Multiresidue analysis of sulfonamides using liquid chromatography with atmospheric pressure chemical ionization mass spectrometry

Sulfonamide antibiotics are widely used to prevent bacterial infections in livestock, and residues are commonly found in milk and meat. This is of toxicological and regulatory concern because sulfamethazine produces thyroid tumors in rodents. Unadulterated milk was purified using solid‐phase extraction and six sulfonamides were added prior to analysis by liquid chromatography coupled to atmospheric pressure chemical ionization mass spectrometry. The mass spectra of all sulfonamides consisted exclusively of protonated molecules at low sampling cone voltage and, under these conditions, estimated detection limits were 0.05–0.5 ng, corresponding to 0.25–2.5 ppb in milk. The response curve for sulfamethazine was linear over a concentration range (1.25–20 ppb) that encompasses the regulatory level of interest in milk (10 ppb). Increasing the sampling cone voltage caused collision‐induced dissociation of sulfonamides that resulted in formation of ions that are diagnostic for the variable heterocyclic base and constant sulfanilyl portions of the molecule. At intermediate sampling cone voltages, multiple fragment ions were formed with reproducible intensity ratios. This process can be used to provide additional confirmation of target sulfonamides. Monitoring a common fragment ion formed at high sampling cone voltage permitted class‐specific multiresidue detection of the six sulfonamides with minimal losses in sensitivity. An analytical method is proposed for the detection, confirmation and quantification of sulfonamides and metabolites in milk with sensitivity equal to or greater than those obtained with less specific methods involving liquid chromatography with a UV detector. The relative simplicity of this technique and the availability of benchtop instrumentation suggests that routine screening and confirmation analyses of sulfonamide contaminants in milk is practicable.