Quantitative evaluation of the sensitivity of library-based Raman spectral correlation methods.

Library-based Raman spectral correlation methods are widely used in surveillance applications in multiple areas including the pharmaceutical industry, where Raman spectroscopy is commonly used in verification screening of incoming raw materials. While these spectral correlation methods are rapid and require little or no sample preparation, their sensitivity to the presence of contaminants has not been adequately evaluated. This is particularly important when dealing with pharmaceutical excipients, which are susceptible to economically motivated adulteration by substances having similar physical/chemical/spectroscopic properties. We report a novel approach to evaluating the sensitivity of library-based Raman spectral correlation methods to contaminants in binary systems using a hit-quality index model. We examine three excipient/contaminant systems, glycerin/diethylene glycol, propylene glycol/diethylene glycol, and lactose/melamine and find that the sensitivity to contaminant for each system is 18%, 32%, and 4%, respectively. These levels are well-correlated to the minimum contaminant composition that can be detected by both verification and identification methods. Our studies indicate that the most important factor that determines the sensitivity of a spectral correlation measurement to the presence of contaminant is the relative Raman scattering cross section of the contaminant.