Identifying and Quantitating Controlled Substances

Whenever a controlled substance is identified, the possibility exists that an individual could be imprisoned or suffer some other legal consequence as a result. There is, therefore, an absolute, uncompromised requirement for certainty in the identification of controlled substances. Prior to 1960, the results of microscopic crystal tests, color screening tests, and TLC were considered definitive. From the 1960s through the mid-1970s, ultraviolet spectrophotometry and GC gained acceptance. It is interesting in 1997 to look back 20 years and contemplate the absolute faith placed in a retention time on a gas chromatogram, or upon the ultraviolet absorption maxima in acidic or basic solutions. In some instances these numerical values were measured with a ruler!

From 1975 through 1985 there were major advances in IR and MS. During those years "specificity", as we understand the term today, was, for the first time, actually attainable in most cases. As the technology continually evolved, with increased Fourier transform peak resolution in IR and NMR, and multi-component separations improved with capillary column gas chromatography, specificity also increased.

In the mid-1980s the advent of "designer drugs" (properly referred to as "controlled substance analogues") resurected the problem of specificity. In attempts at circumventing existing controlled substance laws, clandestine laboratory chemists began to alter chemical structures of controlled drugs by increasingly sophisticated syntheses. By replacing a methyl group with an ethyl group, or by using a five-membered ring instead of a six-membered ring in a synthesis, these clandestine laboratory chemists developed what at the time were non-controlled analogues. The Controlled Substance Analogue and Enforcement Act of 1986 was passed by Congress, largely as a response to this problem. This particular piece of legislation also reinforced the responsibility of the chemist to accurately discriminate between controlled substances and endless lists of possible analogues.

A direct consequence of the new law's passage was the development of analytical procedures in Fourier Transform Infrared Spectrophotometry (FTIR), Fourier Transform Nuclear Magnetic Resonance Spectroscopy (FTNMR), Gas Chromatography/Fourier Transform Infrared Spectroscopy (GC/FTIR), and CE. These instrumental methods have made their way into the forensic science laboratory and now provide the increased specificity required by the courts.

Controlled substances sold on the street are usually mixed with adulterants and diluents in a crude and mostly unspecified manner. In some laboratories, the analysts are required to identify and quantitate both the controlled substance and the adulterant drugs and diluent materials. Color tests, thin layer chromatography, and microcrystal tests of the pre-1960s vintage are still used for screening. These testing procedures were valid then and are still valid today, but today additional instrumental techniques are utilized to make the absolute identification and quantitation.

After the analysis has been completed, it must be documented. The final report must be clear, concise, and accurate, with all conclusions substantiated by analytical data. The data may be in the form of notations on paper in the analyst's writing, or on chromatograms, spectra, or other instrumental printouts. Dates must be checked, and the documented description of the exhibit(s) must be consistent with the actual exhibit. Each time a report is signed, the analyst places his reputation and credibility before the scrutiny of the court and his peers. Discovering a "mistake" after the report has been submitted to the courts is not good.

Cocaine can exist as either the hydrochloride (HCl) salt or as the base. Pursuant to federal law, there are sentencing guidelines based on the identification of cocaine as either the base or as the salt form (usually HCl). Cocaine can be adulterated with benzocaine, procaine, lidocaine, or any combination of these non-controlled drugs, and further diluted with manni-tol, lactose, or other processing sugars. A variety of instrumental techniques can be used to distinguish cocaine HCl from cocaine base. FTIR spectrophotometry is commonly available and used in many laboratories. The IR spectra of cocaine HCl and cocaine base are quite different and easily distinguished. The IR spectrum of a cocaine HCl sample mixed with an adulterant presents a problem. The same sample analyzed by GC/FTIR presents the chemist with a total response chromatogram showing all peaks in a mixture. The resulting IR spectrum and mass spectrum are identifiable. However, in this technique, cocaine HCl and cocaine base cannot be distinguished. At this point, NMR can provide a solution to distinguishing the two forms of cocaine and identifying the adulterants.

The solubility properties of controlled substances can be used to separate different forms of controlled substances. For instance, cocaine base is soluble in diethyl ether, cocaine HCl is insoluble. Therefore, if an analyst is analyzing a material which is believed to be cocaine in a questionable form, he can try placing the material into solution with diethyl ether, separate the ether from the insolubles, evaporate the diethyl ether, and analyze the resulting powder by GC/MS. The resulting cocaine spectrum would indicate the presence of cocaine base because cocaine HCl would not have gone into solution.

Methamphetamine is produced in clandestine laboratories from the reaction of ephedrine with hydriodic acid and red phosphorus, or from the reaction of phenyl-2-propanone (P-2-P) with methylamine. Methamphetamine samples submitted to the forensic science laboratory usually contain precursors from the synthesis, by-products for side reactions, and adulterants such as nicotinamide which has been added by the clandestine laboratory operator. As is true of the mass spectrum of some other phenethylamines, the mass spectrum of methamphetamine may not provide enough specificity for positive identification. The most accurate way to identify many phenethylamines is with IR. However, NMR is at least as specific as FTIR, and it also allows for an identification in the presence of diluents. Unfortunately, NMR is not available in many laboratories. Nicotinamide is one of the more commonly encountered adulterants with methamphetamine and can easily be distinguished from isonicotinamide by NMR spectroscopy.

The IR spectrum of methamphetamine hydrochloride in a potassium chloride salt matrix is very specific, and GC/FTIR is excellent at separating the components of a methamphetamine sample. However, this method requires great care in selecting the optimized temperature and flow parameters, and column selection.

GC/MS is the method most often used for identifying heroin. The mass spectrum of heroin is very specific. Heroin is relatively simple to separate, and identification of the degradation products and the by-products of the heroin synthesis, from morphine and acetic anhydride, is relatively straightforward. Because morphine is derived from opium, many of the by-products from the opium processing are carried over to the final heroin product. Acetylcodeine and acetylmorphine are clearly identified from the corresponding mass spectra. The GC/FTIR also provides excellent spectra for making identifications of heroin, its by-products, degradation products, and precursors. The chloroform insoluble diluents from heroin samples can also be identified in a potassium bromide matrix by FTIR. These materials will usually consist of sugars such as mannitol and inositol. When the heroin has been isolated from diluents and adulterants, FTIR and NMR can be utilized to confirm the salt form of the heroin.

Phencyclidine, more properly identified as phenylcyclohexylpiperidine (PCP), is usually submitted to the laboratory as an exhibit of PCP base in diethly ether, a powder, or sprayed or coated on marijuana. The analysis of PCP is relatively direct by GC/MS. The resulting mass spectrum is specific. The GC/FTIR spectrum of PCP is not as specific when one compares this spectrum with that of PCP analogues and precursors such as phenylcyclohexyl carbonitrile (PCC) and phenylcyohexyl pyrrolidine (PCPy). FTIR spectrophotometry of the solid in a potassium bromide matrix is very specific. A word of caution is in order for anyone handling PCP. PCP is a substance that is believed to be easily absorbed through the skin of the analyst. Minimum handling is recommended.

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Responses

  • konsta
    Can infrared spectroscopy distinguish morphine from heroin?
    7 years ago

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