Simultaneous quantification of an enantiomer and the racemic compound of ibuprofen by X-ray powder diffractometry

An X-ray powder diffractometric method was developed for the simultaneous quantification of the relative amounts of the racemic compound (+/-) of ibuprofen (I) and S(+)-ibuprofen (II), when they occur as a mixture.

The X-ray powder diffraction patterns of I and II show pronounced differences. This formed the basis for the determination of the relative amounts of I and II when they occur as a mixture. X-ray lines with d-spacings of 14.41 and 4.37 A were unique to I and II, respectively. Mixtures containing different proportions of I and II were prepared which also contained lithium fluoride (III) as an internal standard.

A linear relationship was obtained when the intensity ratio (intensity of the 4.37 A line of II/intensity of the 2.01 A line of III) was plotted as a function of the weight fraction of II in the mixture. Similar results were obtained in the case of I. Using these standard curves, the weight fractions of I and II in "unknown" mixtures were determined. The experimentally determined analyte concentration ranged between 98 and 104% of the true value. The relative error in the analyses of individual samples was < 10%. The minimum detectable weight fraction of I and II and II in I were 0.032 (3.2% w/w) and 0.034 (3.4% w/w), respectively. The minimum quantifiable weight fractions were 0.136 for I and 0.112 for II. Since the X-ray diffraction patterns of S(+)-ibuprofen and R(-)-ibuprofen are identical, the conclusions drawn regarding mixtures of I and II will also hold true in the quantitative analyses of mixtures of I and R(-)-ibuprofen.