Drug substances optical rotation

Important to quality control are the comparison and confirmation of drug substance identity, excipients, and packaging components. Techniques such as Fourier transform IR (FTIR), attenuated total reflectance (ATR), NIR, Raman spectroscopy are used with increased regularity. The detection of foreign metal contaminants is essential with inductively coupled plasma spectroscopy (ICP), atomic absorption (AA), and X-ray fluorescence. Also notable is the increased attention to analysis of chiral compounds, as in the synthesis of drug substances. Optical rotation, ORD, and CD are currently the preferred instruments for this practice. The analytical techniques commonly used in the preformulation study are discussed in the following. 

A wide range of physical constants, for instance melting point, boiling point, specific gravity, viscosity, refractive index, solubility, polymorphic forms vis-a-vis particle size, in addition to characteristic absorption features and optical rotation play a vital role in characterization of pharmaceutical chemicals and drug substances. These physical constants will be discussed briefly with typical examples as under ... 

Optical rotation has the dual advantages of historical use and widespread recognition in the compendia. For an enantiopure material, it defines its configuration when used in conjunction with other valid chemical tests. However, optical rotation has been used ineffectively when the primary analytical goal is the determination of stereochemical purity. The limits selected for the specification seem to be unrelated to the purity required by other methods. For example, the compendial monograph for naproxen requires that the drug substance meet a specification of "between -f-63.0 and -1-68.5"" in a chloroform solution. Based on the published specific rotation, this corresponds to a stereochemical purity of 95.5 to 103.7%, compared to the assay limits of 98.5 to 100,5%, determined by titration with sodium hydroxide (5). 

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