Detecting Explosives by Portable Raman Analyzers: A Comparison of 785-, 976-, 1064-, and 1550-nm (Retina-Safe) Laser Excitation

Portable Raman analyzers have great potential for identifying explosive materials associated with improvised explosive devices. However, most commercial analyzers employ 785-nm lasers that can generate fluorescence interference, limiting identification capabilities, and possibly cause permanent eye damage because of the use of high powers and open laser beams. Here we compare the Raman spectra obtained for TNT and RDX using 785-, 976-, 1064-, and 1550-nm lasers. The latter is of special interest, as it falls within the retina-safe spectral range.

Close to 50% of all United States casualties in Afghanistan and Iraq are due to improvised explosive devices (IEDs)(1,2). The United States Army has been investigating the use of portable Raman analyzers to potentially identify such devices, as well as the raw materials used to make them. The choice of Raman spectroscopy is primarily due to its ability to identify virtually any chemical based on its unique spectrum, and the fact that the sample can be measured noncontact without preparation using a point-and-shoot probe. The choice of excitation wavelength plays a critical role in obtaining quality spectra. Nearly all portable Raman analyzers employ 785-nm laser excitation, which can generate fluorescence in samples and diminish, even eliminate, identification (3,4). Of specific concern are TNT, a secondary explosive often used in IEDs, and RDX, a primary explosive and major component in C4 and Semtex, both of which exhibit fluoresce in some samples. Furthermore, highly colored glass containers may also generate fluorescence interference. Longer wavelength lasers, such as 1064 nm, can be used to avoid fluorescence (5), but with a concomitant loss of sensitivity, due to the fact that the Raman signal intensity decreases as the wavelength increases to the fourth power (that is, the υ4 dependence of Raman scattering) (6).

In addition to fluorescence interference and sensitivity, eye safety is also a concern, especially since the end-users are not likely to have extensive experience using high powered lasers (for example, 500 mW). Laser wavelengths at 785 and 1064 nm are transmitted through the cornea and focused on the retina, increasing the power density by several orders of magnitude, and even a few milliwatts of power can cause permanent eye damage (7). Furthermore, all of the current commercial portable Raman analyzers use laser powers exceeding the maximum permissible exposure (MPE) limit of 1–5 mW stated by the American National Standards Institute (ANSI) (7), necessitating the use of sample enclosures or laser glasses by all personnel in the vicinity of the measurement.

Figure 1: Illustration of the human eye showing the focus of visible radiation on the retina and absorption of 1550 nm radiation by the cornea, aqueous, and vitreous humor. Damage to the retina is permanent, while damage to the cornea is temporary, but should still be avoided.
In an effort to address this latter concern, we have developed a portable Raman spectrometer that employs 1550-nm laser excitation. This wavelength falls within the 1400–2000 nm "eye-safe" range (preferably referred to as retina-safe [7]), so-called because the least amount of damage to the eye occurs in this spectral region (8). In contrast to wavelengths below 1400 nm, the retina-safe wavelengths are not focused by the eye, but are absorbed by the cornea, aqueous and vitreous humor (Figure 1). The cornea can handle a 10-s exposure to 1 W/cm2 of a 1550-nm continuous wave laser for 50% of a population before thermal damage (9). Furthermore, the cornea can repair itself in several days. For the retina-safe region ANSI sets the MPE at one tenth this value, 100 mW/cm2 (7).

To our knowledge there has been only one Raman spectroscopy publication using a retina-safe laser (10). This is primarily due the υ4 dependence of Raman scattering. For example, a Raman vibrational mode at 1000 cm-1 will be 21.5 times as intense at 785 nm compared to 1550 nm. In addition, the spectral response of quality detectors is limited to a 0–2000 cm-1 Raman range at this excitation wavelength. Here, the trade-offs in obtaining Raman spectra for TNT and RDX using 785-, 960-, 1064-, and 1550-nm excitation are presented in the context of eye safety.

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