2.8. Molecular Weights Determination

Before the gel permeation chromatograph (GPC) test, the lignin samples were modified by acetylation according to the method used by Glasser and Jain [36], which improved the solubility of lignin in tetrahydrofuran (THF) with almost no effect on molecular weights. In order to ensure lignin molecules disperse uniformly and stably in the solvent THF, the sample solutions (1 to 1.5 mg/mL) were stored for 24 h before testing. The number average molecular weight (M_n), weight average molecular weight (M_w), and molecular weight distribution (M_w/M_n) for EHL and GEHL were determined by an Agilent1100 model GPC, provided by Agilent Technologies Co., Ltd. (Santa Clara, CA, USA), equipped with a G1315B model UV detector and column combined with two 79911GP type columns. The test parameters were set as follows: testing temperature of 30 °C, an amount of injection of 50 μL with a flow rate of 1.0 mL/min, and a column pressure of 2.9 MPa. The columns were calibrated using polystyrene standards with different molecular weights, and the standard curve and fitting equation (Lg(M) = 10.60068 − 0.50533t) was obtained from the GPC system.