Detection of Chemicals with Standoff Raman Spectroscopy

Raman spectroscopy is a powerful analytical technique for detecting a wide range of chemicals in any state (solid, liquid, or gas phase) and is widely used in laboratories for chemical identification. Raman spectra resulting from the vibrational modes of a molecule consist of well-defined sharp spectral lines that are unique to the molecule. This property makes Raman spectroscopy useful as a "fingerprint" technique for the identification of various chemicals, including polymorphs. Recently, Raman spectroscopy is emerging as a standoff technique, using gated ICCD detectors and pulsed lasers, with the capability of identifying chemicals in the daytime from distances ranging from 1 to over 100 m and with a fast detection time of a few seconds. In this article we describe the detection capabilities of a compact standoff Raman system developed at the University of Hawaii (Honolulu, Hawaii) for standoff detection of hazardous chemicals and chemicals used in the synthesis of homemade explosives (HME). The compact standoff Raman system uses a regular 85-mm Nikon camera lens as collection optics and can detect various chemicals from a 50-m distance in daytime with 1–10 s integration time. A slightly bigger system using an 8-in. telescope (203 mm) is shown here to detect chemicals from a 120-m distance using only single laser pulse excitation. These systems are suitable for the real-time detection of various chemicals and would be useful for homeland security and environment monitoring applications.

The recent rise in worldwide terrorist activities has sparked a vast need for standoff technologies capable of detecting hazardous chemicals and explosives at sufficiently safe ranges to avoid and neutralize threats from chemical devices. Standoff Raman spectroscopy can detect several hazardous chemicals from a safe distance without the need to make contact with the sample sealed inside glass and plastic bottles. The ability to distinguish water from colorless flammable organic liquids from remote distances during the daytime would be promising for screening water bottles and various liquid containers at airports. Suspicious chemicals and their residues could also be analyzed on site through their Raman spectra. The fast response time and ability to perform chemical analysis on site during the day and night makes the remote Raman system an attractive instrument for homeland security applications. The Raman spectra representing the vibrational modes of a molecule produce results with a high confidence level and very low incidence of false alarms. The spectral features are unique even for various polymorphs of the same chemical. Raman spectra can detect a wide range of chemicals that are of interest from a public safety point of view, such as explosives (TNT, RDX, C4, TATB, HMX, PETN, various nitrates, chlorates, perchlorates, peroxides, and so forth), hazardous materials (e.g., benzene, naphthalene, acetone, various acids, gasoline, and other flammable materials), toxic gases, and various chemical vapors (1).

At the University of Hawaii (Honolulu, Hawaii), we have been developing a portable remote Raman system suitable for detection of various chemicals, minerals, and explosives. Portable remote Raman instruments previously developed at the University of Hawaii using 5- and 8-in. telescopes have shown the ability to detect various chemicals and minerals from distances of up to 125 m under bright daytime conditions with a short integration time (2–4). Recently, we have significantly reduced the size of our remote Raman system to develop a compact system that could be used to screen for hazardous chemicals at checkpoints and airports. In this article, we show that good quality Raman spectra of various potentially harmful chemicals such as ammonium nitrate, potassium perchlorate, acetone, isopropyl alcohol, nitrobenzene, and various organic and inorganic chemicals could easily be measured from remote distances up to 50 m with a compact remote Raman system utilizing only a regular 85-mm Nikon camera lens as collection optics and with 10 s of integration time. The remote Raman system also uses a small Raman spectrograph developed at the University of Hawaii (5,6).

Experimental Setup and Samples

The schematic of a pulsed time-resolved remote Raman system is shown in Figure 1. A frequency-doubled mini Nd:YAG pulsed laser source (model Ultra CFR, Quantel Laser, Quantel USA, Bozeman, Montana, 532 nm, 30 mJ/pulse, 20 Hz, pulse width 8 ns), is used in an oblique geometry to excite the target located at a remote distance. A 10× beam expander is used after the laser to control the laser spot size at the target. The laser beam is aligned with the field of view of the Nikon camera lens using a folding mirror. A 2-in. diameter 532-nm notch filter is used in front of the 85-mm Nikon camera lens to block off the Rayleigh scattered light. The scattered light generated by the target is collected by the camera lens, which then focuses it onto a 100-μm slit of the compact spectrograph equipped with a gated thermoelectrically cooled intensified charge-coupled device (ICCD) detector.

Figure 1: Schematic diagram of compact time-resolved remote Raman system using an 85-mm camera lens as collection optics and compact Raman spectrograph developed at the University of Hawaii.

Figure 2 shows a photograph of the compact remote Raman system mounted on a pan and tilt stage, which is then mounted on a portable trolley. The system is assembled on an optical breadboard with mounting screws separated by 1-in. spacing. The photograph depicts the size of the overall system. Further miniaturization of the remote Raman system by replacing the ICCD detector with a mini ICCD detector is under development.

