Hygroscopic Tendencies of Substances Used as Calibrants for Quantitative Nuclear Magnetic Resonance Spectroscopy

Abstract

Atmospheric moisture can contaminate calibrants for quantitative nuclear magnetic resonance (qNMR) spectroscopy and cause systematic errors in qNMR measurements. Therefore, coulometric Karl Fischer (CKF) titration was used to evaluate the hygroscopic tendencies of several organic compounds that are commonly used as calibrants for qNMR spectroscopy: benzoic acid, dimethyl sulfone, 1,3,5-trimethoxybenzene, acetanilide, dimethyl terephthalate, and 1,2,4,5-tetramethylbenzene. Samples were placed in a sealed humidity chamber at 100% relative humidity (RH) and a temperature of 295.4 ± 0.9 K. Over the course of months, portions of each sample were analyzed by CKF titration. All the compounds except dimethyl sulfone were resistant to changes in water content and thus are good choices for qNMR experiments. In contrast, dimethyl sulfone absorbed about 25 mass % of water over 5 weeks at 100% RH; such behavior could compromise qNMR experiments under certain conditions.