Confirmation of gentamicin and neomycin in milk by weak cation-exchange extraction and electrospray ionization/ion trap tandem mass spectrometry.

A new procedure for the confirmation of two aminoglycoside antibiotics in milk was developed and validated. This work is among the early applications of ion trap mass spectrometry for regulatory methodology, and it incorporates a novel weak cation-exchange extraction. The procedure was validated for the confirmation of both gentamicin and neomycin at 30 ng ml(-1) and above. Milk is first treated with acid and centrifuged. The supernate, excluding the fat layer, is buffered with sodium citrate to neutral pH. The extract is applied to a weak cation-exchange solid-phase extraction column. Aminoglycosides are eluted with acidified methanol. Following separation by ion-pair liquid chromatography, analytes are ionized with an electrospray interface. Protonated molecular ions are selectively stored in an ion trap mass spectrometer, then collisionally dissociated to yield unique product ion spectra. Confirmation is based on matching spectral responses between samples and comparison standards consisting of a bona fide standard spiked into control extracts. Method performance was demonstrated with replicate samples of control milk, fortified milk, and milk containing incurred residues of each compound.

Gas chromatographic determination of incurred dimetridazole residues in swine tissues.

A gas chromatographic method for determination of 2-hydroxymethyl-1-methyl-5-nitroimidazole (DMZOH), the hydroxy metabolite of dimetridazole, in swime muscle has been developed. The method uses cleanup steps similar to those of an earlier polarographic method. The present method is capable of quantitating levels as low as 2 ppb and detecting less than 1 ppb. Recoveries from 30 control tissues spiked at 1, 2, or 4 ppb averaged 80.4%. Performance of the method in incurred tissue was documented and limited data on the depletion of the metabolite in muscle were generated. The muscle of swine given 150 ppm dimetridazole in feed for 14 days contained less than 1 ppb DMZOH at 12 h withdrawal time.

Pharmacokinetic study of sulfadimethoxine depletion in suckling and growing pigs.

Sulfadimethoxine was administered (IV) to suckling pigs (1 to 2 weeks old) and to growing pigs (11 to 12 weeks old) at a dosage of 55 mg/kg of body weight (single dose). Blood samples were collected over a 48-hour period, and the animals euthanatized were used for measurements of plasma and tissue concentrations of the drug. The blood data were described, using a one-compartment pharmacokinetic model. The blood concentration curves for the two groups of pigs had a consistent depletion pattern with greater than therapeutic concentrations (50 micrograms/ml) of the drug persisting through 12 hours after the drug was given. Sulfonamide blood concentrations were 4 and 11 times that of the method sensitivities in the older (growing) and younger (suckling) pigs, respectively, at 48 hours after treatment. In four of the five pharmacokinetic variables studied, a significantly higher (P less than 0.01) degree of efficiency was observed in the ability of the older pigs to eliminate the drug than in the younger pigs.

Excretion of hydroxylated polychlorinated biphenyl metabolites in cows' milk.

Four lactating Holstein cows were each given a single oral dose of either 0.5 g 2,5,2',5'-tetrachlorobiphenyl, 0.5 g 2,5,2',4',5'-pentachlorobiphenyl, 1.5 g Aroclor 1242, or 1.5 g Aroclor 1254 in gelatin capsules. Milk samples were collected twice daily at the am and pm milkings and were analyzed for polychlorinated biphenyl (PCB) metabolites and PCBs. Excretion of metabolites in the milk paralleled excretion of the parent PCBs, with maximum amounts eliminated 1 day after dosing. Less than 0.06% of the initial doses of the 2 single components appeared as metabolites in milk over a period of 10 days. Most of the metabolites occurred as conjugates. Only the 4-hydroxy metabolites were found in the milk of cows given single components. Milk samples from cows given Aroclor 1242 or Aroclor 1254 contained 10 and 4 monohydroxy metabolites, respectively. Using the same methodology, no monohydroxy metabolites were detected in whole milk samples from herds accidentally contaminated with a polybrominated biphenyl mixture.