Accurate Determination of Trace Water in Organic Solution by Quantitative Nuclear Magnetic Resonance

A new method accompanied by a derived equation for accurate determination of trace water was developed by using quantitative ¹H nuclear magnetic resonance (qNMR) spectroscopy. Given that the response for each chemically distinct moiety is uniformly proportional to the number of the corresponding resonant nuclei within the analyte, it is practicable to directly quantify the water content via its proton number using qNMR. In this study, three water standards with known water contents (e.g., 10.02, 1.006, and 0.103 mg/g), which were accurately determined by a well-established Coulometric Karl Fischer (CKF) titration method, were measured by using the developed qNMR method. An excellent agreement between the results from these two methods was obtained. Then, the water content of Sudan I was determined by high-field NMR (HF-NMR) spectroscopy, and the water contents of acetone and bioethanol were measured by low-field NMR (LF-NMR) spectroscopy. These results were compared with the water content measured by the CKF method to confirm the applicability of the established qNMR method. The developed method can eliminate the influences of environmental humidity and background water in the solvent; subsequently, the results calculated by the derived equation were comparable to the nominal values. Under the optimal conditions, the limit of quantitation of this method was as low as 6.7 μg. The recommended sample sizes for practical samples with various water contents (e.g., 10.02, 1.006, and 0.103 mg/g) were determined to be 5, 50, and 60 mg, respectively, which are much smaller than those required for the CKF method. The new method has a static and stable process without any side reactions, and the traceability to the SI unit can be directly achieved through the NMR internal standard. This method overcomes the limitations of the CKF method, especially for measuring methanol-insoluble substances.