Forensic examination often demands versatile analytical techniques that offer high specificity, high accuracy, and direct measurement; ideally the techniques should be nondestructive and noncontact with little sample preparation. Raman spectroscopy proves to be, by far, the most suitable analytical tool to satisfy these essential criteria. Coupled with a microscope, Raman spectroscopy is capable of identifying trace forensic evidence at the micro-scale with direct chemical specificity.
Although it has been used for almost 100 years, Raman spectroscopy has not been widely applied to forensics, mainly because most “real-world” samples are vivid in color, which can be a background emission (fluorescence) thousands of times stronger than the Raman effect when illuminated by light. This emission blinds the Raman signal carrying the chemical-specific information. These samples include most plant-based (e.g., cloth fiber, paper, leaves, and pollen), bio-based (e.g., tissues, blood, and serum), and synthetic materials (e.g., ink, color dyes, and explosives). When excited with a laser used for Raman spectroscopy, the samples generate fluorescence, which limits the usefulness of Raman in the forensic analysis.
A fundamental way to avoid fluorescence is to move to longer wavelength excitation such as 1064 nm in the near-infrared range, which is typically out of the excitation range of most fluorescent samples. Due to technical limitations in the past, 1064 FT-Raman was the only solution to suppress the fluorescence, but it is relatively cumbersome to use due to its continually moving parts, strict sample preparation requirements, and long acquisition times. Additionally, the FT configuration cannot provide high-resolution spatial information.
Now, with advances in solid-state lasers, efficient gratings, and fast detectors, the Raman technique has seen dramatic improvements, such as ease of operation, fast speed, and lower cost. A new class of 1064-nm dispersive Raman spectrometers and microscopes from BaySpec, Inc. (San Jose, CA) offer highly efficient, patented Volume Phase Gratings (VPG®); fast optics; and deep-cooled, sensitive InGaAs detectors. Without any moving parts, these compact 1064 Raman spectrometers feature high sensitivity, high spectral resolution (up to 4 cm–1), portability, and stability. Several examples are given here.
Information-rich Raman spectra from the real world
In the real world, most materials are colored, which means they are likely to have intense fluorescent emission when illuminated by a laser in the visible range (~400–800 nm); therefore it is not possible to obtain good Raman signals using the laser. The 1064-nm Raman system is well-suited for the Raman spectroscopy of these colored materials.
Figure 1 – Raman spectra of a piece of rhodamine-dyed plastic slide, using different excitation lasers: 532, 785, and 1064 nm. The spectra were taken using the BaySpec
Aglility™ benchtop Raman spectrometer. Inset: The slide taken under 532-nm laser excitation produced intense orange-color fluorescence.
Figure 1 shows an example of Raman spectroscopy using 532-, 785-, and 1064-nm lasers on a common piece of colored plastic. The 1064-nm system produces Raman spectra with rich chemical information about the material, while the 532- and 785-nm lasers excite intense fluorescence that buries the Raman signals.
Figure 2 – Raman spectra of oil paints in different colors taken with the BaySpec
Nomadic™ multiexcitation Raman microscope.
Figure 3 – Chemical analysis and classification via 1064-nm Raman chemical mapping of black ink on paper using a BaySpec
Nomadic confocal Raman microscope.
Most paint and ink, and even some white paper, are highly fluorescent under visible laser illumination. Again, the 1064-nm Raman system produces high-quality, signature-rich Raman spectra for these samples (Figure 2). When coupled with a microscope, the system can be converted to a powerful tool to provide direct chemical mapping of samples at the micro-scale (Figure 3). This is particularly useful for finding and identifying trace evidence for forensic analysis. These 1064-dispersive Raman systems are an effective solution for the most complex forensic analyses, especially when high fluorescence from the samples is a concern.
Also see "A Portable Multiexcitation Dispersive Raman Spectrometer for Environmental Analysis."
The authors are with BaySpec, Inc., 1101 McKay Dr., San Jose, CA 95131, U.S.A.; e-mail: LChandler@bayspec.com.