Am. Kamel et al., Electrospray ionization mass spectrometry of tetracycline, oxytetracycline, chlorotetracycline, minocycline, and methacycline, ANALYT CHEM, 71(5), 1999, pp. 968-977
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.