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Recently, quantitative NMR (qNMR), especially 1 H-qNMR, has been widely used to determine the absolute quantitative value of organic molecules. We previously reported an optimal and reproducible sample preparation method for 1 H-qNMR. In the present study, we focused on a 31 P-qNMR absolute determination method. An organophosphorus compound, cyclophosphamide hydrate (CP), listed in the Japanese Pharmacopeia 17th edition was selected as the target compound, and the 31 P-qNMR and 1 H-qNMR results were compared under three conditions with potassium dihydrogen phosphate (KH 2 PO 4 ) or O-phosphorylethanolamine (PEA) as the reference standard for 31 P-qNMR and sodium 4,4-dimethyl-4-silapentanesulfonate-d 6 (DSS-d 6 ) as the standard for 1 H-qNMR. Condition 1: separate sample containing CP and KH 2 PO 4 for 31 P-qNMR or CP and DSS-d 6 for 1 H-qNMR. Condition 2: mixed sample containing CP, DSS-d 6 , and KH 2 PO 4 . Condition 3: mixed sample containing CP, DSS-d 6 , and PEA. As conditions 1 and 3 provided good results, validation studies at multiple laboratories were further conducted. The purities of CP determined under condition 1 by 1 H-qNMR at 11 laboratories and 31 P-qNMR at 10 laboratories were 99.76 ± 0.43 and 99.75 ± 0.53%, respectively, and those determined under condition 3 at five laboratories were 99.66 ± 0.08 and 99.61 ± 0.53%, respectively. These data suggested that the CP purities determined by 31 P-qNMR are in good agreement with those determined by the established 1 H-qNMR method. Since the 31 P-qNMR signals are less complicated than the 1 H-qNMR signals, 31 P-qNMR would be useful for the absolute quantification of compounds that do not have a simple and separate 1 H-qNMR signal, such as a singlet or doublet, although further investigation with other compounds is needed.

Purity Determination of Cyclophosphamide Hydrate by Quantitative <sup>31</sup>P-NMR and Method Validation