Identity tests optical rotation

Although the majority of current pharmacopoeial methods describe the use of specific optical rotation ([a]) for the determination of stereoisomer identity and/or content, separation techniques such as HPLC and CE are now the most frequently applied technologies during drug development and for new pharmaceutical products entering the market. Indeed, Supplementary Chapter K in Volume IV of the 2004 British Pharmacopoeia (BP) deals with this specific issue [22]. It states that in future when a monograph describes an enantiomer it will include both a test for identity ([a]) and a test to control stereoisomeric purity... 

A relatively common use of optical rotation is as an identity test for a racemate with specification limits that are symmetrical around zero. Such a specification has Little if any regulatory significance. Its validation necessarily depends upon knowledge of the specific rotation and thus requires the resolution of the racemate on a laboratory scale. Furthermore, even with such supporting data, the method is dependent upon the accuracy of the sample preparation, since a solvent blank would also show a rotation of zero. Other analytical methods are far more appropriate for the stereochemically specific identification of racemates. 

In spite of tbe difference in odour between this and caraway oilSp the composition of the two is almcet identical, both consisting nearly entirely of limonene and carvone. Dill oil, however, contains less car-vone than caraway oil, Hence tbe tests given under caraway oil apply here, and screjss should he laid on Ihss sfM cific gravity, optical rotation, and fractionation. Not move than 15 per ctint. should distil below 185", and not less than 40 per cent, above English distilled oils usually... 

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