Page:NIOSH Manual of Analytical Methods - 9106.pdf/28

 METHAMPHETAMINE. . .on Wipes by Liquid-Liquid Extraction: METHOD 9106, Issue 1, dated 17 October 2011 - Page 28 of 31

internal standard will vary with sample desorption volume and the internal standard must be concentrated enough to be measurable where larger volumes of desorption solution are used. E. DRYING COLUMN PREPARATION Using 1 cm i.d. × 12-15 cm long polypropylene columns having a fritted polyethylene disc or equivalent (see EQUIPMENT), add 1 gram (~0.8 cc) of anhydrous potassium carbonate (the bed dimension will be about 1.0 cm dia. × 1 cm long). Add 1 gram (~0.8 cc) anhydrous sodium sulfate on top of the potassium carbonate. Remove any particles clinging electrostatically to the outside surfaces. NOTE:	 Particles of the drying salts must not get into the collection tube, either through the frits or glass wool plugs, or from particles clinging electrostatically to the outside of the columns. Salts appear to inhibit derivatization efficiency. F. DERIVATIZATION: If isopropanol was used as the wetting solvent for the wipes, some of it will be co-extracted into the methylene chloride. In the presence of trace isopropanol, the crystal violet will go through a series of color changes as the extracts are evaporated to dryness. However, if methanol was used as the wetting solvent, the color of the crystal violet will remain blue to blue-violet at all stages of drying. Yet even with methanol, the same color changes can be afforded by adding 0.1 mL of isopropanol to the extracts prior to evaporation. Recoveries of analyte will not be affected in the absence of isopropanol, however, as long as the residues are dry before proceeding to step 11b. With the presence of a small amount of added or co-extracted isopropanol, as each sample concentrates, the color of the solution will go from a blue or violet color rapidly through green to a yellow color as the residue approaches dryness, which is indicative of increasing hydrogen ion concentration in the residual alcohol. Upon continued blowing with nitrogen, the color of the residue turns back to a green or blue hue just at the point of dryness, which is indicative of the loss of excess hydrogen chloride and/or alcohol. At this stage the samples are dry and may be removed. Continued blowing beyond this point may turn the dried residue to a deep blue-violet or violet color. Losses of analyte have not been experienced even after blowing for five additional minutes beyond the violet stage as long as the hydrochloric acid had been added. Color changes will not be as dramatic or will not develop if too much crystal violet is used. As the samples become concentrated, the tubes may be raised up in the water bath so that only the very bottoms of the tubes touch the surface of the water. This makes it easier to observe the color changes. The tubes may be raised out of the water bath, but blow-down times are lengthened. Prolonged heating at high temperatures during derivatization with the acidic conditions of the acid anhydride derivatizing agents promotes mutual isomerization between the ephedrine diastereomers (ephedrine and pseudoephedrine). Dehydration of the ephedrine compounds (ephedrine, norephedrine, and pseudoephedrine) also occurs to some extent to yield β-amino-β-methyl styrenes. Heating during derivatization for longer than one hour is especially not recommended. Thirty minutes is sufficient. 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. It has also been observed that in certain bulk samples, unknown constituents will cause the color of phenolphthalein to fade rapidly so that a purple color cannot be obtained at a pH >9, leaving only the blue color of the bromothymol blue. A quick check with pH paper can confirm that the pH is 9 or greater. G. MEASUREMENT: Recoveries for the laboratory control matrix spike samples (QC and QD) must meet the guidelines of the specific regulatory agency involved, if applicable (80-120% is a reasonable target in the absence of specific guidance).

Method rev. 1.1.1

NIOSH Manual of Analytical Methods (NMAM), Fifth Edition