Qualitative analysis
Gas chromatography and mass spectrometry Using capillary gas chromatography (GC) (operated under a suitable temperature programme), coupled to a mass spectrometer (GC-MS) the drug components of most samples can be separated and identified. The reduced capital outlay now required for such instruments means that it is not uncommon for laboratories to have several instruments working with automatic samplers enabling GC-MS analysis on a 24-hour basis. The use of GC-MS has become the routine method of identification of most drugs. A general GC-MS screen method can be used to separate and/or identify most of the drugs encountered in exhibits. Figure 1 shows the separation achieved of a mixture of the main drugs described here using a general screen method. There are literally dozens of GC methods available for the analysis of drugs of abuse and interested readers should consult Moffat et. al. (2004) for further details. Identification of the various components of a suspect mixture can be made with a search of commercial libraries, but it is important to run a standard of the specific drug being tested (e.g. standard diamorphine). This obviously needs to give a retention time and mass-spectral match.
High-performance liquid chromatography High-performance liquid chromatography is a simple and reliable method of analysis for most drugs. Operated correctly, it is both accurate and precise and thus lends itself to quantitative analysis. It is especially useful for compounds that are thermally labile. HPLC has some advantages over GC because of the variety and combinations of mobile phases that can be chosen. There is also a choice of detectors available for specific applications. HPLC can, however, involve significantly more method development than GC, which is capable of resolving a greater number of substances. No single system is suitable for the optimum separation of all the different drug types, so different systems are used to give optimum separations for specific analysis. The system that is best for the separation of heroin–acetyl codeine–noscapine–papaverine (Huizer 1983) is not the same as the one that separates cocaine from its impurities and processing by-products (Moore and Casale 1994).
Figure 1 Gas chromatographic separation on an HP Ultra-1(cross-linked methylsiloxane) capillary column (12.5m 0.2mm 0.33lm phase thickness). He carrier gas at 1mL/min; 50:1 split ratio; temperature programme 60C for 2 min, then 15C/min to 180C, then 25C/min to 290C and hold 3min. Mass spectrometer operated in electron-impact mode, scanning from 40 to 550 amu (S. D. McDermott, unpublished information.)
A general screen method can be used for the separation of heroin, cocaine, amfetamine and methamfetamine (Fig. 2). The reader is referred to Moffat et al. (2004) for further details of HPLC systems that can be used for the analysis of drugs of abuse.
Using the chromatographic conditions shown in Figure 2, methamfetamine and MDMA co-elute. However, by changing the relative pro portions of the acetonitrile–triethylammonium phosphate buffer mobile phase and reducing the flow rate, amfetamine, methamfetamine, MDMA and MDEA can be separated (Fig. 3). This illustrates the versatility of HPLC – separations can be made between compounds that co elute by altering the elution system. A system such as the above could be used for screening purposes, but identification of substances necessitates a spectroscopic method, such as MS or infrared (IR) spectroscopy.
Figure 2 HPLC analysis of amfetamine, methamfet amine, diamorphine and cocaine on Spheris orb ODS-1 (150 mm X 4.6 mm) at 30C. Mobile phase: acetonitrile–triethylammonium phosphate buffer (pH 2.5) (50:50 v/v) at 1.5 mL/min. Note that methamfetamine and MDMA co-elute on this system.
Figure 3 HPLC analysis of amfetamine, methamfet amine, MDMA and MDEA on Spherisorb ODS-1 (150 mm X 4.6 mm) at 30C. Mobile phase: acetonitrile–triethyl ammonium phosphate buffer (pH 2.5) (20:80 v/v) at 1.0 mL/min. Note that methamfetamine and MDMA are resolved on this system.
Fourier transform infrared spectroscopy
Most modern laboratories are now equipped with Fourier transform IR (FTIR) spectrophotometers, which have many advantages over traditional IR instruments. They are faster and can work with smaller samples (when coupled with a microscope, tiny samples can be analysed). The difficulty with IR analysis of drug samples is the presence of other material that interferes with the spectrum. These interfering compounds could be other drugs that occur naturally in the samples (or from the synthetic process) or adulterants, such as caffeine and paracetamol (acetaminophen). IR analysis can, however, give valuable information on chemicals that are not suitable for GC-MS analysis. Another popular technique is GC-FTIR, because the speed of scanning of the FTIR instrument means it can be used to obtain a spectrum of compounds that have been separated by GC. Like GC-MS, this can provide confirmation of drug identity. In practice, neither spectra nor pure reference samples may be available for comparison for the more unusual substances which can occur in drug samples. In such situations, nuclear magnetic resonance (NMR) spectroscopy may be the method of choice.
