Liquid Chromatograph Mass Spectrometer
Structural prediction of impurities in drugs using MSn data Impurities in an erythromycin sample
Fig.1 and Fig. 2 show the UV and MS chromatograms of Erythromycin A oxime (Ery-AO) which is a macrolide antibiotic produced by a strain of bacteria known as Saccaropolyspora erythraea. Some impurities are indicated.
Fig.3 shows the mass spectra of erythromycin A oxime (Ery-AO). The Ery-AO molecule is shown as m/z = 747.4661. The mass difference between the acquired figure and theoretical figure(747.4643) is approx. 0.002.
The MS/MS spectra of the m/z = 747 is shown in Fig.4. The big peak of the m/z = 571 is considered as the loss of Area A from the Ery-AO structure since mass difference with m/z = 747 is 176. The m/z = 396.2416 is considered as Area C. It is highly reliable figure due to the 0.003 mass difference compared with the theoretical figure.
Fig.6 shows the mass spectra of impurity A. MS/MS spectra indicates a similar structure with Ery-AO.
The mass difference of m/z=396.2406 is 0.002 compared with the theoretical figure of Area C(396.2386). m/z=571 is also detected and it is similar to the MS/MS spectra of Ery-AO. However, the molecular weight of an impurity A is 733, and the mass difference between the impurity A and the loss from that is 162. While the mass difference from Area A of Ery-AO is 14.0144. Using the Accurate Mass Calculator, this figure is considered as CH2 (14.0156), and it is assumed that the impurity A has a structure which is substituted from one methyl group of Area A to one hydrogen.
Using the MS/MS spectrum and accurate mass information, which shows structural information on Ery-AO, other impurity’s structure was also predicted. In addition, considering the MS2 spectrum patterns of the impurity, different from that of Ery-AO, it is assumed that it was externally mixed into the sample.