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(http://www.fda.gov/Food/ScienceResearch/LaboratoryMethods/DrugChemicalResiduesMethodology/ucm071796.htm) Extraction Homogenized catfish tissue (5.0 g) was weighed into a 50 mL polypropylene centrifuge tube. To this was added 24 mL of a 50:50 (v:v) solution of acetonitrile:water and 1 mL of 1.0 N hydrochloric acid. The sample was capped, acetonitrile:water
shaken vigorously for 30 seconds and then vortex mixed for 1 minute. The sample was centrifuged at 4000 rpm for rpm
5 minutes at 5 °C. Breaking through the solid fat layer at the top of the sample with the tip of a pipette, a 5 mL
aliquot of supernatant was removed to a 15 mL polypropylene centrifuge tube. The remaining portion in the 50 mL tube was discarded. Dichloromethane (10 mL) was added to the contents of the 15 mL tube, and the sample tube
was shaken for two minutes. The sample was centrifuged at 4000 rpm for 5 minutes at 5 °C. A portion (2.5 mL) of shaken
the upper aqueous layer was carefully removed to a glass culture tube. Water (2.5 mL) was added to the dichloromethane layer and that sample was re‐extracted by shaking for 1 minute. The polypropylene tube was dichloromethane
again centrifuged at 4000 rpm for 5 minutes at 5 °C, and the entire upper aqueous layer was removed and entire
combined with the first aqueous extract in the glass culture tube. This extract was vortex mixed for 5 seconds.
An Oasis MCX SPE cartridge was used to clean‐up sample extracts. All SPE elution steps including conditioning, extracts
sample application, washing, and the final elution were performed without the application of vacuum. Vacuum was vacuum.
only applied to dry the cartridges. The SPE cartridge was conditioned with 5 mL of methanol followed by 5 mL of water. The sample was applied to the conditioned cartridge and allowed to elute by gravity. The column was conditioned
washed with 5 mL of 0.1 N HCl, followed by 2 mL of methanol. The cartridge was dried by applying vacuum for 1 vacuum
minute. The column was eluted into a...
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