Determination of the relative amounts of anhydrous carbamazepine (C15H12N2O) and carbamazepine dihydrate (C15H12N2O.2H2O) in a mixture by powder x-ray diffractometry.

A powder x-ray diffraction technique has been developed to quantify the relative amounts of anhydrous carbamazepine (C15H12N2O) (I) and carbamazepine dihydrate (C15H12N2O.2H2O) (II) when they occur as a mixture. The theoretical basis of this technique was developed by Alexander and Klug (Anal. Chem. 20:886-889, 1948). The powder x-ray diffraction patterns of I and II revealed that the lines with d-spacings of 6.78 and 9.93 A were unique to I and II, respectively. The ratio of the integrated intensity of the 6.78 A line in a mixture of I and II to the intensity of the 6.78 A line in a sample consisting of only I was calculated as a function of weight fraction of I in the mixture. These ratios were also experimentally determined, and there was a good agreement between the theoretical and the experimental intensity ratios. Similarly good agreements between the theoretical and the experimental intensity ratios for the 9.93 A line of II were obtained. The samples were compressed into tablets and subjected to x-ray analysis. When compressed to a certain pressure, the particles tended to orient the same way in replicate samples resulting in highly reproducible intensity values. Compression into tablets was necessary because the powder samples yielded unsatisfactory results.