The Effect of Sample Preparation Techniques on Lignin Fourier Transform Infrared Spectroscopy

Area of Science:

• Biopolymer Chemistry
• Spectroscopic Analysis
• Materials Science

Background:

• Technical lignin utilization is hindered by challenges in functional group characterization and quantification.
• Existing methods for lignin analysis require improvement in terms of speed, cost-efficiency, and reliability.

Purpose of the Study:

• To evaluate and compare three Fourier Transform Infrared (FTIR) spectroscopy sampling techniques for technical lignins.
• To determine the most effective method for accurate and reliable quantification of functional groups, particularly phenolic hydroxyl groups.

Main Methods:

• Investigated Attenuated Total Reflectance (ATR) of neat lignin powders and DMSO-d6 solutions.
• Assessed transmission FTIR using the KBr pelleting method (0.5 wt%).
• Compared spectral quality (signal-to-noise, resolution) and quantified phenolic OH groups across methods.

Main Results:

• KBr pellet transmission yielded superior spectral smoothness and resolution compared to ATR methods.
• ATR of neat lignins was quick but had limited signal-to-noise.
• ATR of DMSO-d6 solutions at 30 wt% provided acceptable signal-to-noise.
• Best correlation (R² = 0.9995) for phenolic OH quantification was between KBr pellet transmission and neat ATR.
• FTIR results were compared with titration and Size Exclusion Chromatography (SEC) analyses.

Conclusions:

• The KBr pellet transmission method is recommended for reliable and accurate quantification of phenolic hydroxyl groups in technical lignins via FTIR.
• Non-aqueous potentiometric titration may underestimate phenolic OH groups due to pKa differences.
• Lower number-average molecular weight (Mn) correlates with increased phenolic OH content.