Ion Mobility Spectroscopy is routinely used to detect trace explosive residues at security checkpoints, but FTIR is better suited to handle the types of samples that enter a forensic laboratory.
The news media consistently informs the general public of the need for effective identification of explosive materials in areas ranging from field-based identification of unknown powders and security screening at airports, to environmental remediation of contaminated lands. In a parallel popularity jump, forensic science has become a sizzling topic of both reality-based and fictional television shows. A real-life forensic scientist can be faced with a variety of situations in which explosives need to be identified. They range from the identification of material from seized shipments and identification of suspected improvised explosive devices, to explosive residues on innumerable crime scene surfaces. While ion mobility spectroscopy (IMS) is the method of choice for the detection of trace explosive residues at airport security gates, a Fourier Transform Infra-red (FTIR) spectrometer is better suited to handle the types of samples that will enter a forensic laboratory.
Figure 1: These spectra, of common military
explosives, were collected on asingle
reflection, diamond ATR accessory.
The spectroscopic identification of explosive materials by FTIR is attractive due to the inherent capabilities of real-time identification, non-destructive analysis, and minimal sample preparation. These advantages are fully appreciated when reflection sampling accessories are utilized. Previous research has shown that explosives (e.g. TNT, RDX, HMX) can be easily identified from non-energetic materials by their spectral signatures in the mid-IR region. While an in-depth discussion of explosives in terms of the differentiation between primary explosives, secondary explosives, and propellants is beyond the scope of this article, it is safe to say that all three can be identified by FTIR spectroscopy. Infact, as the complexity of the sample increases, in terms of components, the specificity of the identification can improve within the spectral signature as well. Figure 1 presents the distinct FTIR spectral signatures that can be obtained for common military-grade explosives at the mg/mL level with an attenuated total reflectance (ATR) accessory. It is clearly evident to the untrained eye that all three spectra are different in terms of peak position, width, and intensity. Spectra of unknown samples are best identified by searching a spectrum of interest against spectral libraries and then evaluating the quality of possible matches. There are commercially available spectral libraries, such as the Georgia State Crime Lab library, which contains spectra of explosives and drugs. Another common practice is the creation of user constructed spectral libraries of materials/samples that enter the laboratory and can be positively identified by more than oneanalytical method.
FTIR spectroscopy is routinely utilized to harvest the information-rich region of the electromagnetic region, between 4,000 – 400 cm-1, also known as the mid-IR region. 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 characteristic of the entire molecule, it is also known that certain groups of atoms (i.e., 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, for example, rely heavily on functional group analysis for verification and identification of their synthesized products. For a molecule to absorb IR radiation it must have a changeable dipole moment. In practice, the following cannot be measured by FTIR spectroscopy: atoms, mono-atomic ions, or homo-nuclear diatomic molecules. Modern FTIR spectra relate intensity in either transmittance (%T) or absorbance (Abs.) as the ordinate and wave numbers (cm-1) as the abscissa of a spectrum.