METHAMPHETAMINE . . .on Wipes by Liquid-Liquid Extraction: METHOD 9106, Issue 1, dated 17 October 2011 - Page 4 of 31
e. Check the pH which should be about ≤ 4. Adjust the pH with diluted (2.5 to 3 M) sulfuric acid drop-wise, mixing the contents by shaking or inversion a few times after each addition of acid before checking the pH. f. After mixing, transfer 10 mL of supernatant to a 25-mL glass centrifuge tube. NOTE: If cleanup is to be performed on a subsequent day, store samples in a refrigerator. Analytes are stable in the desorption solution for at least one week refrigerated. 8. Cleanup: Potential contamination from oils, triglycerides, plasticizers and other hydrocarbons are reduced through a hexane back-extraction step. a. To each 10-mL aliquot of acid desorbate, add 10 mL of hexane, cap and mix on a rotary mixer for one hour. Allow to stand for 15-30 minutes for the phases to separate. If an emulsion forms, centrifuge the tubes at 1500-2000 rpm for a few minutes. If the emulsion persists, add about 0.5 mL of acetonitrile to the surface of the emulsion and gently mix the layers at the interface of the emulsion. Centrifuge again if necessary. b. Aspirate the (upper) organic layer to waste. Exercise care to not remove any of the aqueous layer. 9. Extraction of analytes into methylene chloride: a. Add 1-2 drops (20-50 μL) of the mixed pH indicator (phenolphthalein + bromothymol blue) solution to each sample. The color of the samples should be yellow, which indicates that the samples are sufficiently acidic for desorption of the analytes from the wipe samples. b. Add 0.5 mL of 10 M sodium hydroxide to each sample. The color of the samples should turn brilliant purple or magenta, confirming that the pH is greater than 9-9.5 (necessary for the extraction of the amines into methylene chloride). If the color remains yellow, or only turns green or light blue, check the pH with pH paper to confirm that it is greater than 9.5. If it is not, add another 0.5 mL of 10 M sodium hydroxide, mix, and check the pH again. NOTE: The color of the solution will gradually fade from purple to deep blue within about 20-30 minutes. This is due to the known tendency of phenolphthalein to fade at high pH. c. Add 10 mL of methylene chloride to each sample. Cap and mix on a rotary mixer for one hour. Allow to stand for 15 to 30 minutes. If an emulsion forms, centrifuge as described above (step 8a). d. Aspirate the aqueous (upper) layer to waste as described above, being careful to not remove any of the lower methylene chloride layer. 10. Removal of water from the methylene chloride extract: a. Prepare potassium carbonate-sodium sulfate drying columns. Note: See APPENDIX E for preparation of drying columns. b. Rinse the packed columns with about 6 mL of methylene chloride. Dry columns afterwards by forcing dry nitrogen or clean air through the top for 10-20 seconds. c. Arrange 14-mL collection tubes (16 × 100 mm test tube) in test tube racks. Add 6 μL of crystal violet solution and 100 μL of 0.3 M hydrochloric acid in methanol to each collection tube. NOTE: Crystal violet is not critical but helps later on as a visualizing aid for monitoring the progress of drying. Hydrochloric acid is critical to prevent loss of the amphetamines during evaporative concentration. d. Position the drying columns over the collection tubes. e. Transfer (decant) the methylene chloride layer into the drying column reservoir. After the last of the sample passes into the bed of the column, rinse the drying column twice with 1 mL of methylene chloride each time and combine with sample eluate. 11. Derivatization: See APPENDIX F for special instructions on derivitization. a. Evaporate the methylene chloride eluates in a nitrogen blow-down apparatus with the water bath set at 35 °C. Rinse the tips of the evaporation needles thoroughly with methanol or acetone between samples to prevent cross-contamination. When the samples are dry, remove and cap the tubes immediately.
Method rev. 1.1.1
NIOSH Manual of Analytical Methods (NMAM), Fifth Edition
