5.1 This guide is designed to assist an examiner in the selection of appropriate sample preparation methods for the analysis, comparison, and identification of fibers using IR spectroscopy. IR spectroscopy can provide additional compositional information than is obtained using polarized light microscopy alone. The extent to which IR spectral comparison is conducted will vary with specific sample and case evaluations.

5.2 IR analysis should follow visible and fluorescence comparison microscopy, polarized light microscopy, and ultraviolet (UV)/visible spectroscopy. If no meaningful differences are noted between the known and unknown samples in optical properties, then proceed to IR spectroscopy as the next step in the analytical scheme, as applicable.

Note 1: IR analysis generally follows the aforementioned techniques since sample preparation (for example, flattening) irreversibly changes fiber morphology.

5.3 IR spectroscopy should be conducted before dye extraction for chromatography due to the semi-destructive nature of the extraction technique. Because of the large number of sub-generic classes, forensic examination of acrylic and modacrylic fibers is likely to benefit significantly from IR spectral analysis (5). Useful distinctions between subtypes of nylon and polyester fibers can also be made by IR spectroscopy.

5.4 IR spectroscopy can provide molecular information regarding major organic and inorganic components. Components in lesser amounts are typically more difficult to identify. Reasons for this include interference of the absorption bands of the major components with the less-intense bands of minor components, and sensitivity issues whereby the minor components are present at concentrations below the detection limits of the instrument.

5.5 Fiber samples are prepared and mounted for microscopical IR analysis by a variety of techniques. IR spectra of fibers are obtained using an IR spectrometer coupled with an IR microscope, ATR, or diamond compression cell with beam condenser.

5.6 IR spectroscopy can be used to obtain spectra for elucidation of the chemical composition of the fiber and for comparison of two or more fiber samples.

5.6.1 When used to characterize the fiber type, the spectrum can be compared to reference spectra obtained from authenticated samples or reference standards.

5.6.2 When used for spectral comparisons, the objective is to determine whether any meaningful differences exist between the samples.

1.1 Infrared (IR) spectroscopy is a valuable method of fiber polymer identification and comparison in forensic examinations. The use of IR microscopes, coupled with Fourier transform infrared (FTIR) spectrometers, has greatly simplified the IR analysis of single fibers, thus making the technique feasible for routine use in the forensic laboratory. This guide provides basic recommendations and information about IR spectrometers and accessories, with an emphasis on sampling techniques specific to fiber examinations. The particular method(s) employed by each examiner or laboratory will depend upon available equipment, examiner training, sample suitability, and sample size.

1.2 This guide is intended for examiners with a basic knowledge of the theory and practice of IR spectroscopy, as well as experience in the handling and forensic examination of fibers. In addition, this guide is to be used in conjunction with a broader analytical scheme.

1.3 If polymer identification is not readily apparent from optical data alone, an additional method of analysis, such as microchemical tests, melting point, IR spectroscopy, Raman spectroscopy, or pyrolysis gas chromatography, should be used. An advantage of IR spectroscopy is that the instrumentation is readily available in most forensic laboratories and the technique is minimally destructive.

1.4 Units—The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.5 This standard is intended for use by competent forensic science practitioners with the requisite formal education, discipline-specific training (see Practice E2917), and demonstrated proficiency to perform forensic casework.

1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.