Confirmation of multiple sulfonamide residues in bovine milk by gas chromatography-positive chemical ionization mass spectrometry.

Gas chromatography-mass spectrometry (GC-MS) using positive chemical ionization was utilized to confirm the presence of 10 ng ml(-1) of nine sulfonamides (SFAs) in bovine milk (50 ml). After the addition of a surrogate and hydroxylamine hydrochloride, the SFAs are extracted with ethyl acetate followed by cyclohexyl solid-phase extraction clean-up. The methylamidotrifluoroacetyl derivatives are prepared and analyzed in selected ion monitoring mode. For regulatory confirmation, the required specificity was achieved by monitoring the molecular ion plus three to five fragment ions for each SFA. Retention times for all SFAs were within 0.1 min of their respective standard. The relative ion abundances were within 10% of those obtained with standards diluted to the same concentration, analyzed on the same day. Concentration was critical, especially for the early eluting SFAs, as the enhancement of the relative abundance of the parent was more pronounced in extracted samples then in the standards. The sensitivity of the mass spectrometer for the different SFAs varied greatly. The amount of SFA necessary to obtain spectra that would meet the confirmation criteria varied from 25 ng on column for the least sensitive to less than 3 ng for the more robust.