Analysis of amfetamine and methamfetamine
As many of the street-level samples submitted to the laboratory are relatively low in purity (5%), pre-concentration of samples may be required for the analysis to be successful. Typically, 100 mg of sample is added to 1 mL of sodium hydroxide solution (0.5 M) and extracted with 1 mL diethyl ether, which is decanted and evaporated to dryness in an airflow without heat. A few drops of methanol can then be added to solubilise the drug.
The Marquis test gives an orange colour for both amfetamine and methamfetamine.
Separation and/or identification It is common practice with primary amines to prepare derivatives such as N-methylb is (tri fluoroacetamide) (MBTFA) or trifluoroacetic an hydride (TFAA) derivative for GC analysis. It is good practice to analyse both derivatised and underivatised samples, since N-hydroxyamines may give the same product as the parent amines.
Using a concentrated and/or base-extracted sample, the m/z 134 and m/z 148 molecular ion peaks for amfetamine and methamfetamine, respectively, can be achieved readily in an underivatised sample. As can be seen from the mass spectra (Fig. 1), the molecular ions for both drugs are of very low abundance and there are few other characteristic high-mass ions in the spectra. The only abundant ions are m/z 44 and 58 for amfetamine and methamfetamine, respectively, which are of low mass. This can cause problems for confirmation of the presence of the drug in a sample and hence it is more common to derivatise these substances to produce a mass spectrum which has more ions for matching against the standard substance similarly derivatised (Fig. 1).
Both amfetamine and methamfetamine have one asymmetric carbon atom that results in a pair of enantiomers in each case (Fig. 2). Depending on the synthetic route, l-, d- and dl-amfetamine or methamfetamine could be encountered in samples submitted to the laboratory for analysis. These optical isomers differ in their pharmacological activity and are subject to different regulatory measures in certain countries. In those countries in which the specific optical isomer needs to be identified, chiral analysis can be undertaken by derivatisation/ GC, by the use of chiral columns (GC and HPLC) and, more recently, by the use of capillary electrophoresis.
Quantitative analysis and profiling of amfetamine and methamfetamine Amfetamine and methamfetamine can be quantified by HPLC or GC. Normally, if GC is used, the samples are base-extracted into an organic solvent and either run directly or derivatised and then run. Using HPLC there is no need to extract; the sample can be dissolved in a suitable solvent, filtered if necessary and injected. In many cases HPLC is the preferred method for quantitative analysis of amfetamine and methamfetamine. Amfetamine produced illicitly often contains impurities that result from the manufacturing process. The presence of these impurities can be used to compare and distinguish samples of amfetamine, since material used in the same manufacturing batch would almost certainly have the same number and relative amount of identical impurities. Samples from the same illicit laboratory produced at different times may show strong similarities, whereas samples from unrelated laboratories are expected to show major qualitative and quantitative differences. Basic extracts into organic solvents are subjected to GC or GC-MS analysis. Samples are compared by visual inspection of the GC trace and by quantitative comparisons. Methamfetamine impurity profiling is also carried out by GC analysis with the impurities also giving information on the synthetic route (Seta et al. 1994).
The purpose of comparison and/or profiling is to identify dealer–user links, establish possible sources (i.e. the clandestine laboratory) and build up databases to allow interpretation in comparison casework.
Figure 1 Mass spectra of (A) amfetamine and (B) methamfetamine (upper panels) and their trimethylsilyl (TMS) derivatives (lower panels). The spectra for the nonderivatised drugs show low-mass ions only, in contrast to the TMS derivatives which produce higher mass ions and more characteristic mass spectra. (Courtesy of NIST/EPA/NIH Mass Spectral Library.)
Figure 2 l and d Isomers of amfetamine.
