The utilization of ultraviolet resonance Raman spectroscopy for the detection and identification of chemical, biological, and nuclear hazards is of great interest due to the sensitivity and specificity afforded by this technique. Detection by means of optical probing is a fast, non-contact method that requires little to no sample preparation and can be performed remotely by an operator. In addition, this method is also well suited for use by an automated monitor or a mobile autonomous system, such as an in-situ environmental detector or a sensorequipped unmanned vehicle. In addition, this technique offers the ability for a single detector to identify multiple species of targets. However, for Raman detection to become practicable as a forensic tool, the method must demonstrate the ability to distinguish the target elements while operating in a complex environment.

By employing a variable frequency laser source to illuminate the anylate, one can interrogate the molecules at a number of distinct wavelengths. The compilation of these single-wavelength illuminations generates a two-dimensional contour map of the functional form I = f(x,y) where x and y are the excitation wavelength and the Stokes Raman frequency shift, respectively.

In addition to the Raman peaks present in the single spectrum that provide information about the target compound’s molecular structure, the twodimensional signature also contains a record of the changes in the molecular resonance cross-sections as the sample responds to variations in the illuminating laser wavelength. This additional dimensionality should create a signal that is much more robust and therefore harder to confuse, resulting in better specificity. This is particularly important when utilizing the device in realistic environments containing multiple target materials and contaminants.

The Swept Wavelength Optical Resonant Raman Detector (SWOrRD) at the Naval Research Laboratory is a stimulated spectroscopy system capable of rapidly acquiring the spectral signatures of both solid and liquid samples over a large range of laser wavelengths. The uniqueness of the SWOrRD system is its ability to both scan over a large range of wavelengths as well as to selectively tune to wavelengths that overlap with the resonant energy bands of the target molecules. As it is impossible to completely determine a priori which wavelengths will be the most effective when attempting to detect a particular signal within a complex and rapidly changing environment, the operation of a system that is flexible in this regard holds distinct advantages over fixed wavelength systems.

In the work presented here, the twodimensional Raman signatures of selected chemicals are measured and then utilized to form a detection basis. These chemicals are then combined in various permutations so as to create unknown mixtures that are then likewise measured. Upon completion of the full experimental run, the raw spectra are Fourier-filtered to remove both high-frequency noise and baseline instrument contributions, and then recalibrated to compensate for power variation, sample absorption, wavelength drifts, system transmission, and detector responses. They are then assembled to obtain a single, multi-run spectrum that is treated as a single unit.

Utilizing a LabVIEW-based detection program, the components of the mixture are rapidly determined through an examination of the signal correlation between the unknown signature and those signatures stored in the chemical library. Once the mixture components have been identified, an estimation of the relative abundances of the tagged chemicals is undertaken by comparison of the functional distance between the unknown signal and a weighted linear superposition of the library elements.

Utilizing a fully contained mixture set, both the components and the possible mixture combinations of those components were measured in the ultraviolet range from 220-260 nm utilizing the SWOrRD spectroscopy system. Reliable identification of mixture components is possible, and a reasonable reconstruction of the fractional abundance of the constituent chemicals is possible while utilizing a linear mixture model.

This work was done by Robert Lunsford, David Gillis, and Jacob Grun of the Naval Research Laboratory; and Pratima Kunapareddy, Charles Manka, and Sergei Nikitin of Research Support Instruments. NRL-0051


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Component Identification in Multi-Chemical Mixtures with Swept-Wavelength Resonant-Raman Spectroscopy

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This article first appeared in the October, 2012 issue of Defense Tech Briefs Magazine.

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