Determination of Ethoxy Content in Ethylcellulose via Relative Quantitative ¹H NMR Spectroscopy

The research team established a quantitative ¹H NMR method to determine the relative ethoxy content (EO%) in ethylcellulose using a CDCl₃/TFA-d solvent mixture. High-field NMR spectroscopy enabled direct measurement without the use of internal or external calibrants by integrating the methyl proton signals (δ 1.15 ppm) and the methylene/methine proton signals (δ 2.5-5.5 ppm), and using their equivalent ¹H numbers and corresponding mass fractions for calculations. The EO% was calculated as: EO% =  $\frac{315r}{486-614r}$  × 100, where r =  $\frac{I_A}{I_A+I_B}$ (I_A methyl ¹Hs integral; Iᴃ methylene/methine ¹Hs integral). Round robin multi-laboratory validation testing showed that the qNMR results deviated by less than 3% from the certificate of analysis (COA) values (44%-51%). The COA values, derived using the pharmacopeial gas chromatography-flame ionization detection (GC-FID) method, may be affected by factors related to incomplete derivatization and the volatility and instability of the product (iodoethane) and the inert internal reference standard (n-octane). Therefore, the qNMR method with its operational simplicity, reduced chemical hazards, and robust performance presents an alternative for quality control of pharmaceutical-grade ethylcellulose.