High-performance liquid chromatography-thermospray mass spectrometry of ten sulfonamide antibiotics. Analysis in milk at the ppb level.

Ten sulfonamide antibiotics including sulfanilamide (SNL), sulfamethazine (SMZ), sulfamethizole (SMTZ), sulfachloropyridazine and sulfaquinoxaline (SQX), were analyzed by thermospray (TSP) mass spectrometry on-line with a high-performance liquid chromatography-UV detection system. Except for the pairs SMZ-SMTZ and sulfadimethoxine-SQX, the standards were resolved in both the UV and TSP profiles. Co-eluting compounds could be differentiated in TSP by their different relative molecular masses. The [M+H]+ ion was the base peak for all the standards except SNL, which showed an [M+NH4]+ ion. Collision-induced dissociation of the [M+H]+ ions afforded daughter ion spectra characterized by common ions at m/z 92, 108 and 156, and ions derived from the amine substituent ([MH-155]+). TSP detection limits [signal-to-noise ratio (S/N) > 3] were below 20 ng (scan mode), 2 ng (selected reaction monitoring, daughter ions from [M+H]+) and 400 pg (selected ion monitoring). UV detection limits were ca. 2 ng (S/N > 5). Results obtained from the multi-residue analysis of spiked cow milk samples at the low ng/ml level are presented.