METHAMPHETAMINE . . . on Wipes by SPE: METHOD 9109, Issue 1, dated 17 October 2011 - Page 30 of 33
2. It is not necessary to know the exact volume of desorption solution added to each sample or the volume of residual wetting alcohol because differences in the volumes are normalized through the use of internal standards added prior to desorption. 3. Alternate strategy for spiking internal standards (spiking strategy B below): By using the exact same volume of internal standard spiking solution in all samples, blanks, QC samples, and calibration standards, regardless of the volume of desorption solution added or residual wetting alcohol, the volume corrections in step 19 (V1/V2 and V5/V2) drop out of the equation. However, the internal standard GC peak areas must still be measurable in samples where larger volumes of desorption solution are used (such as for composite samples). Because of the increased dilution of the internal standard in larger samples, this approach should be limited to desorption solution volumes of about 120 mL or less. NOTE: There are two separate strategies for handling larger samples requiring larger volumes of desorption solvent. These are outlined below as strategies A and B.
Number of Wipes 1 2 4 (e.g., composite)
Size of Shipping Container (mL) 40 to 50 50 100 to 120
Volume of Internal Standard Spiking Solution (μL) Strategy A
Strategy B
Volume of Desorption Solution (mL) (Strategies A and B)
60 80 160
60 60 60
30 40 80
Apply volume Do not apply correction volume correction factors at step factors at step 19. 19.
With either strategy, if two gauze wipes were included in the samples, then use 40 mL of desorption solution. If four gauze wipes were included in the samples, then use 80 mL of desorption solution. a. In strategy A, the volume of internal standard spiking solution is kept at a constant ratio of 2 μL per mL of desorption solution added. This enables larger samples to be desorbed without diminishing the area of the GC peak for the internal standard. However, a volume correction factor (V1/V2 ) is needed in the final calculations in step 19. Therefore, the exact volume of internal standard added to each of the samples relative to that added to the calibration standards must be known. b. In strategy B, the volume of internal standard spiking solution is kept constant for all samples and calibration standards, but need not be exactly 60 μL. This enables the final calculations to be made in step 19 without a volume correction factor. However, the area of the GC peak for the 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. SOLID PHASE EXTRACTION PROCEDURE: Two columns (Clean Screen® and BOND ELUT-CERTIFY®) are based upon a silica support. The other two (Oasis® and Speedisk®) are based upon an organic polymer support. The precision and accuracy data in Tables 8a and 8b apply to the Waters Oasis® MCX 3cc/60 mg column. F. DERIVATIZATION: There are unique advantages and disadvantages in using the mixed MSTFA + MBHFBA reagent. The disadvantages with some possible remedies are listed as follows. 1. A few percent of trifluoroacetyl derivatives of secondary amines are formed (presumably from MSTFA) in competition to the intended heptafluorobutyryl derivatives. Method rev. 1.1.1
NIOSH Manual of Analytical Methods (NMAM), Fifth Edition
