Formation of copper(I) from trace levels of copper(II) as an artifactual impurity in the HPLC analysis of olanzapine.

An analytical artifact peak appearing to be an impurity was observed intermittently among several laboratories performing HPLC analyses of olanzapine drug substance and formulation samples. The artifact peak was identified as Cu(I) that was formed from the reaction of trace amounts of Cu(II) with olanzapine in the sample solution. Unlike Cu(II), Cu(I) was retained under the ion-pairing HPLC conditions used for analysis. A reaction mechanism was postulated whereby Cu(II) present in the sample solution oxidizes olanzapine to a radical-cation, resulting in formation of Cu(I) and three oxidation products of olanzapine including a previously unknown oxidation product that was identified as hydroxy-olanzapine. Acetonitrile in the sample solution was necessary for the reaction to occur. As little as 100 ppb Cu(II) in the sample solution produced a Cu(I) peak, that by peak area, corresponded to about 0.1% relative to the olanzapine peak. The hydroxy-olanzapine oxidation product was also detectable, but the relative peak area was much smaller. To prevent formation of the Cu(I) artifact peak, EDTA was added to the sample solvent to complex any trace Cu(II) that might be present. The addition of EDTA was shown to prevent Cu(I) formation when Cu(II) was present at levels of 4ppm in the sample solution.