METHAMPHETAMINE . . . on Wipes by SPE: METHOD 9109, Issue 1, dated 17 October 2011 - Page 29 of 33
μg/100 cm², which is low enough to be able to determine whether any discrete sample is at or exceeds the action level. Now if a composite of four wipes was taken, each with an area of 100 cm² for a total area wiped of 400 cm², the LOD for that composite sample is not 0.06 μg/400 cm² nor is it 0.015 μg/100 cm²; it is actually several times larger than 0.06 μg/400 cm². First of all it increases relative to the ratio of the volume of desorption solution used to desorb the sample compared to that used for the calibration standards. Secondly it has nothing to do with the AREA that was wiped, because the LOD for the calibration curve is determined in terms of μg per sample, independent of the area. To explain the first point, assume approximately 90 mL was used (for ease in calculation) to desorb the four wipes and 30 mL (the normal amount for a single wipe) was used to desorb each calibration standard. The calculation of the LOD for the four composited samples would be μg/sample x (desorption volume for 4 wipes) ∕ (desorption volume for the calibration standards), or 0.06 μg/sample x (90 mL/30 mL), or 0.18 μg/sample for the composited sample. Since the area wiped for the composite sample was 400 cm², the LOD for that sample could be expressed as 0.18 μg/400 cm². Regarding the second point, this value, 0.18 μg/400 cm², cannot be construed or mathematically reduced to 0.045 μg/100 cm² because it cannot be known whether three of the four wipes were blank and the fourth wipe just under the value of 0.18 μg. Hence, the effective LOD per individual wipe has to be regarded not only as 0.18 μg/400 cm² but also as 0.18 μg/100 cm² because any value determined for entire 400 cm² might have come from just one of those 100 cm² areas. Thus, for composite samples, the LOD must be expressed in terms of the entire area wiped and not extrapolated to some portion thereof. In this example, an LOD of 0.18 μg/100 cm² is above the action threshold of 0.1 μg/100 cm², meaning that this composite sample cannot satisfy the requirement that residual levels be below 0.1 μg/100 cm². It remains for the regulatory agency and not the laboratory to determine how to apply results for composite samples to the established action levels. The same consideration that is given above for the LOD applies to results that are greater than the LOD. To avoid confusion in reporting concentrations for composite samples, it is recommended that the sample concentration (in μg/sample, whatever the sample size) and the total area wiped (in cm²) be reported separately. For example, a result of 0.4 μg/sample for a sample consisting of four separate wipes of 100 cm² each (for a total area wiped of 400 cm²), is to be reported as 0.4 μg/400 cm² and not averaged to 0.1 μg/100 cm². This manner of reporting may be required by some regulatory agencies. 9. For quality assurance purposes, regulatory agencies may require duplicate samples to be taken in the field. If such is the case, an area contiguous with and adjacent to the first area, if possible, should be wiped as described under SAMPLING. Do not re-wipe the previously wiped area. This sample is a blind sample and should not be identifiable by the analytical laboratory as a duplicate of any other sample. These are distinct from the laboratory duplicates of a single sample described in step 14 of the method. Field duplicates are useful for evaluating the consistency of sampling technique, assuming uniformity of contamination on adjacent sampling sites. Laboratory duplicates are useful for evaluating consistency of sample preparation and instrumental analysis. 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 step D3 of APPENDIX), 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.
NIOSH Manual of Analytical Methods (NMAM), Fifth Edition
Method rev. 1.1.1
