Analysis of carboxyl content in oxidized celluloses by solid-state 13C CP/MAS NMR spectroscopy

A noninvasive method to determine the carboxyl content in oxidized celluloses, using solid-state carbon-13 cross-polarization-magic angle spinning nuclear magnetic resonance (13C CP/MAS NMR) spectroscopy, has been developed. Standard samples containing 0, 4, 8, 12, 16, and 20% carboxyl content were prepared by mixing appropriate amounts of powdered cellulose, a non-oxidized cellulose standard prepared from cotton linter by ball-milling for 96 h, and a commercial oxidized cellulose sample that had a 20% carboxyl content. Standard curves were constructed by plotting the percent carboxyl content against the peak area at 171 ppm, normalized with (i) a peak area at 104 ppm and (ii) the sum of peak areas at 171, 62, and 65 ppm. The regression analysis of the curves yielded a linear relationship with correlation coefficient (R2) values of 0.9868 and 0.9863, respectively. To validate the methods, five new samples of oxidized cellulose were prepared and analyzed. The values obtained were comparable to those determined by the calcium acetate method described in the United States Pharmacopoeia (USP), indicating that the solid-state 13C CP/MAS NMR can be used to analyze carboxyl content in oxidized celluloses.