Enantiomeric Separation and Quantitation of ( )-Amphetamine, ( )-Methamphetamine, ( )-MDA, ( )-MDMA, and ( )-MDEA in Urine Specimens by GC-EI-MS after Derivatization with (R)-(-)- or (S)-(+)- -Methoxy- -(trifluoromethy)phenylacetyl Chloride (MTPA)

A new gas chromatography-mass spectrometry (GC-MS) method using (R)-(-)-alpha-methoxy-alpha-(trifluoromethyl)phenylacetyl chloride (MTPA) as a chiral derivatizing agent avoids racemization problems seen with the standard reagent (S)-(-)-trifluoroacetylprolyl chloride (TPC). The MTPA method reliably separates the (R)-(-)- and (S)-(+)-isomers of methamphetamine, amphetamine, MDA, MDMA, and MDEA. In 91% of methamphetamine-positive urine specimens, only the (S)-(+)-isomer—indicative of illicit use—was detected. For MDMA-positive specimens, (R)-(-)-isomer concentrations exceeded (S)-(+)-isomer, suggesting longer bodily retention of the (R)-(-)-form. Quantitation was linear over 25–10,000 ng/mL for most drugs, with correlation coefficients >0.996 and precision within ±11% at 500 ng/mL. The single MTPA method can replace two separate GC-MS tests needed to confirm illicit (S)-(+)-amphetamine and methamphetamine use.

Study at a glance

Abstract

In drug testing, the presence of methamphetamine in urine is generally confirmed by a gas chromatography-mass spectrometry (GC-MS) method. Derivatization of the compound to a perfluoroalkylamide, prior to confirmation, typically yields better chromatographic separation. Once methamphetamine is detected, a second GC-MS test is necessary to distinguish positive results from the use of over-the-counter medication, Vicks inhaler, or from use of a prescription drug, selegiline (Deprenyl). R-(-)-Methamphetamine is the urinary product from legitimate use of these medications. The second GC-MS test is to confirm illicit use of (S)-(+)-methamphetamine. In the procedure, the two methamphetamine isomers are changed to the chromatographically separable diastereomers by a chiral derivatizing agent, (S)-(-)-trifluoroacetylprolyl chloride (TPC). But the method has inherent limitations. Racemization of the reagent produces mixed diastereomers even from pure (S)-(+)-methamphetamine. Instead of using TPC, we utilized (R)-(-)-alpha-methoxy-alpha-(trifluoromethyl)phenylacetyl chloride (MTPA) to prepare the amides of diastereomers of methamphetamine. No racemization was observed with this reagent. The method was extended to resolve GC peaks of (R)-(-)- and (S)-(+)-isomers of amphetamine, 3,4-methylenedioxyamphetamine (MDA), N-methyl-MDA (MDMA), and N-ethyl-MDA (MDEA). Three ions from the drug and two ions from the deuterated internal standard were monitored to characterize and quantitate the drugs. For MDEA, only one ion was used. The quantitation was linear over 25 to 5000 ng/mL for MDEA and 25 to 10,000 ng/mL for all other drugs. Correlation coefficients were > 0.996. Precision calculated as the coefficient of variation at the calibrator concentration of 500 ng/mL was within +/- 11% for all drugs. The method was applied to test 43 urine specimens. In 91% of the methamphetamine-positive specimens, only the (S)-(+)-isomer was detected. In all MDMA-positive specimens, the concentrations of (R)-(-)-isomer were greater than the (S)-(+)-isomer indicating longer retention of (R)-(-)-isomer in the human body. The specimen concentrations (R + S) compared well with that of a non-chiral method that used 4-carboethoxyhexafluorobutyryl chloride as derivatizing agent. But the MTPA method has some advantage. It alone can replace the two GC-MS methods needed to confirm the presence of (S)-(+)-isomers of amphetamine and methamphetamine.