Figure 2: Photo of compact remote Raman system mounted on a portable trolley for outdoor testing. The system components are mounted on a breadboard with mounting screws separated by 1-in. spacing.

Samples of various chemicals in liquid state (water, acetone, ethanol, 2-propanol, benzene, nitrobenzene, and ethyl benzene) and powder form (ammonium nitrate, potassium perchlorate, and potassium nitrate) were contained in glass vials 2.54 cm (diameter) × 5 cm (height) and investigated through the closed glass vial. All samples were analytical grade chemicals obtained from Sigma Aldrich (St. Louis, Missouri) or Thermo Fisher Scientific (Waltham, Massachusetts). A photo of the portable remote Raman system during the 50-m test is shown in Figure 3.

Figure 3: Photograph of the remote Raman system mounted on a portable trolley during 50-m testing.

Detection Capabilities and Discussion

The remote Raman spectra from a distance of 50 m during the daytime with 10 s of integration time of ammonium nitrate (NH₄NO₃), potassium nitrate (KNO₃), and potassium perchlorate (KClO₄) are shown in Figure 4. All major Raman bands necessary for positive identification of these chemicals are clearly observed. These materials are often used in the production of homemade explosive materials. In the spectrum of NH₄NO₃, the intense Raman peak at 1044 cm^-1 is attributed to the symmetric stretching (ν₁) vibration of the nitrate (NO₃^–) ion. The Raman peaks at 715 cm^-1 and between 50 and 250 cm^-1 are, respectively, the in-plane bending (ν₄) vibrations of the NO₃^– ion, and the lattice (translational and rotational) vibrations of the NO₃^– and NH₄⁺ ions (6). The symmetric stretching (ν₁) vibrations of the nitrate (NO₃^–) ion in KNO₃ are observed at 1052 cm^-1. NH₄NO₃ can easily be distinguished from KNO₃ by the presence of a broad Raman band in the high-frequency region near 3137 cm^-1, which is due to symmetric stretching vibration of the NH₄⁺ ion. The asymmetric stretching mode of the ammonium ion gives a Raman band near 3225 cm^-1, which contributes to the broadness of the Raman band in the high-frequency region. In the spectrum of KClO₄, the intense Raman peak at 942 cm^-1 is attributed to the symmetric stretching (ν₁) vibrations of the perchlorate (ClO₄^–) ion, and the shoulder peak at 924 cm^-1 to the first overtone of the symmetric bending (ν₂) vibrations of the ClO₄^– ion. The signal intensity of the latter is enhanced by the Fermi-resonance effect. Fermi resonance happens when the vibrational frequency of an overtone mode is in the vicinity of a symmetric fundamental mode (Raman frequency of the ν₂ fundamental of ClO₄^– is 463 cm^-1). Other Raman peaks of the ClO₄^– ion are 629 cm^-1, attributed to the antisymmetric bending (ν₄) vibrations, and 1087 and 1125 cm^-1, attributed to the antisymmetric stretching (ν₃) vibrations (6).

Figure 4: Remote Raman spectra of ammonium nitrate, potassium nitrate, and potassium per- chlorate from a distance of 50 m with a 10-s integration time. All spectra with 532-nm excitation, 30 mJ/pulse, 20 Hz, 50 ns gate width, as measured. Spectra have been shifted vertically for clarity.

Raman spectroscopy is very sensitive to chemical composition and structure. This is because Raman spectra are representations of the Raman active vibrational modes of a molecule. When a molecule is altered either by adding extra atoms or replacing atoms with a different element, it changes the vibrational frequencies of various modes. The addition of extra atoms to a molecule also changes the total number of Raman active vibrational modes. This results in a significant change in the appearance of the Raman spectrum of the molecule. The process generates fingerprint Raman patterns that are unique to every chemical. Figure 5 shows the standoff Raman spectra of three very similar chemicals — benzene, ethyl benzene, and nitrobenzene — from a distance of 50 m with detection time of 10 s. Benzene and its derivatives are BTEX compounds and have harmful effects on the central nervous system. The strong aromatic C-H stretching Raman line of benzene appears at 3061 cm^-1. The strong Raman band corresponding to the symmetric breathing mode of the benzene ring is observed at 992 cm^-1. This vibrational mode is observed at 1002 and 1003 cm^-1 in ethyl benzene and nitro benzene, respectively. The effect of the addition of the ethyl group to benzene is easily seen in Figure 5 where it shows a broad Raman band in the CH region at 2933 cm^-1, which is characteristic of the aliphatic C-H stretching of the ethyl group. The aromatic C-H stretching mode of the benzene ring is observed at 3053 cm^-1 in ethyl benzene. Similarly, the addition of the nitro group gives a very strong band at 1347 cm^-1 in nitrobenzene due to symmetric stretching (ν_s) vibrations of the NO₂ functional group and the C-H stretching mode of the benzene ring is shifted to 3079 cm^-1 in nitrobenzene. Other Raman peaks of medium intensity in the nitrobenzene spectrum are at 853, 1003, 1109, and 1588 cm^-1, attributed to NO₂ bending, aromatic ring bending, C-N stretching, and aromatic ring stretching, respectively. A number of organic substances that are rich in nitro (NO₂) groups are often used in explosive devices, such as nitroglycerine, trinitrotoluene (TNT), and triaminotrinitrobenzene (TATB) and can easily be identified through remote Raman spectroscopy.

