Page:NIOSH Manual of Analytical Methods - 9109.pdf/31

METHAMPHETAMINE . . . on Wipes by SPE: METHOD 9109, Issue 1, dated 17 October 2011 - Page 31 of 33

a. Remedy #1: This artifact is eliminated by replacing MSTFA with MSHFBA (N-methyl-Ntrimethylsilyl heptafluorobutyramide, Alltech Associates, Deerfield, IL). However, precision and accuracy were not evaluated for NIOSH 9109 using MSHFBA instead of MSTFA. b. Remedy #2: If ephedrine compounds or compounds containing free hydroxyl groups are not to be analyzed, MSTFA might be omitted and MBHFBA used alone. 2. Use of the mixed reagent often results in over-silylation, the production of unintended silylation artifacts [18], particularly of amides. The primary over-silylation artifact with primary amines is the N-trimethylsilyl derivative of the N-acyl derivative. The GC peak area for this artifact can be significant; under certain circumstances it is nearly equal to that of the intended N-acyl derivative. a. Remedy #1: The presence of ammonium chloride from the SPE eluates seems to prevent or greatly reduce over-silylation of amides. These artifacts can be ignored when using the SPE columns with the 80:20:2 methylene chloride:isopropanol:ammonium hydroxide eluent. b. Remedy #2: If ephedrine compounds or compounds containing free hydroxyl groups are not to be analyzed, silylating reagents (MSTFA or its alternate, MSHFBA) might be omitted and MBHFBA used alone. 3. The mass spectrometer may need more frequent cleaning to maintain sensitivity. This is offset by the shorter sample preparation time, especially for large numbers of samples. 4. When the fused silica capillary columns become exposed to the mixed silanization-acylation reagents, the column may become unsuitable for other types of samples. 5. The chromatograms are cluttered with silylation by-products making it difficult to detect low levels of unknown (non-target) compounds if a drug screen for unknown compounds is an objective. For this objective, the liquid-liquid extraction procedure of NIOSH 9106 [4] provides cleaner chromatograms with less interference from reagent by-products. 6. The advantages of the mixed MSTFA+ MBHFBA reagent, when used with SPE, are as follows: a. Faster preparation time (no heating in an oven, no cool-down time, no evaporation or neutralization of the reagents, and no reconstitution with solvent thereafter). b. No heat or acid induced isomerization or dehydroxylation of the ephedrine or other hydroxyl containing compounds (e.g., ephedrine, norephedrine, pseudoephedrine, phenylephrine, etc.). c. The method can be extended to easily hydrolyzed phenolic and polyhydroxy compounds of aryl-alkyl-amines (e.g., Albuterol, epinephrine and metabolites [10], metabolites of MDMA, and phenylephrine) because of the thermal stability of the trimethylsilyl ether groups on phenols and trimethylsilyl ester groups. d. Hindered amines such as MDEA are derivatized more completely but still require an internal standard with structural similarity. G. MEASUREMENT: Recoveries for the laboratory control matrix spike samples (QC and QD) must meet the guidelines of the specific regulatory agency involved (80-120% is a reasonable target in the absence of specific guidance). NOTE 1: 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 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. NOTE 2: 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.

NIOSH Manual of Analytical Methods (NMAM), Fifth Edition

Method rev. 1.1.1