Fiber Guidelines, Chapter 6 (FSC, April 1999)
Infrared Analysis of Textile Fibers
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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 (FT-IR) spectrometers has greatly simplified the IR analysis of single fibers, thus making the technique feasible for routine use in the forensic laboratory.
These guidelines are intended to assist individuals and laboratories that conduct forensic fiber examinations and comparisons in the effective application of infrared spectroscopy to the analysis of fiber evidence. Although these guidelines are intended to be applied to the analysis of single fibers, many of its suggestions are applicable to the infrared analysis of small particles in general.
SWGMAT Quality Assurance Guidelines
SWGMAT Trace Evidence Handling Guidelines
ASTM E1492-92 Practice for Receiving, Documenting, Storing, and Retrieving Evidence in a Forensic Laboratory
ASTM E 131-93 Terminology Relating to Molecular Spectroscopy
ASTM 1421-94 Standard Practice for Describing and Measuring Performance of Fourier Transform Infrared (FT-IR) Spectrometers: Level Zero and Level One Tests
Absorbance (A): the logarithm to the base 10 of the reciprocal of the transmittance, (T).
A = log10 (1/T) = -log10 T
Absorption Band: A region of the absorption spectrum in which the absorbance passes through a maximum.
Absorptivity (a): Absorbance divided by the product of the sample pathlength (b) and the concentration of the absorbing substance (c). a = A/Bc
Absorption Spectrum: A plot, or other representation, of absorbance, or any function of absorbance, against wavelength, or any function of wavelength.
Attenuated Total Reflection (ATR): Reflection that occurs when an absorbing coupling mechanism acts in the process of total internal reflection to make the reflectance less than unity.
Background: Apparent absorption caused by anything other than the substance for which the analysis is being made.
Cellulosic Fiber: Fiber composed of polymers formed from glucose.
Far-Infrared: Pertaining to the infrared region of the electromagnetic spectrum with wavelength range from approximately 25 to 1,000 µm (wavenumber range 400 to 10 cm-1).
Fourier Transform (FT): A mathematical operation that converts a function of one independent variable to one of a different independent variable. In Fourier transform infrared (FT-IR) spectroscopy, the Fourier transform converts a time function (the interferogram) to a frequency function (the infrared absorption spectrum). Spectral data are collected through the use of an interferometer, which replaces the monochrometer found in the dispersive infrared spectrometer.
Fourier Transform Infrared (FT-IR) Spectrometry: A form of infrared spectrometry in which an interferogram is obtained. This interferogram is then subjected to a Fourier transform to obtain an amplitude-wavenumber (or wavelength) spectrum.
Generic Class: A group of fibers having similar (but not identical) chemical composition. A generic name applies to all members of a group and is not protected by trademark registration. Generic names for manufactured fibers include, for example, rayon, nylon, and polyester. Generic names used in the United States for manufactured fibers were established as part of the Textile Fiber Products Identification Act enacted by Congress in 1954 (12).
Infrared: Pertaining to the region of the electromagnetic spectrum with wavelength range from approximately 0.78 to 1,000 µm (wavenumber range 20,000 to 4,000 cm-1).
Infrared Spectroscopy: Pertaining to spectroscopy in the infrared region of the electromagnetic spectrum.
Internal Reflection Spectroscopy (IRS): The technique of recording optical spectra by placing a sample material in contact with a transparent medium of greater refractive index and measuring the reflectance (single or multiple) from the interface, generally at angles of incidence greater than the critical angle.
Manufactured (Man-Made) Fiber: Any fiber derived by a process of manufacture from any substance that is not, at any point in the manufacturing process, a fiber.
Mid-Infrared: Pertaining to the infrared region of the electromagnetic spectrum with wavelength range from approximately 2.5 to 25 m (wavenumber range 4,000 to 400 cm-1).
Spectrometer: Photometric device for the measurement of spectral transmittance, spectral reflectance, or relative spectral emittance.
Subgeneric Class: A group of fibers within a generic class that shares the same polymer composition. Subgeneric names include, for example, nylon 6, nylon 6,6, and poly(ethylene terephthalate).
Transmittance (T): The ratio of radiant power transmitted by the sample, I, to the radiant power incident on the sample, Io . T = I/Io
Wavelength: The distance, measured along the line of propagation, between two points that are in phase on adjacent waves.
Wavenumber: The number of waves per unit length, in a vacuum, usually given in reciprocal centimeters, cm-1.
These guidelines cover identification of fiber polymer composition by interpretation of absorption spectra obtained by microscopical infrared spectroscopy. They are intended to be applicable to a wide range of infrared spectrometer and microscope configurations.
