Investigation of response factor ruggedness for the determination of drug impurities using high-performance liquid...

Quantification of impurities in drug substances and dosage forms using HPLC assays with UV detection is often done by comparison to a standard of the drug itself. Depending on the spectra of the compounds involved, small changes in wavelength may cause response factors to vary from day to day and instrument to instrument. Lack of assay ruggedness with respect to wavelength can lead to poor reproducibility of results. Response factor ruggedness was investigated for potential impurities in LY297802 tartrate, a potent muscarinic agonist. The UV responses of some impurities at 280 nm, the analytical detection wavelength, differ from that of the parent and change significantly with small shifts in wavelength. The ruggedness of response factors was examined on a single detector and among several different detectors. Results varied significantly among the different detectors. The UV spectra of the impurities could be used to predict the effect of wavelength on ruggedness of response. A wavelength system suitability sample is proposed as a way to overcome variability due to small differences in detector wavelength.