The application of near infrared spectroscopy and dynamic vapor sorption to quantify low amorphous contents of crystalline lactose.

PURPOSE: To explore the use of a combined near infrared spectroscopy and gravimetric sorption apparatus in providing an accurate quantification of amorphous contents of predominantly crystalline lactose.
METHODS: Key wavelengths on the near infrared spectra of amorphous and crystalline lactose were used to construct a calibration plot of spectral fit to amorphous content. The extent of water sorption and desorption at 75% relative humidity (RH) was used to quantify the amount of amorphous material in the sample.
RESULTS: Near infrared spectroscopy was used to quantify the amorphous contents of a set of 10 partially amorphous lactose samples using a calibration equation generated from an independent set of 17 samples. The results were found to be accurate to within 1% w/w amorphous content. Dynamic vapor sorption quantification relates the mass of water sorbed and subsequently desorbed during the crystallization process with the amount of amorphous material originally contained within the sample. It was possible to quantitatively detect as little as 1 mg of amorphous content in the sample. The percent amorphous content determination will thus be sample mass dependent, however, assuming a sample mass of 150 mg, the best detection would be ca. 0.7%.
CONCLUSIONS: It has been found that both techniques may be used to quantify small quantities of amorphous material. The combination of the two techniques lends itself to added verification of results and thus increased reliability.