Determination of thyreostatics in animal feed by micellar electrokinetic chromatography

The determination of the thyreostatics 2-thiouracil, its derivatives (4-methyl-2-thiouracil, 4-propyl-2-thiouracil and 4-phenyl-2-thiouracil) and methimazole in manufactured dried animal feed by micellar electrokinetic chromatography (MEKC) is described. A 99 +/- 5% extraction yield at the 20 micrograms g-1 level (n = 8) was achieved by shaking the milled fodder with methanol-1 M NaOH (80 + 20). Aliquots of the supernatant were injected in a 75 microns x 33.5 cm uncoated silica capillary using pressure; separation was performed at 23 degrees C with 15 kV (positive polarity) in a background electrolyte (BGE) containing 40 mM sodium dihydrogenphosphate, 50 mM sodium dodecyl sulfate and 15 mM Tween 20 at pH 9. When the surfactants were added to the BGE, all the thyreostatics were well resolved and the fodder extracts showed lower backgrounds. The peaks appeared within the 2.25-5.2 min range with efficiencies in the 2.5 x 10(4)-8 x 10(4) range; methimazole appeared in the vicinity of the electroosmotic migration time. Calibration curves were linear within the studied range (20-200 micrograms ml-1, r2 > 0.998). Limits of detection in the extracts of spiked fodder samples ranged from 0.25 to 0.4 microgram ml-1, which corresponded to 0.6-1.0 microgram of drug per gram of fodder. Peak area repeatabilities were about 4% at the 20 micrograms ml-1 level.