Spectra may also be obtained by a variety of alternative IR techniques. Other techniques (not covered in the scope of these guidelines) include micro internal reflection spectroscopy (MIR), which differs from attenuated total reflectance (ATR) in that the infrared radiation is dependent upon the amount of sample in contact with surface of the prism (1):
4.1. Diamond Cell (Medium or High Pressure)
This technique is used with a beam condenser (2-4). This combination is frequently used with a spectrometer configured for mid- and far-IR and thus provides a wider spectral range;
4.2.2. Thin films: solvent (5, 6), melt (3), or mechanically prepared (7);
4.2.3. Lead foil technique (5); and
4.2.4. Micro-KBr (or other appropriate salt) pellets (8, 9).
This list is not meant to be totally inclusive or exclusive.
This analytical method covers manufactured textile fibers (with the exception of inorganic fibers) including but not limited to the following:
Anidex (no longer produced in the United States )
Although natural fibers may also be analyzed by IR spectroscopy, they are excluded from these guidelines because no additional compositional information is provided over that yielded by light microscopy.
Fiber samples may be prepared and mounted for microscopical infrared analysis by a variety of techniques. Infrared spectra of fibers are obtained using an IR spectrometer coupled with an IR microscope. Fiber polymer identification is made by comparison of the fiber spectrum with reference spectra.
Consideration should be given to the potential for additional compositional information that may be obtained by IR spectroscopy over PLM alone (see the microscopy guidelines in Chapter 2). The extent to which IR spectral comparison is indicated will vary with specific sample and case evaluations.
The recommended point for IR analysis in a forensic fiber examination is following visible and UV comparison microscopy, polarized light microscopy, and UV or visible spectroscopy but before dye extraction for thin-layer chromatography. This list of analytical techniques is not meant to be totally inclusive or exclusive.
The following generic types of fiber are rarely encountered in routine forensic examinations:
Exemplar data, reference standards, examiner experience, or combinations may be inadequate for their characterization by microscopical and microchemical techniques. For these fiber types, IR spectroscopic confirmation of polymer type may be advisable.
Because of the large number of subgeneric classes, forensic examination of acrylic fibers is likely to benefit significantly from IR spectral analysis (13).
Colorless manufactured fibers are lacking in the characteristics for color comparison available in dyed or pigmented fibers. The forensic examination of these fibers may, therefore, benefit from the additional comparative aspect of IR spectral analysis.
If polymer identification is not readily apparent from optical data alone, an additional method of analysis should be used such as microchemical tests, melting point, IR spectroscopy, or pyrolysis gas chromatography. Infrared analysis offers the advantage of being the least destructive of these methods.
The general handling and tracking of samples should meet or exceed the requirements of ASTM 1492-94 (14).
The quantity of fiber used and the number of fiber samples required will differ according to the following:
6.1. Specific technique and sample preparation;
6.2. Sample homogeneity;
6.3. Condition of the sample; and
6.4. Other case-dependent analytical conditions, concerns, or both.
Sample preparation should be similar for all fibers being compared. Fibers should be flattened prior to analysis in order to obtain the best quality absorption spectra. Flattening the fibers can alter the crystalline/amorphous structure of the fiber and result in minor differences in peak frequencies and intensities. This must be taken into consideration when making spectral comparisons (15). Leaving the fiber unflattened, while allowing crystallinity-sensitive bands to be observed unaltered, results in distortion of peak heights due to variable pathlengths (16). In certain situations, a combination of both approaches may be advisable.
Because flattening the fiber is destructive of morphology, the minimum length of fiber necessary for the analysis should be used. A typical IR microscope is optimized for a 100 µm spot size, thus little beam energy passes through a point that is farther than 50 µm from the center of the field of view. Hence, analytical performance will not necessarily be improved with the use of fibers greater than 100 µm in length.
The flattened fiber may be mounted across an aperture, on an IR window, or between IR windows. Common IR window materials used for this purpose include but are not limited to KBr, CsI, BaF2, ZnSe, and diamond. The choice of window material should not reduce the effective spectral range of the detector being used. Where the fiber is mounted between two IR windows, a small KBr crystal should be placed next to the fiber. The background spectrum should be acquired through this crystal to avoid interference fringes that would arise if the spectrum were acquired of an air gap between the two IR windows.
Where several fibers are mounted on or in a single mount, they should be well separated (microscopically) so that their positions can be unambiguously documented for later retrieval, reanalysis, or both and to prevent spectral contamination from stray light that might pass through another fiber.
It is important that the longitudinal plane (flattened surface) of the fiber be as nearly parallel to the IR window or other mount as possible.
