Methamphetamines and many other illicit drugs do not occur naturally; they are synthesized in clandestine laboratories across the country. Unfortunately, the materials—precursors, catalysts, and solvents—are commercially available, and recipes can easily be found on the Internet. One fundamental role of the forensic chemist is to rapidly screen and identify a large number of seized evidence samples.
Research shows that the spectroscopic identification of these types of materials is attractive due to the inherent capabilities of real-time identification of volatile solvents, nondestructive analysis that allows subsequent analysis by a secondary technique, and minimal sample preparation. This article discusses the advantages of using Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy for the forensic screening of unknown confiscated materials from suspected clandestine drug laboratories.
Using the FTIR method
FTIR spectroscopy is routinely utilized to harvest the information-rich mid-IR region (4000 to 400 cm–1) of the electromagnetic spectrum. When organic molecules are irradiated with light within this region, the radiation is absorbed and converted into molecular vibrations. While it is widely accepted that an IR spectrum is not characteristic of the entire molecule, it is also known that certain groups of atoms (e.g., functional groups) give rise to peaks that occur at or near the same frequency regardless of the structure of the rest of the molecule.
Organic chemists rely heavily on functional group analysis to verify and identify synthesized products. For a molecule to absorb IR radiation, it must have a changeable dipole moment. Modern FTIR spectra relate intensity in either transmittance (%T) or absorbance (Abs) as the ordinate and wavenumbers (cm–1) at the abscissa of a spectrum.
As PC technology increases, FTIR is becoming a key spectrometer in the mid-IR region for forensic laboratories. Three common advantages of FTIR spectrometers over dispersive instrumentation are the Connes, Jacquinot, and Fellgett Advantages. The Connes Advantage uses a laser as an internal standard to provide extremely high accuracy (cm–1). Jacquinot, a throughput advantage, requires no slits or dispersive element, allowing all of the full energy of the source to pass through the interferometer to the sample. FTIR also offers the Fellgett Advantage, where all the wavenumbers in a spectrum are collected simultaneously. As a result, spectral averaging can provide high signal-to-noise spectra in less than 25 sec. Additionally, computerized interferometer alignment and electronic dehumidification have improved the reliability, durability, and ease of use of these systems.
Understanding the chemistry
The clandestine recipes for illegal production involve hydrogenation of the hydroxyl group on the ephedrine or pseudoephedrine molecule. The most common method for small-scale methamphetamine laboratories in the United States is primarily called the “Red, White and Blue Process,” which involves red phosphorus, pseudoephedrine or ephedrine (white), and blue iodine, from which hydroiodic acid is formed. In general, the three top three synthetic routes are: 1) “Red, White, and Blue Red, White, and Blue” (red phosphorus, pseudoephedrine/ ephedrine, blue iodine); 2) birch reduction (metallic lithium as catalyst); and 3) reductive amination of phenlyacetone with methylamine (mercury–aluminum amalgam as catalyst).
It is important for a forensic chemist to be able to identify the majority of the precursors, intermediates, and final products in order to establish grounds for prosecution.
Attenuated total reflectance
The ATR technique is considered a universal sampling accessory to FTIR. It provides reproducible qualitative and quantitative analysis of organic and inorganic liquid, paste, solid, film, and powder samples. In general, the only sample preparation required is to apply pressure to solid samples with the pressure applicator.
Demystifying the “black box”
Figure 1 - ATR application. In ATR, when the IR beam hits the edge of the crystal, it is internally reflected. At this point of reflection, an evanescent wave is created. This wave comes in contact with the sample that lies against the crystal surface, creating the IR spectra.
In an ATR accessory, infrared light is focused into a crystal of high refractive index and propagates the length by total internal reflectance (Figure 1). A sample in close contact with the crystal will interact with the infrared energy and attenuate the total internal reflection. This interaction or “absorption and reflection” process allows the collection of the IR spectrum.
Figure 2 - Differences between ATR and transmission-based techniques.
