Determination of Ethoxy Content in Ethylcellulose via Relative Quantitative ¹ H 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% = <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mfrac><mml:mrow><mml:mn>315</mml:mn> <mml:mi>r</mml:mi></mml:mrow> <mml:mrow><mml:mn>486</mml:mn> <mml:mo>-</mml:mo> <mml:mn>614</mml:mn> <mml:mi>r</mml:mi></mml:mrow> </mml:mfrac> </mml:math> × 100, where r = <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mfrac><mml:msub><mml:mi>I</mml:mi> <mml:mi>A</mml:mi></mml:msub> <mml:mrow><mml:msub><mml:mi>I</mml:mi> <mml:mi>A</mml:mi></mml:msub> <mml:mo>+</mml:mo> <mml:msub><mml:mi>I</mml:mi> <mml:mi>B</mml:mi></mml:msub> </mml:mrow> </mml:mfrac> </mml:math> (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.