On the implications of calibration techniques and detector systems on GPC-based analyses of lignin

One crucial component within the efforts aiming at the valorisation of the so far under-utilized biomass component lignin is the knowledge about 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, etc. etc. 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. As part of our on-going efforts to develop robust protocols for the characterisation of plant-derived polyphenols in general, and especially lignins, we thus systematically evaluated 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). Besides simply comparing the results obtained by the different chromatographic set-ups and calibrations as such, we also compare the different results to data obtained for Mn, Mw, and PD using advanced quantitative NMR techniques for the analysis of lignins.