Similar to fluorescence-labelling for carbonyl groups, the carboxyl-selective fluorescence-labeling with 9H-fluoren-2-yl-diazomethane (FDAM) provides an approach to quantify carboxyl groups in relation to the molar mass distribution of cellulosic substrates (Bohrn et al. 2006).

The FDAM label is synthesized in situ directly prior to labeling and is based on a reactive diazo group (see Figure 3, left). The label reacts specifically with uronic acid groups, and can thus be used to trace glucuronic acids or, alternatively, hemicelluloses in pulps, very selectively (Hutterer et al. 2017). The labelled pulp is dissolved in DMAc/LiCl and analyzed by a SEC-MALLS/fluorescence/RI combination. Importantly, the fluorescence label does not interfere with MALLS detection. The data obtained are a total number of uronic acid groups and their profile related to the molecular weight distribution (Figure 3, right).

Sample amount generally required: 50-100 mg (see above at SEC measurements).

Schematic of the labelling of carboxyl groups in celluloses with FDAM (left). Uronic acid (hemicellulose) profile of a hardwood dissolving pulp (right).

References

Bohrn, R.; Potthast, A.; Schiehser, S.; Rosenau, T.; Sixta, H.; Kosma, P.,
The FDAM Method: Determination of Carboxyl Profiles in Cellulosic Materials by Combining Group-Selective Fluorescence Labeling with GPC.
Biomacromolecules 2006, 7 (6), 1743-1750

Hutterer, C.; Fackler, K.; Potthast, A.,
The Fate of 4-O-Methyl Glucuronic Acid in Hardwood Xylan during Alkaline Extraction.
ACS Sustainable Chem. Eng. 2017, 5 (2), 1818-1823.