Electrospray ionization mass spectrometry of tetracycline, oxytetracycline, chlorotetracycline, minocycline, and methacycline

The effects of mobile-phase additives and analyte concentration on electros pray ionization mass spectra of a series of tetracyclines were investigated in both positive and negative ion modes. Only [M + H](+) and [M - H](-) io ns were observed. The greatest sensitivity as [M + H](+) ions was obtained with 1% acetic acid and the greatest sensitivity as [M - H](-) ions was obt ained using 50 mM. ammonium hydroxide. Sensitivities in the positive ion mo de 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 obse rved for [M - H](-) than for [M + H](+) ions. Both [M + Na](+) and [IM + 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 obser ved 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 formatio n of the ESI sample ions. The utility of mobile phases containing 1% HOAc o r 50 mM NH4OH was demonstrated for chromatographic separations.