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April 1999 Volume 1
Number 1
Chapter 6 of Forensic
Fiber Examination Guidelines
1.0. Scope
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.
2.0.
Reference Documents
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
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3.0. Terminology
Absorbance (A): the logarithm to the base 10 of
the reciprocal of the transmittance, (T).
A = log10(1/T) = -log10T
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.
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4.0.
Summary of Guidelines
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:
Acetate
Lastrile
Polyester
Vinal4
Acrylic
Modacrylic
Rayon
Vinyon
Anidex (no longer produced in the United States [10])
Novoloid4
Saran
Aramid
Nylon
Spandex
Azlon4
Olefin
Sulfar
Fluorocarbon
Polybenzimidazole (PBI)
Triacetate
NytrilPolycarbonate
Rubber
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.
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5.0.
Significance and Use
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:
Anidex Polycarbonate
Lastrile Vinyon
PBI Fluorocarbon
Vinal Novoloid
Azlon Sulfar
Nytril
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.
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6.0.
Sample Handling
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.
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7.0. Analysis
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.
7.6. Focusing
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.
7.8. Resolution
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.
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8.0.
Report Documentation
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.
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9.0. References
(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.
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