By means of different methods, for instance, potentiometry, IR spectroscopy.

Bojidarka B. Ivanova definitely! It will give me some information on the functional groups. But then how to proceed from there? I am looking for a standard protocol to follow. Thanks anyway! :)

FTIR spectroscopy will give you the idea about the presence of -OH,-C=O etc groups in lignin. You can also do NMR (1H,13C,19F) of your sample

i m not very sure about all this but, just to get a very vague idea, u can determine ash content, depending on type of lignin, its ash content may varies.

U can do DSC analysis also, their glass transition temperature may also vary. I hope that can help u.

Thank you everyone for your responses! :)

You will definitely need to apply multiple analysis techniques to determine the delignification process.

1) CHNS elemental analysis --> if sulphur is present, then you should immediatelly think about the Lignosulfonate, Kraft lignin or perhaps the organosolv process with catalytic amounts of sulphur. A rather high amount of sulphur is associated with Kraft lignin while a very low sulphur content is associated more with organosolv lignin.

2) If there is sulphur present, you can try to solubilize lignine in water with different acidity. Lignosulfonate will still be soluble at acidic water with rather low pH compared to Kraft and organosolv lignin.

3) Perform 31P-NMR and determine whether the feedstock is from hardwoods/softwoods/grasses. Not every delignification technique is applicable to every biomass feedstock.

In conclusion, if sulphur is present, you can easily determine the delignification process. When no sulphur is present, then either the lignin is obtained from Soda process or organosolv process and if you gain insight in the biomass feedstock, you might be able to determine the applied delignification process since not every delignification technique is applicable to every biomass feedstock.