Improved impurity determination of pure organic substances by differential scanning calorimetry with a dynamic method.

Differential scanning calorimetry can be used to measure the impurity contents of pure organic substances on the principle of freezing-point depression. Impurity determination by differential scanning calorimetry with a dynamic method, which has the advantages of speediness and convenience, remains to be explored. Here, a series of acetanilide and dibenzothiophene samples with various purities was prepared through zone melting, and the samples were then analyzed by gas chromatography-mass spectrometry. A modified dynamic method, including encapsulating the analyte in a volatile pan through cold welding, remelting the analyte with a low heating rate, calculating the melted fraction considering the area of the tailing under the heat-flow curve, and reducing the error from solid-solution formation, is proposed. Encapsulating with a volatile pan using a proper torque gave an accurate result. Remelting gave a lower impurity content and a more narrow and sooth peak of heat-flow compared with the first melting. The impurity-content results calculated by the modified method were usually higher than those calculated by the ASTM standard method. For acetanilide and dibenzothiophene with impurity contents of less than 0.30%, the modified dynamic method showed good accuracy. The proposed method is applicable to determination of reference materials of organic substances with high purity owing to its accuracy and convenience.