METHAMPHETAMINE . . .on Wipes by Liquid-Liquid Extraction: METHOD 9106, Issue 1, dated 17 October 2011 - Page 29 of 31
NOTE: The QC samples (QC and QD) in this method may be referred to in some guidance documents as matrix spike and matrix spike duplicate samples (MS/MSD), but serve the same purpose. Analyze and report field-equipment blanks as samples. Do not subtract their values from any other sample. Recoveries of Continuing Calibration Verification (CCV) standards must meet guidelines of regulatory agency (80-120% is a reasonable target in the absence of specific guidance). The CCV standards may be referred to in some guidance documents as “QC samples,” but such “QCs” are equivalent to liquid standards (not matrix spiked samples) and serve the same purpose of the CCVs in this method. With the GC/MS it is possible to achieve the lower limit of 0.05 μg or less per sample for methamphetamine in either the scan mode or SIM mode. The scan mode is essential where the identification of unknowns is an analytical objective. If lower limits of detection are desired or difficult to obtain in the scan mode, or for routine target compound only analyses, the instrument may be operated in the SIM mode. H. MAKING DILUTIONS: If the samples exceed the upper calibration range for the analysis, one of the following procedures may be used to estimate the high level concentrations. 1. Dilution Procedure A (dilution of the derivatized sample by reconstitution solvent): This option may be used only if the analytes in the sample were completely derivatized (see NOTE below). If derivatization was complete, transfer an aliquot of the sample from the GC vial (e.g. 0.2 mL for a 1:5 dilution) to a clean GC vial and dilute with reconstitution solvent (e.g. 0.8 mL for a 1:5 dilution), cap vial, mix, and reanalyze. However, dilution also dilutes the internal standard, and this procedure is useful only if the GC peak area for the internal standard is sufficiently measurable and the calibration curve is reasonably linear. Dilutions probably should not exceed a factor of 10. If this approach is used it is not necessary to enter a dilution factor in step 19 (V3/ V4) since both internal standard and analyte are diluted equally. The accuracy of this dilution procedure depends upon the linearity of the calibration curve in the extrapolated region beyond the upper end of the calibration curve. NOTE: Determination of Incomplete Derivatization: Incomplete derivatization can be caused by water, glycols, a large excess of analyte, or other contaminants that interfere with or compete for the derivatization reagent. If any one of the following symptoms appears, use Dilution Procedure B described below. a. An “oily” film (i.e., apparently viscous liquid) or unusual residue (e.g. grit) remains after being blown-down under nitrogen after derivatization (step 11d). This may be due to the presence of water, glycols, detergents, salts, or other contaminants. Incomplete derivatization has been observed with such residues. b. A very large (off scale) GC peak for any one of the derivatives (e.g. pseudoephedrine, a precursor for methamphetamine) indicates the possibility of incomplete derivatization for other analytes (e.g. methamphetamine) due to competition for the derivatization reagent. c. A smaller than usual GC peak area for the internal standard (<50% of the average) in undiluted samples suggests that something was competing for or inhibiting the derivatizing reagent. Such inhibition or competition for the internal standard will be experienced by the target analyte as well. d. Incomplete derivatization can be confirmed by the obvious presence of a GC peak for an underivatized target analyte. Underivatized analytes are not always detectable. Ephedrines usually do not show up on DB-5 capillary columns in this method, but GC peaks for underivatized secondary amines (e.g. methamphetamine) and for high levels of underivatized primary amines (e.g. amphetamine) can be detected, usually as irregularly shaped GC peaks, depending upon GC column conditions.
NIOSH Manual of Analytical Methods (NMAM), Fifth Edition
Method rev. 1.1.1
