Surface properties correlate to the digestibility of hydrothermally pretreated lignocellulosic Poaceae biomass feedstocks

Table 5 ATR-FTIR assignments of wavenumbers used to measure peak area

Wavenumber (cm⁻¹)	Asssignment^a		Estimated penetration depth^a (μm)
895	Holocellulose	Anomeric C-groups, C₁-H deformation, ring valence vibration (cellulose, wood, holocellulose) [71]	1.85
1508	Lignin	Aromatic skeletal vibrations [71, 72]	1.10
1732	Hemicellulose	C=O stretch in unconjugated carbonyl groups of carbohydrate origin (side chain acetylation in mannan, carboxylic acid side chain in xylan, and ester groups in lignin-carbohydrate complexes) [71, 72]	0.96
2918	Wax	Asymmetric CH₂ stretching from cuticular waxes [73]	0.57

^aCalculated based on the formula (Eq. 3):
\(d_{\text{p}} = \frac{\lambda }{{2\pi n_{1} \sqrt {\sin^{2} \theta - \left( {n_{2} /n_{1} } \right)^{2} } }},\) (3)
where d _p, λ, θ, n ₁, and n ₂ are penetration depth, wavelength, incident angle, ATR crystal refractive index, and sample refractive index, respectively. The values of θ and n ₁ are known specifically to be 45° and 2.40, respectively, for diamond ATR. The refractive index of biomass samples is estimated to be 1.4 which is a common value for organic polymer, e.g., in wood cell wall [74]

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