Fiber Analysis
Physical Aspects of Forensic Science
Taken in part from a presentation by Sheila Cowart &Linda
Cummins, Physical Aspects of Forensic Science, Spring, 2001.
Textile Fiber Defined
• Defined as the smallest part of a textile
material
– Many objects in our environment (clothing,
ropes, rugs, blankets, etc.) are composed of
yarns made of textile fibers
Textile Fiber Categories
• Animal (hairs)
– Wool, cashmere, silk
• Vegetable
– Cotton, kapok, linen
• Mineral
– Asbestos
• Manmade
– Acetate, rayon, nylon,
acrylic, polyester, and
olefin
Animal Fibers
• Woolen fibers occupy less than 1% of all
fibers used in production of textile
materials
• Wool has a microscopic structure that is
characteristic of hair
• The cuticle (outer covering) is made of
flattened cells, commonly called scales
Animal Fibers (continued)
• The scales resemble shingles of a roof and
are one of the most useful features to ID
an unknown textile fiber as wool
• Other animal hairs are not as frequently
encountered so they can be quite valuable
if they occur as evidence
– Include goat (cashmere, mohair), llama
(alpaca, vicuna, guanaco), and camel hair
Animal Fibers (continued)
• Cattle and rabbit hair are found in the
manufacture of certain kinds of felts
– Felts are made from water suspensions of
randomly arranged fibers. When the fibers
settle out, the water is removed and the mass
of fibers is pressed to form the felt
– Some modern felts are no longer made
exclusively from hairs but are mixtures with
other fibers
Animal Fibers (continued)
• Silk places a distant second to wool in
occurrence, and its use has decreased
since development of artificial fibers
• Silk fibers are not very often encountered
in crime investigations, probably because
silk fabrics do not shed very easily
Vegetable Fibers
• Only cotton is found in any large extent
in items of clothing
• Approximately 24% of total US textile
fiber production was cotton in 1979
• Other plant fibers, such as jute and sisal,
are seen in various types of cordage and
baggings
Vegetable Fibers (continued)
• Cotton fibers have a distinctive flattened, twisted
microscopic appearance, which is quite
characteristic
• The fibers resemble a twisted ribbon
– In mercerizing process, fibers are treated with alkali,
making them swell up and become more rounded and
less twisted in appearance.
– This process results in improved texture and feel, but the
fibers are still recognizable as cotton under the
microscope
Vegetable Fibers (continued)
• Undyed cotton
fibers are so
common they
have little value as
physical evidence
• Almost any
surface or dust
sample will be
found to contain
white cotton fibers
Household Dust
Mineral Fibers
• Asbestos fibers are the only natural fibers
found in the mineral category
• Seldom used in items of clothing or
household objects, they are rarely found
in either the composition of or the debris
from items seen in crime labs
Mineral Fibers (continued)
• Asbestos minerals are all crystalline materials
in which the chemical bonds are much
stronger in one direction (along the fiber axis)
than in the other 2 directions
• Therefore, they fracture or cleave, to form
long thin rods and this process can be
continued until extremely thin fibers have
been produced and may be truly
submicroscopic and easily airborne
Mineral Fibers (continued)
• Asbestos fibers can be valuable as physical
evidence and are easily transferred from
one surface to another
• Forensic microscopists can group the fibers
which are large enough to be examined
according to the type of asbestos from
which they were derived
– Principle kinds are crysotile (form of the
mineral serpentine) and crocidolite (form of
amphibole)
Manmade Fibers
• Represent approximately 75% of total
textile fiber production in US
• Can be defined as a fiber of a particular
chemical composition that has been
manufactured into a particular shape and
size, contains a certain amount of various
additives, and has been processed in a
particular way
Manmade Fibers (continued)
• Within the 6 most seen of the 21 generic
classifications established by the US
Federal Trade Commission, there are well
over a 1,000 different fiber types
• Therefore, numerous fiber types can be
present in the composition of textile
materials
– This is true before even considering differences
in color
Importance of Fiber Evidence
• Perpetrators of crimes are not always aware
or able to control the fibers they have left
behind or picked up
Importance of Fiber Evidence
• In contrast to hair, fibers offer much
greater evidential value because they
incorporate numerous variables
– Number of fibers in each strand, diameter of
strands and fibers, direction and number of
twists, type of weave, and dye content, as well
as foreign material embedded or adherent to
the fiber
Important to Remember:
• It is important to collect evidence from both
complainants and suspects as soon as
possible
• Studies show that some 80% of fibers can
be expected to be lost in four hours, with
just 5-10% remaining at the end of 24 hours
Methods of Examination
• In the recent past, the ID and comparison of
fibers were at a relatively simple level which
relied heavily on microscopy
From Less than 1 cm of a 20 mm Diameter
Fiber It is Possible to Determine:
•
•
•
•
•
•
•
Generic class
Polymer composition
Finish--bright/dull
Cross-sectional shape
Melting point
Refractive Indices
Birefringence
•
•
•
•
•
Color
Fluorescence
Absorption spectrum
Dye class
Dye Components
Microscopy
• Microscopic examination provides the
quickest, most accurate, and least
destructive means of determining the
microscopic characteristics and polymer
type of textile fibers.
Microscopic View
Acetate
Dacron
Stereomicroscope
• Should be used first to examine fibers.
• Physical features such as crimp, length,
color, relative diameter, luster, apparent
cross section, damage, and adhering debris
should be noted.
• Fibers are then tentatively classified into
broad groups such as synthetic, natural, or
inorganic.
Comparison Microscope
• If all of the characteristics are the same under
the stereoscope, then the comparison
microscope is used.
• A point-by-point and side-by-side
comparison provides the most discriminating
method of determining if two or more fibers
are consistent with originating from the same
source.
