Liquid chromatographic determination of depletion of bithionol sulfoxide and its two major metabolites in bovine milk

A liquid chromatographic method is described for determining bithionol sulfoxide and its metabolites, bithionol and bithionol sulfone, in milk. Samples are treated with HCl to precipitate proteins and to permit extraction of bithionol sulfoxide in nonionized form. Tetrahydrofuran is added to the organic phase to facilitate extraction in diethyl ether; the dried residue is dissolved in chloroform, hexane, and sodium hydroxide and subjected to LC analysis. Residues of bithionol sulfoxide and its 2 metabolites were determined in milk of lactating cows. Holstein-Friesian dairy cows were administered a single oral dose of bithionol sulfoxide (50 mg/kg). Milk samples were analyzed with a reliable detection level of 0.025 microgram/mL for each compound. Residues of bithionol sulfoxide and bithionol were detected during 30 and 16 milkings, respectively; bithionol sulfone was never present at detectable levels.