Forensic chemists should not overlook the few differences between ATR and transmission-based techniques. A transmission measurement collects a spectrum that is an average of the bulk properties of the sample. ATR measurements only measure the surface properties of the sample. The depth of penetration is usually only a few microns and can be calculated with the refractive index of the ATR crystal and the sample. The observed intensity of the absorption bands is less than those collected during a transmission measurement. This can be attributed to the reduced pathlength and the decreased overall throughput of the ATR phenomena (Figure 2).
Accessories and advantages of ATR
ATR sample techniques and accessories allow forensic laboratories to focus more on achieving results and less on sample preparation. ATR accessories are available from the majority of companies that specialize in FTIR accessory production. For analyzing solids and powders such as precursors or intermediates used to make illicit drugs, a pressure anvil or press is used to create close contact between the sample and the crystal.
To minimize the chances of crystal damage, a slip-clutch feature is ideal for forensic applications where multiple technicians use the instrument. A swivel tip and a single-reflection ATR element provide low sample size requirements and are cleaned easily with an appropriate solvent and a cotton swab. A variety of pressure tips such as concave, PTFE-coated, and flex pivot are available to accommodate different sampling needs. Many of these accessories provide troughs for analyzing liquid samples.
Generally, the only sample preparation required is applying pressure to solid samples with the pressure applicator. When analyzing liquid samples, there is no need to apply pressure with the anvil.
When selecting an ATR crystal, the user should take into account chemical vulnerability, physical susceptibility to scratching and cracking, depth of penetration, and measurement range. A diamond ATR is the best choice for forensic laboratories based on its combined chemical and physical ruggedness. Diamond crystals are chemically inert to mineral acids, corrosive materials, and strong bases used in methamphetamine production.
Spectral libraries for FTIR-ATR analysis
Forensic chemists should not assume unknowns are pure, since most drugs are synthesized from multiple materials. Spectra of unknown samples are best identified by searching a spectrum of interest against spectral libraries and then evaluating the quality of possible matches. Using spectral libraries allows a nonspectroscopist to perform identifications.
Each comparison generates a match number often referred to as a hit quality index. The library should contain a comprehensive collection of precursors, intermediates, final products, cutting agents, and common household solvents/chemicals. A good match score is more than 800 out of 1000 for a pure compound.
Figure 3 - After setting a variety of search parameters to perform a library search, users should visually inspect the data by overlaying the unknown with the match.
With spectral library searches of unknowns, the user can set a variety of search parameters such as maximum number of hits, the searching algorithm, and spectrum search or peak search. Figure 3a and b show the comparison of an unknown powder (black line) to a methamphetamine (red line) in the library.
Commercial spectral libraries, such as the Georgia State Crime Lab library, are available and contain spectra of explosives and drugs. Another common practice is the creation of userconstructed spectral libraries of materials/samples that enter the laboratory and can be positively identified by more than one analytical method.
Also, minor spectrum preprocessing can vastly improve results. Correction should be performed for offset baseline, sloping baseline, noise, endpoints, solvent peaks, CO2, H, H2O, and accessory correction.
There is another dimension of infrared analysis for forensic chemists. While FTIR microscopy is beyond the scope of this article, readers should be aware of the technique’s ability to create a two-dimensional spatial map or “image” of materials on crime scene evidence, which is invaluable to forensic scientists. For example, a surface at a seized laboratory gets swabbed for drug residue, but the results may be inconclusive because of interfering substances on the swab. Removing the evidence from the crime scene, instead of swabbing, and mapping it with FTIR microscopy in the forensic laboratory avoids this scenario.
Conclusion
Anyone can produce methamphetamines with instructions found on the Internet and easily acquired chemicals. As the number of clandestine laboratories increases across the U.S., it is vital for forensic chemists to provide real-time, reliable results. FTIR-ATR is an important tool for the forensic laboratory due to its ease of use and cleaning, lack of sample preparation, and chemical/physical ruggedness of diamond sampling crystals. In addition, the technique offers better sampling reproducibility by eliminating the possibility of sample contamination during sample preparation.
The authors are with Shimadzu Scientific Instruments , 7102 Riverwood Dr., Columbia, MD 21046, U.S.A.; tel.: 800-477-1227; fax: 410-381-1222; e-mail: [email protected]. The authors wish to acknowledge PIKE Technologies and Fiveash Data Management, Inc. for their assistance in the production of this piece.