Fast atom bombardment mass spectral study of tetracycline antibiotics

In the fast atom bombardment mass spectra of tetracyclines, ammonia elimination fragment ions, [M + H – NH 3 ] + and [M + H – NH 3 – H 2 O] + , appeared with considerable abundances. A plausible mechanism for the loss of ammonia from [M + H] + was discussed based on results by measurements on various types of tetracyclines and benzamides as the model compounds and B/E linked scanning. The results indicated that the loss of ammonia occurred in the carboxyamide moiety in the A ring and was accelerated by an ortho ‐hydroxy group. The same fragmentations were observed in the electrospray mass spectra of tetracyclines with a skimmer‐capillary voltage differential of 150 V.