Introduction
Solid-state 13C NMR characterization of soil and sediment samples is difficult for several reasons. Molecules in the solid state are “frozen” in a variety of different physical conformations. Since NMR gives information on the chemical environment of an atom, slightly different ordering of the molecules throughout a sample, even if it contains only one molecule, will result in a broadening of the resonances in an NMR spectrum that makes it difficult to interpret the spectrum. This broadening is called the chemical shift anisotropy and it is a fundamental limit in solid-state NMR. It is overcome using what is known as cross-polarization magic angle spinning or CPMAS. A CPMAS experiment consists of a series of radio-frequency pulses and delays. The radio-frequency pulses that impart energy to the atoms in a sample, “exciting” them while the delays (T1 relax time) allow the excited atoms to “relax’ to their original state before the next acquisition sequence begins. A NMR spectrum results from the measurement of energy that is given off by the excited atoms during their relaxation. Thus, determining the length of this delay is critical to obtaining spectra that provide quantitative determinations of the different types of carbon present in a sample when the spectrum is integrated.
Because natural organic matter (NOM) is an extremely complex mixture of organic substances, samples typically contain carbon atoms of many different structural types. To compound this difficulty even further, the different carbon types in NOM have different relaxation times in the NMR experiment; aliphatic carbon relaxes very rapidly, anomeric carbon atoms relax rapidly, aromatic carbon atoms relax slowly, and carboxyl and ester carbons relax very slowly. Thus, if the relaxation time in a CPMAS experiment is too short the spectrum will appear to be more aliphatic than it should be because slower relaxing carbon atoms are not allowed the opportunity to return to their original state before the next pulse sequence begins and thus their contribution to the overall spectrum is underestimated.
In previous studies, researchers have used a variety of T1 times ranging from milliseconds (1-4), 1-2 seconds (5-14), about 6 seconds (15&16), and some even longer around 10-15 seconds. A short relax time is in milliseconds, intermediate relax time is aprox.1-2 seconds, long relax time is aprox.5-6, and a quantitative relax time is aprox.10-12.5.The longer delay chosen, the closer the spectra get to the actual chemical makeup, as shown by Mao (10). Thus in order to obtain a spectra representable of the sample, the T1 time needs to be adequately long to allow for complete relaxation of carbon atoms.
Methods & Materials
The focus of our research was four polymers [Poly (benzyl Methacrylate), Poly (butyl Methacrylate), Ethyl Cellulose, and Cellulose Acetate Butyrate] and four soil samples [Lenardite, Guanella peat, extracted peat, and S.W. M.G.] . The polymers chosen, each displayed a specific functional region of interest. The delay times used were based on the literature and Cross Polarization (CP) Magic-angle-spinning (MAS) T1 Total Suppression of Spinning Sidebands (TOSS). The chosen delay times were as follows 500 milliseconds, 1 second, 5 seconds, 2/3X, and X. X being equal to the T1 as determined by the CP MAS T1 TOSS. X varied for each sample.
The spectra in our study were acquired through DPMAS combined with T1 corrections obtained from CP/T1-TOSS. Both 13C DPMAS and 13C CP MAS spectra were obtained on a 100 MHz (2.1 T) wide-bore solid-state NMR spectrometer with a Doty Scientific probe, using a MAS spin rate of approximately 3 KHz with a 13C resonance frequency of 25.19 Hz in a 9 mm zirconium dioxide rotor with kel-F caps, the pulse sequence is described by Moa (10). The scans for the spectra ranged from 2500-50000, with scan times ranging from 8 to 300 hrs.
The chemical shift scale was calibrated with adamantine at 37.85 ppm, and for integration our spectra were divided into four chemical shift ranges; 0-50 ppm (aliphatic), 51-110 ppm (anomeric), 111-160 ppm (aromatic), and 161-190 ppm (carboxyl).
Results & Discussion
It is clearly shown in Fig 1 that the shorter delay does not allow complete relaxation, therefore the spectra displays a higher concentration in the aliphatic region when it is clearly displayed that as the delay time is increased the concentration is partially shifted to the carboxyl and anomeric region. Furthermore leaving the initial delay of .5 seconds a very inaccurate display. The spectrum of Poly (butyl methacrylate) in figure 1 also shows how noticeable changes in spectra occur with slight adjustments in T1 delay. In turn this shows us that if a T1 Delay time is not chosen correctly Figure 1: we will be analyzing a misrepresented spectrum.
Poly (butyl methacrylate), 13C DPMAS 6000 scans
Table 1: Actual percentages of carbon groups of Poly (Butyl Methacrylate)
Our data consistently shows a direct correlation between the T1 delay time and semi- quantitative 13C spectra. As the T1 time increases the area distribution for certain carbon groups changes dramatically. It appears that if an insufficient T1 delay time is used you will not get a correct representation or distribution of the sample
Our results show that if an insufficient T1 delay time is used the spectra will be far from quantitative, therefore leaving the data unacceptable for scientific applications
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