Comparison Microscopy
• Side-by-side
Comparison
• Bright Field
Adjustment
Comparison Microscopy
• Characterization
• Fluorescence
– Chemical factors
– Environmental factors
Comparison Microscope
• Comparisons should be made under the
same illumination conditions at the same
magnifications.
• This requires color balancing the light
sources.
• A balanced neutral background color is
optimal.
Fluorescence Microscopy
Kevlar fibers in
complex composite
material strongly
fluoresce.
• The sample is
illuminated by
ultraviolet light,
causing some
phases to fluoresce
so they can be
observed, counted,
sized and mapped.
Polarized Light Microscope
• Perhaps the most versatile
of all microscopes; allows
the analyst to actually see
and manipulate the sample
of interest.
• Refractive indices,
birefringence, and
dispersion can all be
quantitatively determined.
Microspectrophotometry
• To the unaided eye, 2 dyes
may be identical.
• Using a grating
spectrometer, light
absorbed by or reflected
from a sample is separated
into its component
wavelengths, and intensity
at each wavelength plotted.
Microspectrophotometry
• Microscope linked to a
Spectrophotometer
– IR Absorption spectrum
– UV/VIS Absorption Spectrum
Microspectrophotometry
• IR spectography identifies generic subtypes
indistinguishable by microscopic exam
• Use of IR microscopes coupled with Fourier
transform infrared (FT-IR) spectrometers
has greatly simplified the IR analysis of
single fibers
Microspectrophotometry
• Advantages
– Nondestructive
– Not limited to sample size
• Disadvantages
– Reactive dyes
– Chemical composition
– Tentative identification
Scanning Electron Microscopy
• SEM with energy dispersive
spectroscopy(EDS) is used as an imaging
and microanalytical tool in characterization
of fibers.
• Surface morphology can be examined with
great depth of field at continually variable
magnifications.
Thin-Layer Chromatography
• An inexpensive, simple, well-documented
technique that can be used (under certain
conditions) to complement the use of visible
spectroscopy in comparisons of fiber
colorants.
• Dye components are separated by their
differential migration caused by a mobile
phase flowing through a porous, adsorptive
medium.
TLC (continued)
• Should be considered for single-fiber
comparisons only when it is not possible to
discriminate between the fibers of interest
using other techniques, such as comparison
microscopy (brightfield and fluorescence)
and microspectrophotometry in the visible
range
TLC (continued)
• Technique
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Extraction of dyes
Solid stationary phase
Liquid moving phase
Capillary action
Chromatogram
TLC (continued)
• Interpretation
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–
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Rf (retention factor)
Color
Proportions
Scanning densitometer
• peak height ratios
– Fluorescence
TLC (continued)
• Analysis of Chromatograms
– Positive association
– Exclusion
– Inconclusive
TLC (continued)
• Advantages
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–
–
–
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Highly discriminatory
Inexpensive
Dye batch variation
Reactive dyes
Optically isomeric pair
TLC (continued)
• Disadvantages
– Destructive
– Pale fibers
– Tentative identification
Pyrolysis Gas Chromatography
• Pyrolysis is a destructive analytical
method.
• When the heat energy applied to the
polymer chains is greater than the energy
of specific bonds in that polymer chain,
these bonds will fragment.
PGC (continued)
• In PGC, the fragments generated by
pyrolysis are introduced into a gas
chromatograph for separation and
characterization
• PGC can be used to ID the generic type of
an unknown fiber, and in some cases it can
ID subclasses within a generic class
Analysis of Fibrous Materials
• The analyst should perform a combination
of methods that extract the greatest potential
for discrimination between samples.
• A minimum of 2 of the analytical
techniques must be performed for each
category.
Case Study
• On Saturday, September 19,1987, the body
of an 18-year-old was found on the side of a
road in Finland
• The deceased was completely naked from
the waist down, but her jeans, stockings,
and underwear were bundled on her chest
Case Study (continued)
• TI team took as samples the clothing of the
deceased, fingernail scrapings, head hair
and pubic hair combings, and known hair
standards
• Fiber evidence was found in the victim’s
hair
• Semen was found on vaginal and anal
samples taken at autopsy
Case Study (continued)
• Eleven brown uniform and multicolor
acrylic fibers were found in the combed hair
samples
• Similar fibers were searched from the tapelifted samples taken from the clothes--about
200 fibers were found in the clothes
Case Study (continued)
• With help of reference samples, it was
deduced that these fibers most likely
originated from the seat cover of a car
• Police were advised the lab was especially
interested in brown automobile seat covers
made from a pile-type material, and they
were asked to give special attention to red
textiles while searching cars
Case Study (continued)
• TI team examined 12 cars during a period
of 2 months--samples did not match
• The 13th car to be examined to a person
who had been charged with rape 9 years
earlier but released because of
insufficient evidence
Case Study (continued)
• The seat cover material of his car matched
the fibers from the victim
• In addition, the samples taped from the seat
covers contained similar red-colored
viscose and woolen fibers found in samples
taken from the victim
– These proved to be the same as cloth fibers
found of the pillowcase found on suspect’s sofa
Case Study (continued)
• Other fiber evidence was also liked to the
suspect’s residence
• Accused did not confess to the crime but
admitted that he alone used his car
• He was convicted and sentenced to 10 years
on the basis of evidence produced by the
fiber investigation
Evidential Value
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Establish a sequence of events.
Link murder weapon with victim or suspect.
Help corroborate victim’s account.
Provide leads about surroundings at time of
incident.
• Link together a number of different victims or
criminal activities.
• Establish a high probability that contact or other
association has taken place between people
and/or objects.
Conclusions of Fiber Analysis
• The unknown fibers could have originated
from the known.
• The unknown fibers could not have
originated from the known.