Quantitative NMR (qNMR), particularly 1H-qNMR, is useful for determining the absolute purity of organic molecules. However, identifying the target signal(s) for quantification is difficult, because of the overlap and complexity of organic molecules. Therefore, we focused on the 31P nucleus, owing to the simplicity of its signals, and investigated the 31P-qNMR absolute determination method by using organophosphorus drugs, water-soluble cyclophosphamide hydrate (CP), and water-insoluble sofosbuvir (SOF). The optimized and reproducible 31P-qNMR conditions, such as qNMR sample preparation [i.e., selecting suitable deuterated solvents and a reference standard (RS) for 31P-qNMR], hygroscopicity and solution stability of the analyte and RS, and qNMR measurements-such as acquisition time, relaxation delay time, and spectral width-were examined. The CP purities determined using 31P-qNMR agreed well with those for the established 1H-qNMR method in D2O. In contrast, the SOF purity determined using 31P-qNMR was 1.6% higher than that for 1H-qNMR in the protic solvent CD3OD. Therefore, using a protic solvent, such as CD3OD, was not suitable for 31P-qNMR; the deuterium exchange with the RS for 31P-qNMR (i.e., phosphonoacetic acid) resulted in a small integrated intensity. Consequently, the aprotic solvent DMSO-d6 was employed to determine the SOF purity. The data revealed that the SOF purities determined using 31P-qNMR agreed well with the established 1H-qNMR values, indicating that the absolute quantification of SOF using both 31P-qNMR and 1H-qNMR is possible in DMSO-d6. Thus, we established an optimized and reproducible 31P-qNMR method in validation study across multiple laboratories.
Keywords: absolute purity; certified reference material; cyclophosphamide hydrate; deuterated solvent; quantitative 31P-NMR (31P-qNMR); sofosbuvir.