5 Reasons FTIR and Raman May Not Identify Explosive Materials in the Field

Many bomb technicians and others use handheld FTIR instruments and/or Raman instruments to successfully identify explosive and precursor materials in the field. Although it can be frustrating, no single spectroscopy technique can “see” all chemicals. However, if you understand what the lack of response means and how that can suggest the identity or characteristics of an unknown material, you will be better informed.

Lack of signal cannot identify a compound, but it can be characterized. Sometimes a Raman signal is overwhelmed by (1) high fluorescence or heat, but more commonly lack of Raman signal suggests (2) water, (3) metals, (4) elements, and/or (5) simple ionic compounds could be the issue. Similarly, lack of an FTIR signal suggests the sample may be made of metals, elements, and /or simple ionic compounds.

When a sample fails repeatedly to respond to both technologies, here are some tactical considerations to review:

Raman spectroscopy is a technology that enables users to safely analyze explosive materials through sealed translucent containers without disturbing the sample. Chemical Identification analyzers using Raman can recognize thousands of potential explosives including: TATP (triacetone triperoxide), ammonium nitrate, TNT (trinitrotoluene), RDX (cyclonite) and HMTD (hexamethylene triperoxidediamine). Additionally, these instruments are capable of identifying explosive precursors in liquid mixtures including: hydrogen peroxide, fuel oil, acetone, sulfuric acid and more.

To aid in identification, the device collects the molecular fingerprint of an unknown sample, and then compares the substance against the onboard chemical library, typically providing results in a matter of seconds. Raman instruments excel at identifying liquids, gels, pastes and light colored solid materials. (Raman technology utilizes a focused laser, which can heat some energetic materials, so proper safety protocols should be followed to ensure operator safety.)

This technology can provide specific actionable data to first responders, EOD and CBRNE teams, because explosives, precursors and improvised explosive devices can be identified quickly, right in the hazard zone. (Read more:  Explosives Identification Using Raman Technology.)

However, Raman does not see water, is blinded by heat or dark colors (fluoresce), and simple fuels and oils may fluoresce. Also, Raman cannot see metals (aluminum, iron, magnesium, mercury, zinc, etc.), elements like carbon or charcoal, iodine, phosphorous, or simple ionic compounds, like sodium chloride, potassium hydroxide, ammonium hydroxide, and calcium chloride.  It can identify sulfur, potassium cyanide and sodium cyanide.

Because Raman cannot see purely ionic acids in water [like hydrochloric acid, hydrofluoric acid, hydroiodic acid, sodium hydroxide (base), ammonium hydroxide (base),  and potassium hydroxide (base)], you may want to consider pH test for all liquids without a Raman signal. Complex anions that can be seen in ionic compounds include nitrate, sulfate, carbonate, perchlorate, chlorate, and cyanide.

FTIR has similar issues. FTIR stands for Fourier Transform InfraRed, the preferred method of infrared (IR) spectroscopy. When IR radiation is beamed to a sample, some radiation is absorbed by the sample and some passes through (is transmitted). The resulting signal at the detector is a spectrum representing a molecular ‘fingerprint’ of the sample. The usefulness of infrared spectroscopy arises because different chemical structures (molecules) produce different spectral fingerprints.

FTIR spectrometers are exceptionally useful for identifying unknown chemicals of a variety of colors. They will not generate heat during the sampling process, which makes it an ideal tool for verifying substances such as: smokeless powders and colored materials.  (Read more:  Explosives Identification Using FTIR Technology.)

However, with FTIR, one must open the container and the test requires pressure on solid samples, which could be dangerous. FTIR is sometimes confused or blinded by water.  Like Raman, it cannot see elements, simple ionic compounds, and purely ionic acids in water. 

For a quick comparison of advantages and limitations of Raman and FTIR analysis, here is a chart of Raman/FTIR Tactical Considerations.

Be aware that there are instruments that integrate both technologies in one instrument. Using Raman and FTIR technology together provides confirmatory results and a broader range of unknown substance identification.

⇒ Editor’s Note: Much of the content, including the chart, used in this article was provided by Houghtons, Inc., a small privately held company that provides training, test methods and technical assistance to public and private sector organizations responsible for emergency response You can contact principal Rick Houghton at https://hazardid.com 

For more unknown substance identification information, visit the Safety and Security Threat Detection section of our website.