Introduction
Amphetamines are analogues of β-phenethylamine and have a stimulant effect on the central nervous system. Prolonged use results in addiction and dependence. Changes in the structural framework of the basic amphetamine nucleus results in different forms of amphetamines.
Absence of modifications on the aromatic nucleus results in a potent subgroup of amphetamines such as amphetamine and methamphetamine. Amphetamines may occur in various forms such as salts, which can be phosphates, hydrochlorides or sulphates. These forms necessitate the use of a myriad of analytical methods to facilitate proper identification and classification of amphetamines (Stinson & Berry, 1974).
Analysis
Analysis of amphetamine and methamphetamine is carried out in a series of steps that entails employment of numerous tests for conclusive identification of this group of stimulants.
Presumptive Analysis
These are autonomous tests that are employed in the initial rapid screening of samples suspected to contain amphetamine and methamphetamine. They aimed at alleviating false positives and as such are dependent on supplementary analytical tests for accurate determination of presence, level and type of amphetamine present. These analytical tests encompass a myriad of test methods aimed at mapping out the different characteristics of amphetamine and methamphetamine (Stinson & Berry, 1974).
Color Analysis
These tests offer high sensitivity and are rapid in nature. However analyst perceptions should be considered in interpretation of these tests. They entail use of special reagents and include three analytical methods namely Simon’s and Chen’s methods. Simon’s is specific to amphetamine and methamphetamine since the two are secondary amines. The results are confirmed by use of the marquis test. Amphetamines elicit an orange color in presence of marquis reagent (Stinson & Berry, 1974).
Analysis of anions
These tests are based on the differences in solubility properties of different amphetamines in presence of different solvents such as ethanol and water. Amphetamine and amphetamine salts are soluble in diethyl ether, ethanol and chloroform. However, they are only slightly soluble in water. Similar behavior is observed with methamphetamines and their salts.
Microcrystal Analysis
The principle behind these tests results from the ability of the amphetamine base to yield crystals in the presence of a particular chemical entity. The obtained product is subjected to analysis by use of polarizing microscope (Fulton, 1969).
Thin Layer Chromatographic Analysis
This method of analysis has gained acceptance due to its rapid nature, high sensitivity and its versatility in analysis of amphetamines and methamphetamines. The method entails comparison of the Rf of the amphetamine or methamphetamine compound with the Rf of a reference standard. The presence of the amphetamines is confirmed when the two Rf values are analogous (Stead, Gill, Wright, Gibbs & Moffat, 1982).
Gas Chromatographic Analysis
Gas chromatographic analysis of amphetamines employs the narrow bore columns with diameters of 0.2nm-0.32nm. Both qualitative and quantitative analysis can be achieved by gas chromatography. The retention time of the suspected amphetamine is judged against to the retention time of a reference standard. When qualitative analysis is carried out, amphetamine elutes earlier when compared to methamphetamine (Karl, Hans & Armin, 2000).
Quantitative gas chromatographic analysis entails utilization of three methods. These are the unit standard technique, multiple-standard technique exclusive of derivatization and multiple-standard technique with derivatization. Detection may either utilize the flame ionization detector or the mass spectrometer. In both cases the retention time of the sample of the amphetamine or methamphetamine is matched to the retention time of the reference standard (Karl, Hans & Armin, 2000).
High Performance Liquid Chromatography
This method is the mainstay for forensic analysis of drugs of abuse. It employs a versatile column called the octadecyl silica column (C18) in analysis. By utilizing visualization methods such as UV light and diode array, the retention times of the sample of the drug is matched to that of the reference standard. Amphetamine and methamphetamine are eluted after other stimulants such as norephedrine and ephedrine (Malone, 1998).
Fourier Transform Infrared (FTIR) Spectroscopy
To determine the structure of amphetamine and methamphetamine, the Infra red technique is utilized. It can either be applied qualitatively or quantitatively. The method finds use in fast testing of tablets of the two drugs. The basis of identification is the matching of the spectra of the drug with the spectra of the reference standard.
Optical Isomer determination
Since amphetamine and methamphetamine contain an asymmetric carbon, they are capable of occurring in more than one enantiomeric variety. These optical isomers are regarded as different chemical and pharmacological entities hence the need for a method to accurately identify the various forms of occurrence of these drugs.
Optical isomer determination entails the use of melting points and such it has been shown that the hydrochloride salts of the d- and l-methamphetamine exhibit similar melting points (170-175°C), but on mixing the racemic mixture has been shown to have a lower melting point (130-135°C).
In addition, analysis of micro-crystals can be carried out and both the isomers have been shown to produce structurally similar micro-crystals. However, the racemate results in crystals structurally different from the enatiomers facilitating identification (Cunningham, 1973).
Conclusion
Amphetamine and methamphetamine have of late become among the major drugs all over the world. Due to the potential for abuse, proper identification of these drugs is critical. To achieve this, proper and reproducible analytical techniques need to be employed to properly and accurately identify these drugs.
References
Cunningham, M. D. (1973). Rapid and sensitive technique for the differentiation of the optical isomeric forms of methamphetamine and amphetamine. Microgram, 6(6), 87-95.
Fulton, C. C. (1969). Modern microcrystal tests for drugs. New York, NY: Wiley-Interscience.
Karl, P., Hans, M., & Armin, W. (2000). Mass spectral and GC data of drugs, poisons, pesticides, pollutants and their metabolites. New York, NY: Wiley-Interscience.
Malone, J. V. (1998). HPLC quantification of clandestinely manufactured mixtures of amphetamine and methamphetamine. Microgram, 31, 304-307.
Stead, A. H., Gill, R., Wright, T., Gibbs, J. P., & Moffat, A. C. (1982). Standardized thin layer chromatographic systems for the identification of drugs and poisons. Analyst, 107, 1106-1168.
Stinson, S.B., & Berry, M.R. (1974). Separation and identification of amphetamine or methamphetamine in combination with ephedrine or caffeine. Microgram, 7(4), 51-54.