Lignin monomer analysis by GC and GC–MS

Lignin monomers resulting from hydrogenolysis were initially identified by GC–MS and then quantified by GC⁹. Before the GC and GC–MS analyses, the sample was derived with BSTFA. Specifically, 20 μl of the solution containing dodecane (an internal standard) was directly sampled into a 2-ml GC vial, followed by the addition of 200 μl of pyridine and 500 μl of BSTFA. The mixed solution (0.7 ml) was heated in an oven at 80 °C for 1 h. The silylated products were identified by GC–MS (Techcomp) using a Scion 436C series GC instrument equipped with an HP5-MS capillary column (30 m × 0.45 mm) and a Scion 436-GC-SQ series MS detector. The injection temperature was 300 °C. The column temperature programme was: 50 °C (5 min), 10 °C min⁻¹ to 300 °C, and 300 °C (5 min). The monomer products were directly identified according to the MS spectra (Extended Data Fig. ​Fig.2).2). The identified products were further quantified by a GC instrument (Scion 436C series, Techcomp) equipped with an HP5 capillary column and a flame ionization detector (FID). The operation conditions were the same as those used for the GC–MS analysis.

For convenience, the quantification of monomer (Extended Data Figs. ​Figs.11 and ​and2)2) was based on an internal standard (dodecane) and the effective carbon number (ECN) method according to a previous report¹⁹ (Extended Data Fig. ​Fig.2).2). The detailed yield calculations were as follows:

in which n_dodecane (mmol) is the molar amount of the internal standard (dodecane), n_product (mmol) is the molar amount of the lignin monomers, A_product is the peak area of monomers in the GC–FID chromatogram, A_dodecane is the peak area of dodecane in the GC–FID chromatogram, ECN_dodecane is the effective carbon number of dodecane, ECN_product is the effective carbon number of the lignin monomers, Y_product is the molar yield of monomers calculated on the basis of the molar amount of lignin monomeric units (n_theoretical); an average molecular weight of 210 g mol⁻¹ for lignin monomeric units was used for the calculations.