At present, with the continuous occurrence of various terrorist incidents, safety and security work has received more and more attention. There are many kinds of explosives and various hidden means. The inspection work is very difficult, so, the timely and effective detection of explosives that have been camouflaged and hidden in the baggage has become an urgent and arduous task in the world.

Common explosive detection methods include: ion mobility spectroscopy, ultraviolet fluorescence, mass spectrometry, nuclear magnetic resonance, neutron technology, X-ray imaging, γ-ray imaging, etc., but each method has some advantages and disadvantages. Raman spectroscopy can realize the remote, high-sensitivity, non-damage and micro-trace detection of explosives, and has a huge application market in the field of explosives.

In this article, Raman spectroscopy technology is applied to the field of explosives detection, focusing on the application of confocal Raman microscopy, surface-enhanced Raman spectroscopy and portable Raman spectroscopy in the field of explosives detection and analysis.

1．Raman spectroscopy

The Raman scattering effect is an inelastic scattering caused by molecular and lattice vibration. In 1928, the Indian physicist Raman first discovered the phenomenon of the frequency of the scattered light, which is called Raman scattering. Raman spectroscopy is divided into the following types: Fourier transform Raman spectroscopy, confocal Raman microscopy, laser resonance Raman spectroscopy, high temperature Raman spectroscopy, surface-enhanced Raman spectroscopy and portable Raman spectroscopy , etc.

Each Raman spectroscopy has its unique advantages in its application. Among them, confocal Raman microscopy can realize the profile analysis of sample micro-area. Space-shifted Raman spectroscopy can effectively suppress the Raman interference of packaging materials, it realizes analysis of samples at different depths in transparent or translucent media. Surface-enhanced Raman spectroscopy can realize trace detection of explosives. Portable Raman spectrometer can be used for on-site monitoring and has the advantages of fast, convenient, high accuracy and high safety.

2．Confocal Raman microscopy

Confocal Raman microscopy is an analytical technique that combines Raman spectroscopy with microscopic analysis. The confocal module is used in the optical path to eliminate stray light interference in the defocus area of the sample, to ensure that only the sampled focus thin layer micro-area signal is collected. Laser confocal micro-Raman spectrometer is the most common Raman instrument in the laboratory. It is generally equipped with a microscope confocal imaging system, which is convenient for users to observe the microscopic shape of the object, and has a large volume, which is suitable for high-precision measurement.

The Finder Vista laser micro-confocal Raman spectrometer is independently developed and designed according to the principle of confocal by Beijing Zhuoli Hanguang Instrument Co., Ltd. By adjusting the depth of laser focus, material analysis of the surface and inside of the sample can be achieved. The laser configuration high-power optical microscope not only can directly obtain the microscopic two-dimensional shape of the explosive, but also can focus the laser spot diameter to the order of micrometers, eliminate the interference of surrounding impurities, and obtain accurate analysis of the micro-area. The use of scientific research deep-cooling CCD detectors ensures high sensitivity and high resolution of the spectrum, ensuring that no spectral information is missed.

In 2009, E. Al et al. reported the in-situ detection of explosives (PETN, TNT, ammonium nitrate) and explosive intermediates (HMTA, pentaerythritol) hidden in clothing using confocal micro-Raman spectroscopy.

Esam M．A．et al. uses confocal Raman spectroscopy to measure TNT in clothing fibers. The detection accuracy can reach the order of picogram without the pretreatment of the sample, and the detection time is less than 90s.

Wang Junsheng used the laser micro-confocal Raman spectrometer commonly used in the laboratory to detect bulk explosives and assigned the vibration modes corresponding to the Raman peaks of different explosive molecules.

In 2009, Leonardo C studied the TNT, dinitrotoluene (DNT), RDX remote Raman and micro-Raman using the 514.5nm, 488nm laser wavelength generated by an argon-ion laser. The result shows that the Raman spectroscopy is consistent with the Raman spectrum when the test distance is 7m, which confirms the accuracy and feasibility of the long-distance test.

3．Surface-enhanced Raman spectroscopy (SERS)

SERS uses trace molecules to adsorb on Cu, Ag, Au and other nanoscale metal materials, and its Raman spectral signal can be enhanced by 104-106 times. In recent years, the potential of SERS for trace analysis has been rapidly developed, and the detection limit has been reduced to a very low level, resulting in better trace detection results.

In the mine-sweeping work, , the position of the mine is accurately located by detecting the Raman spectrum of the nitro explosives degradation products 2,4-dinitrotoluene and 1,3-dinitrotoluene in the air, which is a non-contact detection method for locating mines.

K. Spencer's research team used electrochemical roughening of gold foil as a SERS substrate to accurately identify 2,4-dinitrotoluene vapor at a concentration of 10 ug/L and achieve a detection limit of 5ug/L with specific data processing methods. S. Banker et al. obtained the Raman spectrum of TNT, NG (Nitroglycerin) and TATP (Triacetone peroxide) by using a commercially available metal silver film as a SERS substrate. The detection limit of TNT is 200pg, NG is 400ng, and TATP is 400ng. It can be seen that SERS technology is widely used in the detection of trace explosives.

SERS technology can significantly improve the sensitivity and detection limit of Raman technology, but many aspects of its basic principles are still in research and debate. At present, the research hotspots of SERS technology in explosives identification are mainly in basic research, practical instrument development and Application and other aspects.

4．Portable Raman spectroscopy

Homeland defense needs to confirm a large number of unknown chemicals online. At present, the rapid detection technology and means for unidentified sealed solids, liquid samples and toxic and hazardous substances in water are still in the blank. The hand-held Raman spectrometer is small in size and weighs about 1kg. It can be used to detect unidentified sealed liquid or solid hazardous chemicals on the spot. It has the advantages of fast, convenient, high accuracy and high safety. Because the operator often does not have very professional knowledge of chemical and spectral analysis during on-site inspection, reducing manual intervention in Raman spectroscopy and automating Raman spectroscopy data processing and analysis are the key to handheld Raman spectroscopy.

Beijing Zhuoli Hanguang Instrument Co., Ltd. independently developed and designed the “Finder Edge” handheld Raman spectrometer according to the requirements of on-site law enforcement, and quickly identified drugs, precursor chemicals, explosives, explosive chemicals, and contraband. The software functions include detection, database comparison, data transmission, Bluetooth transmission and printing, cloud computing and big data processing. The instrument has no requirements for the test environment, can be used in both water and air environments, and its environmental adaptability and various analytical capabilities fully meet the requirements of homeland security and defense.

5．Conclusion

Raman spectroscopy technology has matured with the development of science and technology, and has been widely used in food detection, chemical detection, biomedicine, physical materials, administrative identification, jewelry and jade. Because Raman spectroscopy has non-contact and non-invasive in-situ detection, the safety of inspectors and physical evidence is guaranteed to a large extent. The security application of Raman spectroscopy is also gradually becoming marketized. The use of Raman spectroscopy for explosives detection has become an international research hotspot and has a good development prospect in the detection of explosives. With the emergence of new technologies and new instruments of Raman spectroscopy, it will play a greater role in the qualitative and quantitative analysis of explosives.