Abstract
The amount of hydroxyl groups is a crucial parameter when characterizing lignin as a raw material and when developing lignin-based applications. Currently, the most common method used for quantitative hydroxyl group determination is 31P nuclear magnetic resonance (NMR) spectroscopy after phosphitylation of the hydroxyl groups. The method relies on an internal standard for the quantification. The NMR signals of the internal standard have been shown to either overlap with signals from the lignin or to have poor stability due to degradation of the internal standard. To overcome these drawbacks, we have used PULCON (pulse length-based concentration determination) for the quantification of phosphitylated lignin. The method is based on the fact that the product of the intensity of an NMR signal and the length of the 90° pulse is directly proportional to the concentration of the sample. We conclude that the PULCON method could be used for hydroxyl group determination of lignin, and the results obtained are compa...
Highlights
The most common method used for quantitative hydroxyl group determination is 31P nuclear magnetic resonance (NMR) spectroscopy after phosphitylation of the hydroxyl groups
We conclude that the PULCON method could be used for hydroxyl group determination of lignin, and the results obtained are comparable to those based on utilization of an internal standard
Due to the complex structure of lignin, which varies between plant species and is altered during technical isolation methods, multiple analytical methods have been developed for structural characterization.[1]
Summary
Due to the complex structure of lignin, which varies between plant species and is altered during technical isolation methods, multiple analytical methods have been developed for structural characterization.[1]. We present a method for quantifying the hydroxyl groups of lignin using 31P-based PULCON (pulse length-based concentration determination).[11] The method does not require an internal standard, and the concentration determination is based on the fact that the signal strength obtained with an NMR probe is inversely proportional to the 90° pulse length for the probe.[12−14] Due to this property, the principle of reciprocity, the product of the signal intensity, and the 90° pulse are directly proportional to the concentration of the sample. The method presented could improve and simplify the standard procedures of the determination of hydroxyl groups in lignin by 31P NMR spectroscopy in both academia and industry
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