Electrospray ionization collision-induced dissociation tandem mass spectrometry of amoxicillin and ampicillin and their degradation products.

RATIONALE

Detailed analysis of the literature results on the electrospray ionization mass spectrometry (ESI-MS) fragmentation of amoxicillin and ampicillin, and their comparison with our results, have revealed some incorrect suggestions or incomplete interpretations of mass spectra of these compounds. Therefore, this paper contains a comprehensive discussion devoted to the ESI-MS/MS of ampicillin and amoxicillin as well as their degradation products, namely products of hydrolysis and methanolysis.

METHODS

Electrospray ionization collision-induced dissociation tandem mass (ESI-CID-MS/MS) spectra and accurate mass measurements were made on a quadrupole time-of-flight (Q-tof) mass spectrometer. Hydrolysis of the antibiotics was performed by heating, for a few hours, their aqueous solutions adjusted to pH 10. Methanolysis of the antibiotics was performed by heating their methanol solutions for a few minutes. Additionally, mass spectra of isotope-labeled compounds were also obtained.

RESULTS

A number of fragment ions, previously wrongly interpreted or not interpreted, have been rationalized. For example, formation of an abundant fragment at m/z 208 originating from the protonated amoxicillin molecule (ion Amox + H) was previously rationalized as a result of breaking of two bonds of the β-lactam ring. We found that this fragment ion had to be formed by the loss of ammonia and breaking of three bonds of the bicyclic system.

CONCLUSIONS

The discussion presented enables a better understanding of the MS decompositions of amoxicillin and ampicillin as well as their degradation products. MS decomposition is used for the determinations of these compounds, when the so-called multiple-reaction monitoring is applied during liquid chromatography (LC)/ESI-MS analysis. Thus, better understanding of MS decompositions of the above compounds seems to be important.