Determination of liquid chromatographic peak purity by electrospray ionization mass spectrometry

A technique is described whereby the purity of an HPLC peak can be determined using electrospray liquid chromatography-electrospray ionization mass spectrometry. Electrospray mass spectra acquired across an HPLC peak are summed and examined for co-eluting impurities. The mass spectrometer is set up to produce solely cationised molecules and background noise is minimized so that minor coeluting impurities can be observed down to a level of <0.1% of the major component. This method offers advantages over using diode-array UV detection (LC-DAD) for the determination of HPLC peak purity, namely components with similar UV spectra can be distinguished, the molecular mass of the impurity can be determined and structural data can be obtained by using tandem mass spectrometry (RIS-MS). The effectiveness of the technique is demonstrated with drug substances of pharmaceutical interest which have been chromatographed on an HPLC system designed to intentionally co-elute a number of impurity standards with these compounds.