Detection and structural elucidation of preservative-excipient isomeric esters of benzyl alcohol and citric acid in a topical hydrogel formulation on accelerated stability by LC-MS and benchtop NMR spectroscopy.

During routine accelerated stability testing of assay and related compounds for a marketed hydrogel formulation by ultra-high pressure liquid chromatography (UHPLC), two new peaks were detected at or near the ICH Q3B(R2) identification threshold. Preliminary examination of the extracted UV spectra for these peaks by the Quality Control laboratory showed that they contained an aromatic moiety (absorbance ∼255 nm), which the active component and its related compounds do not have. However, a preservative (benzyl alcohol) is present in the formulation and showed a similar extracted UV spectra as the unknown peaks. Analysis by liquid chromatography-mass spectrometry (LC-MS) determined that these peaks were isomeric monobenzyl-citrate esters. Synthesis, isolation, purification by fraction collection and analysis by benchtop nuclear magnetic resonance (NMR) determined the identity of each of the ester isomers seen in the stability testing.