A mid-infrared spectrometer (FT-IR is the current standard, but dispersive IR is not excluded) and an infrared microscope that is compatible with the spectrometer are recommended. The lower frequency cutoff varies with the microscope detector used (preferably no higher than 750 cm-1).
Useful sample preparation accessories include but are not limited to sample supports, infrared windows, presses, dies, rollers, scalpels, and etched-tungsten probes.
7.1. Equipment Readiness
All spectrometer and microscope components should be turned on and allowed to reach thermal stability prior to commencement of calibration and operational runs. This may take up to several hours. It should be noted that most FT-IR instruments are designed to work best when left on or in the standby mode 24 hours a day. Analysts should refer to the manufacturer's guidelines for the optimum performance of their instruments.
7.2. Instrument Performance and Calibration
It is essential that instrument performance and calibration be evaluated routinely, at least once a month, in a comprehensive manner.
The preferred performance evaluation method is in accordance with ASTM 1421-94, Sections 1-7, 9.5, and 9.5.1 (17). In brief, this includes evaluating the following:
7.2.1. System throughput;
7.2.2. Single-beam spectrum;
7.2.3. 100% T line; and
7.2.4. Polystyrene reference spectrum.
Dispersive instruments should be checked according to manufacturers' recommendations. Instrument performance records may be maintained on hard copy, computer disk, or both. Report documentation may vary by laboratory but should include the date, the operator, the system parameters, and the original instrumental output data.
7.3. Sample Illumination and Detector Measurement Apertures
The apertures that control the areas (fields) of sample illumination and detector measurement in an IR microscope may be of fixed or variable size and may be either rectangular or circular in shape. Variable rectangular apertures are recommended because they can be more closely matched to the fiber shape. Light throughput, stray light reduction, and aperture focus in the sample image plane are some of the considerations in selecting aperture parameters and positioning. Fiber width, flatness, and linearity will usually limit the size of the illumination and detector apertures used for analysis. In general, the illuminating and detector fields should lie within the boundaries of the fiber edges.
Not all systems provide for the control of both illumination and detector measurement fields. The following recommendations can be modified to suit the constraints of a particular system design.
7.4. Objective and Condenser Adjustment
The objective, condenser, or both should be adjusted, if possible, for any IR window that lies between the optic and the sample in the beam path. This compensation reduces spherical aberration and permits more accurate focus.
7.5. Polarization Bias of the Infrared Spectrometers and Microscopes
Infrared spectrometers and microscopes exhibit a polarization bias. This fact, coupled with the pleochroism associated with most fibers, makes it essential that fiber alignment be consistent throughout an analysis and preferably for all fiber analyses performed on a given system. A vertical or north-south alignment is typically used.
Samples should be focused as close to the center of the sample volume as possible and centered on the optical axis of the system. The condenser should be focused and recentered if necessary. This is best accomplished using a circular field aperture.
The detector measurement aperture width should be adjusted to just slightly less than the width of the fiber but preferably not less than 10 µm (18). The aperture length may vary with sample geometry but should not be so great as to allow the detector to be saturated when acquiring a background spectrum. The illuminating field aperture should be adjusted so that the image of its edges coincide with those of the detector measurement aperture. The size and position of the apertures should not vary between sample and background data acquisition for a given analysis.
7.7. Background Spectrum
A background spectrum refers to a reference absorption spectrum, which includes the absorbance contributions of all system components except the sample of interest. The IR window or windows with KBr crystal are all considered part of the system. The system parameters for background spectra should be identical to the parameters used for sample spectra (with the possible exception of gain and number of scans). These parameters include resolution, mirror velocity, and spectrometer aperture size.
Resolution should be set at 4 cm-1 (one data point every 2 cm-1 ). Higher resolution may be used. The additional data points, however, typically yield no further analytical information for polymer samples. Because the apertures are adjusted to fit the sample, it is usually most convenient to acquire the sample spectrum prior to acquiring the background spectrum.
7.9. Sample and Background Scans
Sample and background scans should be run under the same conditions. If necessary, parameters can be subsequently modified and new sample and background spectra acquired.
7.10. Data Storage
It is generally useful to save all data on disk after it is generated and prior to any modification. Consideration should be given to storing the original interferogram data prior to Fourier transformation. Data that is damaged during subsequent processing can then be restored from saved files.
7.11. Identification of Fiber Polymers by IR Spectra
Successful identification of fiber polymers by IR spectra depends on experience and familiarity with fiber reference spectra. Spectral identification is accomplished by comparison with spectra of known reference standards. A library of reference IR spectra is essential. A library of reference fiber IR spectra obtained using the same technique used for the unknown fiber is desirable. It is also desirable to have available authentic samples of the fibers to be identified.
