METHAMPHETAMINE . . . on Wipes by SPE: METHOD 9109, Issue 1, dated 17 October 2011 - Page 33 of 33
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 3”x3” 12-ply (or 4”x4” 8-ply) cotton gauze 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. 3. Dilution procedure C (dilution of desorbates from dried samples): Dilution errors for over-range samples may be corrected by knowing the exact amount of residual alcohol in the samples. The volume (or weight) of residual solvent in each gauze wipe might be determined by the difference between a wet weight and dry weight. Better yet, the error might be eliminated for diluted samples by adding, after the samples are dried (without taking any weight), the same known volume of wetting alcohol that is added to the calibration standards (i.e. 3 mL). Thereafter, if any samples need dilution, there will be no dilution errors due to differences in residual alcohol, because all samples and standards will have the same volume of alcohol and total volume of desorption solution. However, air drying of the samples is not recommended because of the possible loss of methamphetamine due to its volatility when it is not in the salt form, which form cannot be assured in field samples. Also, manipulating the samples for weighing and drying might introduce contamination. Drying is not recommended as a procedure for analytes having a vapor pressure high enough to be lost in the process, or that tend to form azeotropes with alcohols, especially when the critical action levels for remedial cleanup are at the lower end of the method calibration range. Drying is not an option if the samples have already been desorbed.
NIOSH Manual of Analytical Methods (NMAM), Fifth Edition
Method rev. 1.1.1