Figure 5: Remote Raman spectra of benzene, ethyl benzene, and nitrobenzene during the daytime from a distance of 50 m with a 10-s integration time. Experimental conditions are the same as in Figure 4.

Highly volatile and flammable organic compounds are used as solvents and reagents in the chemical and materials industry (for example, acetone, diethyl ether, benzene, methanol, and pyridine). These colorless liquids could be concealed in water bottles and pose significant fire hazards for airlines. Presently, there are restrictions on carrying water and other clear liquids through airport check points, mostly due to the lack of detection capability of screening technologies in distinguishing water from various other hazardous clear liquid chemicals. Figure 6 shows that a standoff Raman system provides distinct identification of water from other flammable organic chemicals. The remote Raman spectra of water, ethanol, 2-propanol, and acetone were obtained through glass vials from a distance of 50 m with a 10-s integration time.

Figure 6: Remote Raman spectra of water, ethanol, 2-propanol, and acetone through glass vials from a 50-m distance showing that water can be easily identified from organic compounds. Experimental conditions are the same as in Figure 4.

Water gives very strong Raman signals in the 3100–3600 cm^-1 spectral region and is easily identified from its broad Raman bands. In the water sample, strong broad Raman bands near 3228 and 3423 cm^-1 are from the symmetric (ν₁) and antisymmetric stretching (ν₃) vibrational modes of the water molecule, respectively. In contrast, the C-H bonds in most organic chemicals give very strong sharp Raman peaks in the 2800–3000 cm^-1 region and make the high-frequency region (>2500 cm^-1) especially useful in distinguishing water from organic compounds. For example, in the spectrum of 2-propanol, the most intense Raman peaks are from the symmetric and asymmetric stretching vibrations of the methyl (CH₃) group: 2884 and 2972 cm^-1, respectively. The symmetric stretching mode of the CH₂ group is observed at 2923 cm^-1. In addition, a very broad, weak band extending from 3100 to 3600 cm^-1 can be found in various kinds of alcohols, attributed to the hydroxyl (OH) stretching vibrations. The most intense Raman peak of acetone is located at 2923 cm^-1, also attributed to the ν_s vibrations of the methyl groups. Another strong peak at 787 cm^-1 is attributed to the C-C stretching vibrations. Furthermore, a signature peak from the C=O stretching vibrations can be found in all carbonyl compounds, located between 1600 and 1800 cm^-1 (1711 cm^-1 in acetone).

The data presented here show the ability of a portable compact remote Raman system utilizing an 85-mm Nikon camera lens as collection optics to detect various chemicals from a distance of 50 m during the daytime with 10 s of integration time (equivalent to 200 laser pulses excitation). The rapid detection and identification of chemicals could be important in many homeland security applications, such as identifying materials on a conveyor belt or emanating from a fast-moving object. At shorter distances, chemical analysis could be done at a very fast rate using only single laser pulse excitation because the Raman signal is inversely proportional to the square of the target distance. Standoff Raman detection of various chemicals at 10 m distance using single-pulse excitation has been demonstrated earlier by our group using a 5-in telescopic Raman system (7). With a slightly bigger 8-in. telescopic system, single-pulse detection could be done at the much longer range of 120 m as shown in Figure 7. A 100-mJ/pulse energy was used for exciting the targets of ammonium nitrate and potassium perchlorate placed inside a 1-in. glass vial. The figure shows five single-pulse measurements of each target, illustrating good reproducibility and high signal-to-noise detection capability.

Figure 7: As-measured stand-off spectra of ammonium nitrate and potassium perchlorate powders with a single laser pulse from a 120-m distance using an 8-in. remote Raman system. Spectra measured with single 532-nm excitation, 100 mJ/pulse and with 100-ns gate width. Five measurements for both chemicals are shown to demonstrate the good signal reproducibility of the system.

Conclusion

A standoff Raman system capable of detecting various chemicals with a very high degree of confidence can be useful for homeland security applications where chemicals sealed inside glass and plastic bottles could be identified from a remote distance in real time without need for handling the targets. Standoff Raman spectroscopy clearly identifies explosive and hazardous chemicals in the daytime using fingerprint vibrational modes unique to each chemical.

Acknowledgment

This work was supported in part by grants from NASA and ONR. The authors would like to thank Nancy Hulbirt and May Izumi for their valuable help with figures and editing.

References

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Anupam K. Misra, Shiv K. Sharma, Tayro E. Acosta, and David E. Bates are with the Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, Hawaii.