For identification, the positions of the absorption bands according to wavelength or wavenumber and their relative intensities must be compared with those of a known reference spectrum. It is desirable to confirm the identification by other methods (i.e., previously performed analytical techniques such as PLM).
The generic class of manufactured textile fibers can be unequivocally identified with the exception of rayon versus lyocell (these generic classes may be differentiated by their optical properties). The subgeneric class of synthetic manufactured fibers may be identified.
Similarity or dissimilarity in the IR spectra can be noted when making a fiber comparison.
(1) Bartick, E. G., Tungol, M. W., and Reffner, J. A. A new approach to forensic analysis with infrared microscopy: Internal reflection spectroscopy, Analytica Chimica Acta (1994) 288:35-42.
(2) Read, L. K. and Kopec, R. J. Analysis of synthetic fibers by diamond cell and sapphire cell infrared spectrophotometry, Journal of the Association of Official Analytical Chemists (1978) 61:526-532.
(3) Hartshorne, A. W. and Laing, D. K. The identification of polyolefin fibres by infrared spectroscopy and melting point determination, Forensic Science International (1984) 26:45-52.
(4) Gál, T., Ambrus, I., and Urszu, S. Forensic analysis of textile fibres by Fourier transform infrared diamond cell technique, Acta Chimica Hungarica (1991) 128:919-928.
(5) Cook, R. and Paterson, M. D. New techniques for the identification of microscopic samples of textile fibres by infrared spectroscopy, Forensic Science International (1978) 12:237-243.
(6) Garger, E. F. An improved technique for preparing solvent cast films from acrylic fibers for the recording of infrared spectra, Journal of Forensic Sciences (1983) 28:632-637.
(7) Carlsson, D. J., Suprunchuk, T., and Wiles, D. M. Fiber identification at the microgram level by infrared spectroscopy, Textile Research Journal (1977) 47:456-458.
(8) Fox, R. H. and Schuetzman, H. I. The infrared identification of microscopic samples of man-made fibers, Journal of Forensic Sciences (1968) 13:397- 406.
(9) Grieve, M. C. and Kotowski T. M. Identification of polyester fibres in forensic science, Journal of Forensic Sciences (1977) 22:390-401.
(10) Hatch, K. L. Textile Science. West Publishing, Minneapolis/St. Paul, 1993, pp. 84.
(11) ASTM E 131-93: Terminology Relating to Molecular Spectroscopy. American Society for Testing and Materials, West Conshohocken, Pennsylvania.
(12) Federal Trade Commission Rules and Regulations Under the Textile Products Identification Act, Title 15, U. S. Code Section 70, et seq. 16 CFR 303.7.
(13) Grieve, M. C. Another look at the classification of acrylic fibres: Using FTIR microscopy, Science and Justice (1995) 35:179-190.
(14) ASTM 1492-94: Standard Practice for Receiving, Documenting, Storing and Retrieving Evidence in a Forensic Science Laboratory. American Society for Testing and Materials, Philadelphia.
(15) Tungol, M. W., Bartick, E. G., and Montaser, A. The development of a spectral data base for the identification of fibers by infrared microscopy, Applied Spectroscopy (1990) 44:543-549.
(16) Bartick, E. G. Considerations for fiber sampling with infrared microspectroscopy. In: The Design, Sample Handling, and Applications of Infrared Microscopes, ASTM STP 949. Ed., P. B. Roush. American Society for Testing and Materials, Philadelphia, 1987, pp. 64-73.
(17) ASTM 1421-94: Standard Practice for Describing and Measuring Performance of Fourier Transform Infrared (FT-IR) Spectrometers: Level Zero and Level One Tests. American Society for Testing and Materials, Philadelphia.
(18) Messerschmidt, R. G. Minimizing optical nonlinearities in infrared microspectrometry. In: Infrared Microspectroscopy: Theory and Applications. Eds. R. G. Messerschmidt and M. A. Harthcock. Marcel Dekker, New York, 1988, pp. 1-19.
(19) Tungol, M. W., Bartick, E. G., and Montasser, A. Forensic examination of synthetic textile fibers by microscopic infrared spectrometry. In: Practical Guide to Infrared Microspectroscopy. 2nd. ed. Ed., H. Humecki. Marcel Dekker, New York, 1995.
(20) Hartshorne, A. W. and Laing, D. K. The identification of polyolefin fibres by infrared spectroscopy and melting point determination, Forensic Science International (1986) 26:45-52.
FORENSIC SCIENCE COMMUNICATIONS APRIL 1999 VOLUME 1 NUMBER 1