|
April 1999 Volume 1
Number 1
Chapter 2 of Forensic Fiber Examination Guidelines
1.0. Scope
This section describes guidelines
for microscopical examinations employed in forensic fiber characterization,
identification, and comparison. Several types of light microscopes
are used including stereobinocular, polarized light, comparison,
fluorescence, and interference. In certain instances, the scanning
electron microscope yields additional information. The nature
and extent of the fiber evidence will dictate which tests or
techniques are selected and performed.
2.0. Reference
Documents
SWGMAT Quality Assurance
Guidelines
SWGMAT Trace Evidence Handling Guidelines
Back to the top
3.0. Terminology
Anisotropic: An object that has properties that
differ according to the direction of measurement when viewed
in polarized light.
Barrier Filter: A filter used in fluorescence microscopy
that suppresses unnecessary excitation light that has not been
absorbed by the fiber and selectively transmits only the fluorescence.
Becke Line: The bright halo near the boundary
of a fiber that moves with respect to that boundary as the microscope
is focused through best focus.
Becke Line Method: A method for determining the refractive
index of a fiber relative to its mountant by noting the direction
in which the Becke line moves when the focus is changed. The
Becke line will always move toward the higher refractive index
medium (fiber or mountant) when focus is raised and will move
toward the lower refractive index medium when focus is lowered.
Birefringence: The numerical difference in refractive
indices for a fiber, given by the formula: nll
- n^. Birefringence can be calculated
by determining the retardation (r) and thickness (T) at a particular
point in a fiber and by using the formula: B = r (nm)/1,000 T
(mm).
Comparison Microscope: A system of two microscopes positioned
side by side and connected via an optical bridge in which specimens
are examined simultaneously in either transmitted or reflected
light.
Compensator: Any variety of optical devices that
can be placed in the light path of a polarizing microscope to
introduce fixed or variable retardation comparable with that
exhibited by the fiber. The retardation and sign of elongation
of the fiber is then determined. Compensators can employ a fixed
mineral plate of constant or varying thickness or a mineral plate
that is rotated to alter the thickness presented to the optical
path (and retardation introduced) by a set amount.
Compensator, Full Wave
(Red Plate): A compensator
using a plate of gypsum, selenite, or quartz, which introduces
a fixed retardation between 530-550 nm (approximately the retardation
of the first order red color on the Michel-Lévy chart).
Compensator, Quarter Wave: A compensator, usually with a mica
plate, which introduces a fixed retardation between 125-150 nm.
Compensator, Quartz Wedge: A wedge, cut from quartz, having
continuously variable retardation extending over several orders
of interference colors (usually 3-7).
Compensator, Sénarmont: A quarter-wave plate inserted above
the specimen in the parallel 0" position with a rotating
calibrated analyzer. Measures low retardation and requires the
use of monochromatic light.
Compensator, Tilting (Berek): A compensator typically containing
a plate of calcite or quartz, which can be rotated by means of
a calibrated drum to introduce variable retardation up to about
ten orders.
Cortex: The main structural component of
hair consisting of elongated and fusiform (spindle-shaped) cells.
The cortex contains pigment grains, air spaces called cortical
fusi, and structures called ovoid bodies.
Crimp: The waviness of a fiber.
Cross-Over Marks: Oblique flattened areas along silk
fibers caused by the overlapping of extruded silk fibers before
they have dried completely.
Cuticle: The layer of scales composing the
outer surface of a hair shaft. Cuticular scales are normally
classified into three basic types: coronal (crown-like), spinous
(petal-like), and imbricate (flattened).
Delustrant: A pigment, usually titanium dioxide, used
to dull the luster of a manufactured fiber.
Dichroism: The property of exhibiting different
colors, especially two different colors, when viewed in polarized
light along different axes.
Dislocations: Concerning natural fibers (e.g.,
flax, ramie, jute, and hemp) where distinct features in the shape
of X's, I's, and V's are present along the fiber cell wall. These
features are often useful for identification.
Dispersion of Birefringence: The variation of birefringence with
wavelength of light. When dispersion of birefringence is significant
in a particular fiber, anomalous interference colors not appearing
in the regular color sequence of the Michel-Lévy chart
may result. Strong dispersion of birefringence also interferes
with the accurate determination of retardation in highly birefringent
fibers.
Dispersion Staining: A technique for refractive index
determination that employs central or annular stops placed in
the objective back focal plane of a microscope. Using an annular
stop with the substage iris closed, a fiber mounted in a high-dispersion
medium will show a colored boundary of a wavelength where the
fiber and the medium match in refractive index. Using a central
stop, the fiber will show colors complimentary to those seen
with an annular stop.
Dyes: Soluble substances that add color
to textiles. Dyes are classified into groups that have similar
chemical characteristics (e.g., aniline, acid, and azo). They
are incorporated into the fiber by chemical reaction, absorption,
or dispersion.
Excitation Filter: A filter used in fluorescence microscopy
that transmits specific bands or wavelengths of energy capable
of inducing visible fluorescence in various substrates.
