4.3. Analytical Methods

Analyses of the transformation products (HPLC Agilent Technologies 1200 Series) were performed using a DAD detector (detection wavelength, 210 nm) with a C18 column (3.9 mm × 150 mm) as the chromatographic column. The mobile phase was methanol/water = 43:57 (v/v), the flow rate was 0.6 mL/min, the detector temperature was 35 °C, and the injection volume was 1 μL.¹⁵ The following formula was used to calculate syringaldehyde yield:

where m_lignin is the dry weight of lignin (g), V_EA is ethyl acetate volume (mL), C_SY is the syringaldehyde concentration (mg/mL).

Characterization of perovskite oxides was performed via X-ray diffraction (XRD) pattern analysis. The Cu Kα source was adopted, the current was 20 mÅ, the tube voltage was 36 kV, the scanning angle was 20–80°, and the scanning rate was 4°/min.

The lignin sample (100 mg) was dissolved in 2% NaOH solution and diluted 10 times in 0.1 mol/L NaAc solution. After filtration with the pore size of 0.45 μm, the molecular weight was determined by GPC. The GPC analysis was carried out with an Agilent1260 series equipped with RID and diode array detector (DAD). The chromatographic column was a TSKgel G-3000PWxl (300 × 7.8 mm). The mobile phase was 0.1 M NaAc with a flow rate of 0.6 mL/min. The temperature of the column temperature box was 35 °C, as described in the previous study.¹⁶ The differential distribution of molecular weight W(log M) = dwt/d(log M) and W(log M) × d(log M) can be used to determine the polymer mass content in a certain molecular weight range and correspond to the intercept of the ordinate corresponding to a certain range of abscissa on the cumulative molecular weight distribution map. To a certain extent, the ordinate represents the normalized mass ratio. Therefore, the y axis is dimensionless.

The lignin sample (0.002 g) was mixed with KBr in the agate mortar in the proportion of (lignin/KBr = 1:100 w/w). The functional groups of lignin were measured by an FT-IR-600 plus spectrometer (JASCO Corp., Tokyo, Japan). The average scanning time was 32, the scanning wavelength range was 400 to 4000 cm^–1, and the resolution was 1 cm^–1. Cross-linked lignin (CLL) was calculated by using the peak value at 1500/1600 in FT-IR data.