Demonstrated Wavelength Portability of Raman Reference Data for Explosives and Chemical Detection

Abstract

As Raman spectroscopy continues to evolve, questions arise as to the portability of Raman data: dispersive versus Fourier transform, wavelength calibration, intensity calibration, and in particular the frequency of the excitation laser. While concerns about fluorescence arise in the visible or ultraviolet, most modern (portable) systems use near-infrared excitation lasers, and many of these are relatively close in wavelength. We have investigated the possibility of porting reference data sets from one NIR wavelength system to another: We have constructed a reference library consisting of 145 spectra, including 20 explosives, as well as sundry other compounds and materials using a 1064 nm spectrometer. These data were used as a reference library to evaluate the same 145 compounds whose experimental spectra were recorded using a second 785 nm spectrometer. In 128 cases of 145 (or 88.3% including 20/20 for the explosives), the compounds were correctly identified with a mean “hit score” of 954 of 1000. Adding in criteria for when to declare a correct match versus when to declare uncertainty, the approach was able to correctly categorize 134 out of 145 spectra, giving a 92.4% accuracy. For the few that were incorrectly identified, either the matched spectra were spectroscopically similar to the target or the 785 nm signal was degraded due to fluorescence. The results indicate that imported data recorded at a different NIR wavelength can be successfully used as reference libraries, but key issues must be addressed: the reference data must be of equal or higher resolution than the resolution of the current sensor, the systems require rigorous wavelength calibration, and wavelength-dependent intensity response should be accounted for in the different systems.

Design Considerations for a Portable Raman Probe Spectrometer for Field Forensics

“…Our tests of oxidants such as sodium chlorate, which have a high elastic Mie-Tyndall scattering cross-section, became a simple test to highlight the limitations of reduced spectral resolution. In Figure 2, the region around the extremely dominant Raman peak was studied as a function of averaging time and also compared to a spectrum obtained under high resolution and extended averaging using a 1064 nm-based FT-Raman system [31].…”

“…a secondary FT-Raman spectrum of decanoic acid, a fairly stable persistent organic, used to test and then calibrate the Headwall/Andor prototype system. The FT spectrum was taken at 2.0 cm −1 resolution with 1064 nm excitation [31]. For this calibration work, the FT system had the interferometer frequency calibrated with a stabilized HeNe laser and the results were compared against the known 1064 nm excitation laser; that is, calibration appears satisfactory as is seen when comparing the corresponding Stokes/anti-Stokes line shifts.…”

“…Other than a few low frequency bands, the positions of most of the peaks are within the stated FWHM resolution of the Headwall spectrograph (4.7 cm −1 with 26 μm pixel pitch), indicating good wavelength calibration across the spectrum. Figure 10 shows expanded FT-Raman spectra for RDX and PETN in the fingerprint region from the FT-Raman system [31]. Figure 11 shows Raman Stokes spectra from the FT-Raman database for a sample of Semtex-H which can have a nearly 1 : 1 ratio of RDX to PETN, although this is variable with up to a maximum of 65.5% RDX and a minimum of 25% PETN according to Moore et al [32].…”

“…Instrument resolution was set to 2 cm −1 . Both spectra were obtained with a Bruker FT-Raman system using the protocols described in [31]. The FT-Raman spectra of the pure materials were scaled from 0 to 2.0 for the largest band.…”

“…The details of those measurements can be found in Ref. [31]. After experimenting with the two demo systems we assembled our prototype from a Headwall Photonics Raman Explorer 785 and an Andor Newton camera as were used in the demo system described above.…”