Page:NIOSH Manual of Analytical Methods - 9106.pdf/27

 METHAMPHETAMINE. . .on Wipes by Liquid-Liquid Extraction: METHOD 9106, Issue 1, dated 17 October 2011 - Page 27 of 31

D. DESORPTION FROM MEDIA: 1.	An internal standard spiking solution volume of 60 μL was selected for ease in scaling from 60 μL per 30 mL to 80 μL per 40 mL of desorption solution. In either case the rate of 2 μL internal standard spiking solution per mL desorption solution was used. However, any convenient volume of internal standard spiking solution (e.g. 50 μL) that can be delivered reproducibly is acceptable. Whatever volume is chosen, there must be no variation in the volume of the internal standard spiking solution used in preparing each of the calibration standards. If spiking Strategy A is used (see APPENDIX D3), it is critical to know the exact volume of internal standard spiking solution that is applied to each sample (V1), the media blanks (V5), and the calibration standards (V2), since these volumes are used for internal standard spiking solution volume corrections in step 19. 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. Volume of Internal Standard Spiking Solution (μL)

Number of Wipes

Size of Shipping Container (mL)

Strategy A

Strategy B

Volume of Desorption Solution (mL) (Strategies A and B)

1 2 4 (e.g., Composite)

40-50 50 100-120

60 80 160

60 60 60

30 40 80

Apply volume correction factors at step 19.

Do not apply volume correction factors at step 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

NIOSH Manual of Analytical Methods (NMAM), Fifth Edition

Method rev. 1.1.1