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In criminal investigations, matching textile fibers from a crime scene to the clothing of a suspect is a key factor in placing a suspect at the scene of a crime. A fiber analyst compares shape, dye content, size, chemical composition and microscopic appearances of the fiber. Examination of fibers for fluorescence and color characteristics to compare with the known fiber samples may provide important information.
Current fiber analysis techniques include microspectroscopy and fluorescence microscopy. In microspectroscopy an expert simply collects a spectrum of a specific region of interest (ROI) within the sampled fiber. Because this method does not offer any spatial information about the sample, photography is recommended to capture the spatial features of the ROI for future reference.
- Immediate analysis and side-by-side comparisons
- Little to no sample preparation required, thus decreasing the chances of possible contamination
- Sample composition, structure and architecture are clearly revealed with enhanced sensitivity and detection
- The ability to discriminate evidence that cannot be discriminated by traditional techniques
Fluorescence microscopy incorporates sets of excitation and barrier filters at discrete wavelengths to determine whether the samples exhibit fluorescence. Barrier filters are used to suppress excitation light that has not been absorbed by the fiber. The color and intensity of fluorescence of known and questioned fibers are noted at each excitation wavelength. Such analysis is subjective and slight variations in emission intensity and band position often go undetected.
Figure 1 shows three similarly colored, red polyester fibers and corresponding fluorescence chemical images with fluorescence spectra. The fluorescence maximum shifts from 580 nm (shown in green) to 640 nm (shown in blue) clearly indicate a similar origin for fibers A and C and a different orgin for fiber B.
Absorbance (Colorimetric) Chemical Imaging is performed in the same manner as fluorescence, but with the sample illuminated by white light. The absorbance images and spectra of the fibers can be compared and minor differences in response can be easily noted.
ChemImage’s CI-TRACE™ System, equipped with a mercury/xenon light source is used for fast and accurate fiber analysis in either absorbance or fluorescence mode. In a typical experiment, images arecollected from 400 to 720 nm in 3 nm steps. The camera exposure time usually ranges from 0.5 sec to 3 sec per frame. Because images are recorded as a function of the wavelength, a spectrum is generated for every pixel within the image. The resulting contrast in the image is due to the varying amounts of absorption or emission within the sample.
CI Xpert™ software allows easy data collection, image processing and interpretation. All instrument components are software-controlled, so the parameters for a series of experiments need only to be set-up once. Statistical image processing algorithms can be applied to the data sets to create enhanced contrast within and between images.
Figure 2 shows results of colorimetric fiber analysis on two cotton samples as well as a chemical image extract at 650 nm, clearly illustrating the differences in absorbance at this particular wavelength. The corresponding absorbance spectra confirm the difference in absorbance between the two samples.
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