### Experimental Techniques  
As already mentioned, most SSNMR studies on OM samples are performed by exploiting the \({}^{13}\)C nucleus. \({}^{13}\)C has a natural abundance of 1.11% [39; 40], which results in a good balance of sensitivity and spectral \"simplicity\" since complex interactions among \({}^{13}\)C nuclei can be neglected. The most important nuclear property used for the identification of functional groups is chemical shielding. On the basis of this property, nuclei in different chemical environments give rise to signals at different resonance frequencies. Experimental techniques providing high-resolution \({}^{13}\)C SSNMR spectra, similar to those in solution, are at present routinely available on SSNMR instruments, the most important being Magic Angle Spinning (MAS) and high-power decoupling from abundant \({}^{1}\)H nuclei. In the most basic experiment, usually called direct polarization MAS or direct excitation MAS (DE-MAS), these techniques are combined with a radiofrequency excitation pulse on \({}^{13}\)C. The \({}^{13}\)C DE-MAS experiment gives the most accurate quantification of the different species in an OM solid sample, provided that it is performed with a sufficiently long delay between consecutive scans (five times the spin-lattice relaxation time, \(T_{1}\), of the \({}^{13}\)C nuclei). Since \(T_{1}\) values for \({}^{13}\)C nuclei in the solid state can be considerably long (even hundreds of seconds), thus requiring a recycle delay of many seconds or even minutes, extremely (and sometimes prohibitively) long measurement times are required for acquiring \({}^{13}\)C DE-MAS spectra of OM samples that commonly need thousands of scans to be accumulated for obtaining an acceptable signal-to-noise ratio. Due to these long measurement times, \({}^{1}\)H-\({}^{13}\)C cross polarization MAS (CP-MAS) experiments are most commonly used in the investigation of OM samples. In CP-MAS experiments, the \({}^{13}\)C signal is created by magnetization transfer from the abundant \({}^{1}\)H nuclei, with an enhancement by up to a factor of four. Moreover, in CP experiments the recycle delay is determined by the longitudinal relaxation of the \({}^{1}\)H nuclei, which is usually much faster than that of \({}^{13}\)C. This allows recycle delays of a few seconds to be used, with a substantial reduction in the experimental time. It is worth noting that, while CP-MAS is an excellent technique for investigating the qualitative composition of a sample, the relative peak intensities may not be fully preserved; thus, CP is generally considered a \"semiquantitative\" technique. In fact, the quantitative reliability of CP depends on several factors related to both the experimental conditions and the nature of the sample. Low CP efficiency of non-protonated and mobile carbons or moieties having short \({}^{1}\)H \(T_{1\text{o}}\) (proton spin-lattice relaxation time in the rotating-frame) can indeed make \({}^{13}\)C CP-MAS spectra non-quantitative. In fact, in CP experiments the trend of \({}^{13}\)C magnetization with contact time (also called CP dynamics) depends on the cross-polarization time, during which there is transfer of magnetization from \({}^{1}\)H to \({}^{13}\)C spins, and on the \({}^{1}\)H \(T_{1\text{o}}\), which determines the loss of \({}^{1}\)H magnetization. Examples of CP dynamics profiles in OM samples are reported in [41]. A careful adjustment of the acquisition conditions, and in particular of the contact time between \({}^{13}\)C and \({}^{1}\)H magnetizations, to the specific requirements of the investigated sample must be performed in order to minimize these problems. For example, [PERSON] et al. [42], after measuring \({}^{13}\)C and \({}^{1}\)H \(T_{1\text{o}}\)s and studying CP dynamics for a compost of municipal solid waste, reported that in the \({}^{13}\)C CP-MAS spectrum with a contact time of 1 ms all spectral regions show a similar intensity loss (<10%) with respect to the quantitative spectrum, confirming the acceptability of these CP conditions for relative quantitation.