CELL 70 - Why 'laboratory standard' is not good enough in GPC-analyses of lignins

One crucial component within the efforts aiming at the valorisation of lignin is a rather precise knowledge regarding its structural features in terms of the amounts of aliphatic and phenolic hydroxyl groups, the amount of non-phenolic end groups, and the nature of interunit bonding motifs including their abundances. For achieving a complete characterisation, these data are best to be interpreted in connection with the molecular mass key data describing the oligomeric lignin chains, namely the number average molecular weight Mn, the weight average molecular weight Mw, and the polydispersity PD. Especially the latter one is often referred to when the discussion comes to the quality of polymeric or oligomeric sample: the smaller the PD, the better the polymer with respect to subsequent (chemical) modifications. Within the GPC-based determination of the above-mentioned key data like the PD for a given lignin sample, a meaningful calibration of the instrumentation, as well as the use of suitable detectors, are of paramount importance: a misled estimation of the “expected” molecular weight of a lignin sample might lead to a poor choice of the range of molecular weights of the standards used for calibration; the wrong detector-type lacks sensitivity. The range of the molecular weights covered in the calibration, however, does affect significantly the results obtained in the measurements of the lignin samples especially with respect to the polydispersity, and a non-suitable detector neglects the extremes. So-obtained astonishingly good, or unexpectedly bad polydispersities eventually lead to wrong conclusions and decisions concerning future actions. We have developed a transparent and robust protocol for the gel-permeation chromatography-based characterisation of plant-derived polyphenols in general, and lignins in particular. After systematically evaluating i) the influence of the range of molecular weights used for the calibration of the analytical set-up, ii) the influence of the flow-rate on the molecular mass-related key figures of certain lignin samples, and iii) the influence of the nature of the detector used for recording (UV-based of refractive index-based), we propose a generally applicable method and set-up for determining molecular weight key-figures based on GPC, that correspond to molecular weight data obtained via NMR spectroscopy-based methods.