Molecular heterogeneity in lignin remains poorly understood due to the lack of analytics robust enough to determine the precise structure of individual molecules in isolated lignin samples. Recently, we showed that MALDI-TOF MS analysis, combined with a range of NMR techniques, facilitated comprehensive structural studies of oligomer populations present in milled wood lignin from spruce, without prior fractionation. Here, the developed methodology is applied to study populations in milled wood lignin from birch. We report a dominance of linear aryl ether homo-oligomers subdivided into three categories: oligomers exclusively of sinapyl units, oligomers exclusively of guaiacyl units, and oligomers with both guaiacyl and sinapyl units. Linkage progressions in oligomeric structures with the other common interunits are also elucidated. Unlike previous reports, the study enabled the determination of molecule-specific syringyl/guaiacyl ratios. All of the elucidated oligomers contain enone or enal groups at the aliphatic end and phenolic hydroxyls on the other, highlighting homolytic cleavage reactions that occur during lignin procurement. Overall, the study provides a comprehensive framework for an atomistic understanding, offering significant potential for both fundamental and applied research.