**3. Characterization of lignin**

It is very important to know the composition and structure well before planning to use them as precursor for any of the applications. Since lignin can be recovered from various plant resources and can be separated well by numerous methods, identification of its unique characteristics is very important [8, 9]. The functional group analysis in lignin is conducted by using FT-IR spectroscopy and Raman and fluorescence spectroscopy [10]. The stretching frequencies for ether bond, methoxy, and hydroxyl groups can be easily identified using FT-IR spectroscopy [11]. Macromolecular structure of lignin is determined by electron microscopy such as SEM-EDX (scanning electron microscope with energy dispersive X-ray spectroscopy) and TEM (transmission electron microscopy). UV-visible spectroscopy shows good absorption band owing to its aromatic nature. The shape of UV-visible graph is very sensitive to its particular type, pH, and solvent used [12].

Many structural aspects of lignin regarding its composition and reactivity can be assessed by using advanced NMR techniques like 2D-NMR and 2D HSQC-NMR. The basic and rough estimation of lignin structure can be done by [13] C-NMR and [1] H-NMR, but the spectra obtained is much overloaded and complex. For better resolution, hyphenated NMR techniques gives best results [13–15]. Gel polymer chromatography (GPC) is a type of size exclusion chromatography which is utilized in measuring the molecular weight of lignin polymers. Various aqueous and nonaqueous solvents can be used based on hydrophilic and hydrophobic nature of lignin molecule [16, 17].
