Identification of Pseudopolymorphism of Magnesium Stearate by Using Low-Frequency Raman Spectroscopy

Magnesium stearate (Mg-St), which is currently available on the market, has a wide variety of properties, including pseudopolymorphism, relative content of stearic acid in fatty acid, and particle size. These properties of Mg-St influence manufacturing processes of pharmaceutical products, and therefore, it is necessary to control the quality of Mg-St from suppliers. The purpose of this study was to evaluate the low-frequency region of Raman spectroscopy for identification of pseudopolymorphism in Mg-St. Ten samples of Mg-St obtained from different suppliers were measured by powder X-ray diffraction (PXRD) and thermogravimetry and differential thermal analysis (TG-DTA) to identify the pseudopolymorphism of Mg-St. Then we investigated the relationship between their Raman spectra, including the low-frequency region, and pseudopolymorphism. The results were categorized as four types of Mg-St, namely, mono-, di-, and trihydrate and their mixture. The conventional region of the Raman spectrum (greater than 200 cm –1 ) was able to identify pseudopolymorphism to a certain degree, but it was not easy to completely distinguish pseudopolymorphism for the mixture of Mg-St. In contrast, the low-frequency region of the Raman spectrum (less than 200 cm –1 ) was able to clearly distinguish them. These data suggest that Raman spectroscopy, especially in the low-frequency region, is an effective method for rapid identification of pseudopolymorphism in Mg-St.