Characterization of the water-insoluble fraction from fast pyrolysis liquids (pyrolytic lignin) - Part III. Molar mass characteristics by SEC, MALDI-TOF-MS, LDI-TOF-MS, and Py-FIMS

Matrix assisted laser desorption ionisation-time of flight-mass spectrometry (MALDI-TOF-MS) laser desorption ionisation-time of flight-mass spectrometry (LDI-TOF-MS) and temperature resolved analytical pyrolysis field ionisation mass spectrometry (Py-FIMS) have been applied for the first time on two pyrolytic lignins (PL's), precipitated from different aged bio oil, and four PL-fractions for molar mass characterization. The results were compared with data from size exclusion chromatography (SEC). SEC was the only method that allowed a mathematical calculation of molar mass characteristics such as average molecular weight (Mw), dispersity (D), and the molar mass at the peak maximum of the elugram (Mp). The SEC Mp values of PL-fractions differ from visually interpolated MALDI-TOF-MS measurements by 20%. MALDI-TOF-MS spectra showed detailed structures of the molar mass distribution (MMD) of PL and PL-fractions. Especially, the spectrum of one PL showed various local maxima with intervals of 170-200 Da. The size of these intervals could represent the average absolute molar mass of PL-monomers. MALDI-TOF-MS was limited by the influence of superposing matrix signals in the spectrum at low molar masses. LDI-TOF-MS showed clearer spectra than MALDI-TOF-MS in mass ranges below 400 Da. No signals were obtained for fractions with higher masses or whole PL. Intervals between main signals were mostly 14-16 Da. In spectra of different PL-fractions, corresponding main signals can vary between 2 and 4 Da. These mass differences indicate variations in the aliphatic region of the PL molecules. Py-FIMS spectra contained masses of thermally ejected, but unfragmented monomers and dimers. It was the only method, which allowed partial identification of monomeric and dimeric structures of all samples. The detected monomers correspond to known lignin derived compounds in bio oil, the detected dimers have some similarities to phenylcoumaran structures. PL from aged bio oil showed an increased content of higher oligomers and a higher average molecular weight. (C) 2006 Elsevier B.V. All rights reserved.