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Table 1 Characterization techniques to assess lignins quantitatively and qualitatively
Method
Description
Limitations
Ref.
76 and 77
Calibration required with samples of known concentrations. Large dataset (training and test sets) needed for reliable quanti cation. Training samples and prediction samples cannot di ff er greatly. Analyses are sensitive to sample preparation techniques NMR experiments are expensive, instruments found at specialized institutions and universities. Both experiments and data processing can be highly time-consuming Full derivatization of OH-groups is essential for proper quanti cation. Inverse gated decoupling pulse sequence needed for quanti cation: reduced sensitivity and increases relaxation time of analysis Less accurate than 31 PNMR. A ff ected by incomplete ionization of functional groups. Presence of other ionizable groups can a ff ect results Heterogeneity of COOH- and OH- groups distorts in ection point. Limited to quanti cation of COOH- and OH-groups (and possibly other ionizable groups) Time consuming calibration required. Samples must be within linear range. Acetylation is o en used for increased solubility prior to analysis Variations in inherent metal contents greatly a ff ects the pyrolysis reaction of the biomass Interference from other aromatic compounds (phenylalanine, tyrosine) can distort image. Not suited for imaging all tissue types Prone to artefact-generation arising from sample preparation. Fragmentation of lignin during imaging due to high-energy ion bombardment
FTIR spectroscopy
Popular technique to combine with chemometric methods such as principal component regression (PCR) or partial least squares (PLS) regression. Have been used for successfully determining lignin content in biomass samples Extremely detailed information about inter-unit linkages can be obtained. Has allowed for the assignment and quanti cation of over 80% of linkages in lignin oil from reductive catalytic fractionation of pine wood Di ff erentiation of the phenolic OH content of the three monolignols is possible from experiments a er derivatization of the OH groups Cruder determination of phenolic OH content is possible by comparing the di ff erences in absorption at speci cmaxima between neutral and alkaline solutions A fast and cheap alternative to wet- chemical methods for determining both phenolic OH group and carboxylic acid contents Popular technique for obtaining weight and number average molecular weights, M w and M n , and for further calculating the polydispersity index (PI) of samples Analysis of biomass composition, quanti cation of volatiles, bio-oil and biochar. Can be coupled with TGA and FTIR. Label-free method with high sensitivity and chemical selectivity for imaging of lignin in e.g. plant cell walls Visualization of monolignol distribution on plant sample cross- sections
1 H/ 13 C 2DNMR
78 and 79
31 PNMR
78
78 and 80
UV-vis spectroscopy
81 and 82
Simultaneous conductometric and acid – base titrations
Size exclusion chromatography (SEC)
78 and 83
84 and 85
Gas chromatography – pyrolysis (GC-Py)
Coherent anti-Stokes Raman scattering (CARS) microscopy
58 and 86
87
Time-of- ight secondary ion mass spectrometry (ToF-SIMS)
hydrophobization are frequently mentioned for lignin, 34,77,78 which would normally fall into the second category, unless the purpose is to protect the underlying substrate from degradation by water. The di ff erent applications will be discussed more in detail in this chapter. The end-use usually determines the manner, in which mixtures and coatings must be formulated. In principle, four di ff erent approaches can be distinguished, which are (1) application of neat lignin, (2) blends of lignin with other active
3. Formulations and applications of lignin-based surfaces and coatings The coatings and surface modi cations in this review most o en ful ll one of two purposes. Firstly, they may seek to protect the underlying substrate, e.g. , from mechanical wear, chemical attack (corrosion), or UV radiation. Secondly, they add functionality such as antioxidant, controlled substance release, or antimicrobial properties. Reduced wetting and
RSCAdv. , 2023, 13 , 12529 – 12553 | 12535
© 2023 The Author(s). Published by the Royal Society of Chemistry
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