C atoms line up parallel to that field, either spin-aligned or spin-opposed, while C-12 atoms are unaffected by the magnetic field.

If a sample of carbon is subjected to a magnetic field of increasing strength, at some point all the 13C atoms will align themselves to that field, a point known as resonance. However, because carbon atoms form chemical bonds with hydrogen, oxygen, nitrogen and other carbon atoms, the nature of those bonds determines when resonance is achieved for individual carbon atoms. Thus, by placing a sample of mixed carbon compounds in a magnetic field and recording the energy levels at which 13C atoms attain resonance, it is possible to deduce some of the chemical properties of the carbon compounds present. That analytical technique is known as 13-C nuclear magnetic resonance or 13 Carbon-nuclear magnetic resonance. This is one of the analytical techniques that have been used to investigate the chemical properties of DOC in groundwater (Aiken et al., 1985; Grőn et al., 1996; Murphy et al., 1989; Aravena et al., 2004). While the underlying principle of NMR, with 13C atoms acting as tiny magnets lining up with a magnetic field is easy to grasp, the physics behind exploiting those properties is quite complex. Furthermore, because non-magnetic 12C accounts for 98.9 percent of all carbon atoms, NMR applies only to a small minority of the carbon present. That, in turn lowers the sensitivity of the method.

The NMR technique that has seen the most use for studying DOC in groundwater (Aiken et al., 1985; Grőn et al., 1996; Aravena et al., 2004) is known as solid-state cross polarization/magic angle spinning (CP/MAS). The first step in this procedure is to collect solid samples of carbon (hence the term “solid state”). This can be done by precipitating the humic and/or fulvic acid fractions of DOC at low pH with subsequent drying. It might take as much as a hundred liters of groundwater to provide enough humic or fulvic acids for solid-state NMR analysis. Next, these samples are packed tightly into rotors which are then spun at rates from 1 to 35 kHz in a spectrometer. Magic-angle spinning introduces artificial motion by placing the axis of the sample rotor at the magic angle (54.74o) with respect to an external magnetic field. This technique provides a way to obtain quantitative solid-state 13C-NMR spectra of organic materials with an acceptable signal-to-noise ratio. As the magnetic field strength increases, the number of 13C atoms reaching resonance is recorded on the y-axis as a spectrum. Because different 13C functional groups achieve resonances at different field strengths, it is possible to estimate the relative abundance of those functional groups by quantifying the area under the spectral curve.

An example of how 13C-NMR has been used to characterize DOC in groundwater was provided by Aravena and others (2004). These researchers were studying the Alliston aquifer, a regional confined aquifer in southern Ontario, Canada. The aquifer is of glacial origin and is composed of sand and gravel lenses confined above by thick clay tills and below by Paleozoic bedrock. The groundwater chemistry of the Alliston aquifer is characterized by relatively high concentrations of DOC (~10 mg/L) and high concentrations of dissolved methane (~20 mg/L). These high DOC concentrations made it possible to extract sufficient solid-phase fulvic acids from approximately 100 liters of groundwater for solid-state 13C-NMR analysis.

Some of the results reported by Aravena and others (2004) are shown in Figure 14. These results indicate the relative abundance of carboxyl (COOH) groups, aromatic (carbon ring structures, and aliphatic carbon (carbon chains) present in the DOC. Note that the horizontal scale has a zero value. That zero point is where resonance is achieved by the 13-C atoms in tetramethylsilane (TMS) which serves as a standard to which the resonance characteristics of other compounds can be compared. In this study, the results indicate that the fulvic acid fraction of DOC is dominated by aliphatic carbon (~65 percent) with lesser amounts of aromatic carbon (~25 percent) and carboxyl carbon (~10 percent).

The relative proportion of aliphatic, aromatic, and carboxyl carbon in the fulvic acid fraction of DOC (Figure 14) seems to vary between different hydrologic systems. In a study of three different aquifers of marginal marine origin in Denmark (Grőn et al., 1996), aromatic carbon predominated (51 percent) in DOC from one aquifer (the Fjand aquifer). In contrast, DOC from two other aquifers (the Skagen and Tuse aquifers) was composed of predominantly aliphatic carbon (51 percent). These observed differences probably reflect the source materials (marine versus terrestrial) for the DOC as well as their diagenetic history.

Image showing a 13C-NMR spectrum

Figure 14  An example of a 13C-NMR spectrum showing the approximate relative abundance of carboxyl, aromatic, and aliphatic carbon present in the fulvic acid fraction of DOC. Data are from Aravena and others (2004).


Dissolved Organic Carbon in Groundwater Systems Copyright © 2022 by Francis H. Chapelle. All Rights Reserved.