Absolute molar mass of lignins by size exclusion chromatography and MALDI-TOF mass spectroscopy

The absolute molar mass of lignins has been determined by Size Exclusion Chromatography (SEC) in combination with off-line Matrix-Assisted-Laser-Desorption-Ionization Time Of-Flight Mass Spectroscopy (MALDI-TOF-MS). The lignins were fractionated with SEC into fractions with narrow molar mass distributions and the absolute molar mass of each fraction was determined by MALDI-TOF-MS. The absolute average molar masses (Mn and Mw) and the polydispersity index (Mw/Mn) of the lignins were calculated using the SEC distribution curve and the absolute molar masses of the fractions (MALDI calibration method). Absolute molar masses were determined for lignin standards previously used in an international Round-Robin test on lignin analysis organized by the International Energy Agency. Hardwood and softwood lignins isolated from kraft pulping liquors were also successfully analyzed. The absolute molar masses measured by the MALDI calibration method were compared with the apparent molar masses obtained on the same lignins using a polystyrene standard calibration method. The deviations in the Mn and Mw values obtained by the two methods vary somewhat depending on the type of lignin analyzed, but they never exceeded a maximum deviation of about 20%. Thus, to accurately determine the average molar mass a separate calibration (MALDI calibration method) for each type of Lignin was needed.