Trace levels of oxytetracylcine spiked into commercial milk samples are
extracted, cleaned up, and preconcentrated using a C18 solid-phase extraction column. The
extract is then analyzed by a high-performance liquid chromatography (HPLC) instrument
equipped with a UV detector and a C18 column (150 mm × 4.6 mm × 3.5 μm). An isocratic
mobile phase of 0.010 M oxalic acid/acetonitrile/methanol (in 15/2/2 ratio by volume) is
employed with an average elution time of 5 min. The method has been successfully used in
an upper-class instrumental analysis laboratory course to determine the concentration of
oxytetracycline at 50 μg/g in the spiked milk sample. Students’ results show an RSD of 18%
and a percent error of −1%.