Forensic Analysis of Fibers Using FTIR Microscopy

Using FTIR in Forensic Analysis

Forensic analysis commonly involves the collection of textile fibers when examining crime scenes. Characterizing and identifying micro-fibers is vital because they provide extensive information relating to a crime and can help tie a suspect to a location. Microscopic fibers can present problems in terms of both size and quantity of sample. Fourier transform infrared (FTIR) spectroscopy is a technique already being used during the forensic analysis of physical evidence (such as controlled substances, fibers, and paint).

FTIR spectroscopy is emerging as the technique of choice for nondestructive analysis of the chemical composition of unknown biological stains. This method is advantageous over most current biochemical tests because of its high specificity, universality, nonconsumptive nature, and potential for quick in situ analysis where the stain is examined directly on the substrate.

A significant advantage of this technique is its nondestructive nature and specific signature for different sample types based on their chemical composition. And when coupled with microscopy, FTIR enables forensic examiners to identify both detailed visual microscopic information and simultaneous material identification.

According to Douglas Deedrick of the FBI, “The likelihood of two or more manufacturers duplicating all aspects of the fabric type and color exactly is extremely remote.” [1] This further exemplifies the importance of fibers at a crime scene and the need to characterize them. IR spectroscopy is invaluable for determining the material properties of single fibers. [2,3] Whether the fibers of interest are synthetic, natural or a hybrid in composition, they will exhibit unique spectral features when interrogated under infrared light.

Fundamental vibrations caused by the absorption of infrared light correspond to structural composition that make up the fibers and dyes commonly used. In this application note, we present how the SurveyIR™ FTIR accessory can assist forensic examiners with their forensic analysis when it comes to characterizing microscopic fibers.

Results & Discussion

Traditional FTIR microscopes are costly, complex systems that require well-trained personnel to operate. They commonly involve the use of liquid nitrogen to cool the mercury cadmium telluride (MCT) detectors. The SurveyIR provides a less costly, simplified microscope accessory that can be paired with most commercially available benchtop FTIR spectrometers. By using the instrument-mounted detector, the need for liquid nitrogen for cooling the MCT detector may be eliminated. The live image feed from eSpot™ software allows for simultaneous viewing and spectral data collection. Due to the accessory’s all reflective design, the spectral range is limited by the spectrometer optics exclusively. The SurveyIR can switch between reflectance and transmittance collection modes, as well as a clip-on view-through attenuated total reflection (ATR) accessory.

The images in Figures 1 and 3 represent fibers extracted from woven fabric. The fibers were cut into small lengths (13 mm), rolled flat via roller knife on plain glass microscope slides, and then transferred to Low-E glass microscope slides for analysis. The left image of polyamide (Fig. 1, a) shows a transparent fiber about 60 μm in diameter. The right image of polyester (Fig.1,b) is slightly less than 60 μm wide. Due to the sample preparation technique used, structural appearance of these two fibers is very similar as both fibers appear to be similar in size and transparency. But looking at their corresponding IR spectra, we can clearly see two different materials represented.

FTIR spectra were collected by reflectance at 8cm-1 resolution and coadded 64 scans. The bottom spectrum (Figure 2) of a polyamide, exhibits spectral features consistent with nylon 6,6, poly(hexamethylene apidamide). The top spectrum (Figure 2) is consistent with polyethylene terephthalate (PET). Both fibers can be differentiated using FTIR spectroscopy and identified using spectral library searching methods.

Cotton and silk are well-known textiles and some of the oldest known. Cotton has been found in archeological sites dating back as early as 5000-6000 BC. Likewise, silk has history dating back to ancient Chinese civilizations, usually in the hands of the emperors or elite members of society. Both materials have very different textures. Cotton is known to be soft and fluffy, whereas silk is very smooth and lustrous. Unlike the synthetic fibers, the images in Figure 3 exhibit apparent differences, as silk (d) appears very smooth compared to cotton (c) which has small inconsistencies. The composition of the two is completely natural; however, cotton is comprised of almost completely of cellulose and silk protein.

The IR spectra seen in Figure 3 of silk (red) has distinct peaks associated with N-H structural units around 3300cm-1 and amide bands I and II at 1661 and 1532cm-1, respectively. Cotton (Figure 3, blue) on the other hand has an absence of amide bands I and II (peaks 1661 and 1532cm-1) and a broadened peak at 3380cm-1 due to extensive hydrogen bonding in the cellulose structure. The broaden peak at 1085cm-1 (blue) is associated with cellulose structure as a combination of C-C, C-OH, C-O-C stretches. The ability of FTIR to differentiate the two natural fibers is apparent and can be referenced against a library for identification purposes.

Conclusion

FTIR microspectroscopy with the Czitek SurveyIR instrument proves to be a useful method for identifying and interrogating microscopic fibers. Single fiber analysis can be accomplished without sample destruction and reference images captured for later presentation and sample position validation. The resulting spectra from both natural and synthetic fibers were identified utilizing library searches and further verified by key characteristic fundamental IR vibrations. The uniqueness of IR vibrational spectroscopy is proven in both synthetic and natural fibers analyzed, which provides certainty for identification purposes. To the forensic examiner, certainty is imperative for evidence that is to be used in the court. The ability of the SurveyIR instrument to switch between different sampling methods (ATR, Transmittance, and Reflectance) presents an effective, easy to use toolset for forensic examinations.

References:

1. Deedrick, Douglass W. “Hairs, Fibers, Crime, and Evidence Part 2: Fiber Evidence.” Forensic Sci. Comm., 2, 3 (2000).
2. ASTM E2224 – 10, “Standard Guide for Forensic Analysis of Fibers by Infrared Spectroscopy,” ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA.
3. “A Forensic Fiber Examiner Training Program,” Scientific Working Group for Materials Analysis (SWGMAT), https://archives.fbi.gov/