In the development of organic reference materials, a purity evaluation by the freezing point depression method, which is regarded as one of primary methods of measurement, is used to ensure traceability to the International System of Units (SI). However, because this method is difficult to apply to some organic compounds, a subtraction method based on chromatographic techniques under ISO Guide 35 : 2006 was developed for purity evaluation. The subtraction method was based on subtracting every impurity detected from 1. Since the detection of every impurity is impossible, validation of the subtraction method is necessary. To validate our subtraction method based on chromatographic techniques, the purities of 4-
n-nonylphenol, 4-
n-heptylphenol, and 2,4-dichlorophenol were determined using both the subtraction and freezing point depression methods, and then compared. The purities obtained by the subtraction method were comparable, within the range of uncertainties, to the purities determined using the freezing point depression method. Moreover, validation of our subtraction method was confirmed using international comparisons, which play an important role in ensuring mutual recognition of national reference materials, related to purity evaluation. Therefore, these data suggest that our subtraction method used for purity evaluation of alkylphenols is a reliable technique.
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