Electrospray Ionization Mass Spectrometry of Tetracycline, Oxytetracycline, Chlorotetracycline, Minocycline, and Methacycline

Publisher Website
Google Scholar
Abstract
The effects of mobile-phase additives and analyte concentration on electrospray ionization mass spectra of a series of tetracyclines were investigated in both positive and negative ion modes. Only [M + H]+ and [M − H]- ions were observed. The greatest sensitivity as [M + H]+ ions was obtained with 1% acetic acid and the greatest sensitivity as [M − H]- ions was obtained using 50 mM ammonium hydroxide. Sensitivities in the positive ion mode were greater than those in the negative ion mode. The sensitivity as [M + H]+ showed no systematic variation with pH; however, the sensitivity as [M − H]- did increase with increasing pH. A larger linear range was observed for [M − H]- than for [M + H]+ ions. Both [M + Na]+ and [M + H]+ ions were observed with 0.5 mM sodium acetate and sodium iodide, but no adduct ions were observed with ammonium acetate. Some M2H+ ions were observed at higher concentrations. Cluster ions, Na(NaOAc)n+ or Na(NaI)n+, but no sample ions were observed using 5 mM salts. The data suggest that mechanisms in addition to solution ionization are involved in the formation of the ESI sample ions. The utility of mobile phases containing 1% HOAc or 50 mM NH4OH was demonstrated for chromatographic separations.