Famotidine is in a class of medications called histamine H2-receptor antagonists and is being used to treat ulcers and gastroesophageal reflux disease. Azithromycin (Zythromax) is a semi-synthetic macrolide antibiotic which is effective against a wide variety of bacteria organisms. Azithromycin has a long half-life allowing for once a day dosing and for shorter treatment courses for most infections.

Mass spectrometry has played a significant role in the quantitative analysis of famotidine and azithromycin. However, detailed CID studies of these drugs have not been reported.

The present study was conducted to assess the structures and mechanisms of formation of principal fragment ions in the ESI mass spectra of these three drugs. The CID mass spectra of famotidine and azithromycin and their corresponding deuterated analogs have been studied in both positive and negative ion modes. Decomposition mechanisms are proposed for the principal fragment ions using H/D exchange and the combination of multiplestage CID at low collision energy with high-resolution mass measurement. The mass spectra of famotidine and azithromycin can serve as useful models for structural determination of chemically or biologically modified famotidine and azithromycin or related compounds.

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