METHAMPHETAMINE . . .on Wipes by Liquid-Liquid Extraction: METHOD 9106, Issue 1, dated 17 October 2011 - Page 30 of 31
e. The problem of incomplete derivatization can be minimized by the use of an isotopic (e.g. deuterated) analog for each of the target analytes as the internal standard for that compound. This allows quantification in spite of incomplete derivatization. 2. Dilution Procedure B (dilution of a smaller aliquot of the original desorbate): If the sample was not completely derivatized or if large dilutions are needed (e.g. greater than about 1:5), the following procedure can be used. See NOTE in part 1 above. The procedure may also be used if derivatization was determined to be complete. a. Dilute an aliquot of the original aqueous acid desorbate of the wipe sample to 10 mL with desorption solution from a simulated sample blank, and re-extract. Add both the aliquot to be diluted and the diluting solution from the simulated blank directly to a clean 25-mL glass centrifuge tube (step 7f ) and proceed to step 8. For example, to make a 1:10 dilution, transfer 1 mL of original desorbate to the 25-mL tube and dilute with 9 mL from a simulated sample blank. b. The simulated sample blank should be prepared identically to the sample being diluted, using the same volumes of internal standard spiking solution and desorption solution that were used with the sample in the original desorption. For example, if the original sample was desorbed with 40 mL desorption solution with 80 μL of added internal standard spiking solution, then prepare the simulated blank in the same way. The volume of wetting alcohol is estimated (e.g. about 3 mL per 3”×3” (7.5 cm x 7.5 cm) 12-ply cotton gauze wipe). Include a dilution factor (V3/ V4) in the calculations in step 19 (e.g. V3/ V4 = 10 mL divided by the volume in mL of original desorbate diluted to 10 mL with solution from the simulated blank). The dilution factor in the above example is 10 mL/ 1 mL or 10. c. Correct for differences in internal standard spiking solution volumes in step 19 (if applicable) using for V1 the volume of internal standard spiking solution which was added to the original undiluted sample. NOTE: This dilution procedure gives quantitative results only if the residual volume of methanol (or isopropanol) used for wetting the sample wipes was exactly the same as the volume used in preparing the calibration standards (normally about 3 mL, see Table 7). Deviations of a few milliliters in residual wetting alcohol will not affect the results for undiluted samples, but will amount to an error of a few percent in the final results of samples that are diluted. d. The potential error due to differences in residual wetting solvent can be estimated for specific volumes of desorption solution and wetting alcohol. Assume the sample wipes and calibration standards are both desorbed in 30 mL of desorption solution and 3 mL of alcohol is added to the calibration standards. The potential error in volume (and final results) in the samples is approximately ±3 % (inversely proportional) per mL difference in the residual alcohol in the samples (i.e., ±1 mL difference in 33 mL). For 40 mL of desorption solution and 4 mL of alcohol added to the calibration standards, the error is ±2 % for every mL difference (i.e., ±1 mL difference in 44 mL). However, since the volume of residual wetting alcohol is not known and cannot be determined once the sample wipe has been desorbed, the actual error cannot be determined. However, the maximum possible error can be calculated. Since the maximum amount of alcohol that a recommended wipe can hold is about 6 mL when saturated (dripping wet), there can only be a deviation of plus or minus 3 mL from the 3 mL alcohol added to the calibration standards. Therefore, the maximum error in a result due to differences in the volume of residual alcohol in a cotton gauze sample compared to the standards can only be three times the error for a 1 mL difference in volume. Since the error for ±1 mL is ±3.03%, the maximum error for ±3 mL is three times larger, or ±9.1%. In practice, the error will be less than this because it is unlikely that the gauze samples will be completely dry or completely saturated after squeezing out the excess alcohol and wiping a surface. The practical amount of alcohol that remains in the wipes when the excess is squeezed out is between 1 and 2 mL. This translates into an error that is between +3% and +6% in the final results for diluted samples. Undiluted samples will not be affected. This error is within the overall accuracy for the method for methamphetamine.
Method rev. 1.1.1
NIOSH Manual of Analytical Methods (NMAM), Fifth Edition