Inorganic Fibers: A class of fibers of natural mineral
origin (e.g., chrysotile asbestos) and manmade mineral origin
(e.g., fiberglass).
Interference Colors: Colors produced by the interference
of two out-of-phase rays of white light when a birefringent material
is observed at a nonextinction position between crossed polars.
The retardation at a particular point in a birefringent fiber
is determined by comparing the observed interference color to
the Michel-Lévy chart.
Isotropic: An object that is identical in all
directions and invariant with respect to direction.
Light Microscope: A microscope that employs light
in the visible or near-visible portion of the electromagnetic
spectrum.
Lignin: The majority noncarbohydrate portion
of wood. It is an amorphous polymeric substance that cements
cellulosic fibers together. The principal constituent of woody
cell walls.
Lumen: The cavity or central canal present in many natural fibers
(e.g., cotton, flax, ramie, jute, and hemp). Its presence and
structure are often a useful aid in identification.
Luster: The gloss or shine possessed by
a fiber, resulting from its reflection of light. The luster of
manufactured fibers is often modified by use of a delustering
pigment.
Manufactured Fiber: A class name for various families
of fibers produced from fiber-forming substances, which are synthesized
polymers, modified or transformed natural polymers, and glass.
Medulla: The central portion of a hair composed
of a series of discrete cells or an amorphous spongy mass. It
may be air-filled and, if so, will appear opaque or black using
transmitted light or white using reflected light. In animal hair,
several types have been defined: uniserial or multiserial ladder,
cellular or vacuolated, and lattice.
Michel-Lévy Chart: A chart relating thickness, birefringence,
and retardation so that any one of these variables can be determined
for an anisotropic fiber when the other two are known.
Microscopical: Concerning a microscope or the use
of a microscope.
Modification Ratio: A geometrical parameter used in
the characterization of noncircular fiber cross sections. The
modification ratio is the ratio in size between the outside diameter
of the fiber and the diameter of the core. It is also called
aspect ratio.
Natural Fibers: A class name of fibers of vegetable
origin (e.g., cotton, flax, and ramie), animal origin (e.g.,
silk, wool, and specialty furs), or of mineral origin (e.g.,
asbestos).
Pigment: A finely divided insoluble material
used to deluster or color fibers (e.g., titanium dioxide and
iron oxide).
Plane Polarized Light: Light that is vibrating in one
plane.
Pleochroism: The property of exhibiting different
colors, especially three different colors, when viewed in polarized
light along different axes.
Polarized Light: A bundle of light rays with a single
propagation direction and a single vibration direction. The vibration
direction is always perpendicular to the propagation direction.
It is produced by use of a polarizing filter, from ordinary light
by reflection, or double refraction in a suitable pleochroic
substance.
Polarized Light Microscope: A microscope equipped with two polarizing
filters, one below the stage (the polarizer) and one above the
stage (the analyzer).
Privileged Direction (of
a Polarizer): The
direction of vibration to which light emerging from a polarizer
has been restricted.
Refractive Index: For a particular transparent medium,
the ratio of the speed of light in a vacuum to the speed of light
in that medium.
Relative Refractive Index: The estimate of the refractive index
of a fiber in relation to the index of its surrounding medium.
Retardation (r): The actual distance of one of the
doubly refracted rays behind the other as they emerge from an
anisotropic fiber. Dependent upon the difference in the two refractive
indices, n2 – n1, and the thickness of the fiber.
Sign of Elongation: Referring to the elongation of a
fiber in relation to refractive indices. If elongated in the
direction of the high refractive index, the fiber is said to
be positive, and if elongated in the direction of the low refractive
index, it is said to be negative.
Spherulites: Spheres composed of needles or rods
all oriented perpendicular to the outer surface or a plane section
through such a sphere. A common form of polymer crystallization
from melts or concentrated solutions.
Stereomicroscope: A microscope containing two separate
optical systems, one for each eye, giving a stereoscopic view
of a specimen.
Surface Dye: A colorant bound to the surface
of a fiber.
Synthetic Fibers: A class of manufactured polymeric
fibers that are synthesized from chemical compounds (e.g., nylon
and polyester).
Technical Fiber: A bundle of natural fibers composed
of individual elongated cells that can be physically or chemically
separated and examined microscopically for identifying characteristics
(e.g., hemp, jute, and sisal).
Thermoplastic Fiber: A synthetic fiber that will soften
or melt at high temperatures and harden again when cooled.
Thickness (T): The optical path through the fiber
used for the calculation of birefringence, typically measured
in micrometers.
Back to the top
4.0.
Summary of Microscopy Guidelines
Textile fibers are examined
microscopically. They are mounted on glass microscope slides
in a mounting medium under a cover slip. The fibers are then
examined microscopically with a combination of various illumination
sources, filters, and instrumentation attached to a microscope
to determine their polymer type and record any microscopic characteristics.
Known and questioned fibers are then compared to determine if
they exhibit the same microscopic characteristics and optical
properties.
Back to the top
5.0. Significance
and Use
Microscopic examination provides
the quickest, most accurate, and least destructive means of determining
the microscopic characteristics and polymer type of textile fibers.
Additionally, a point-by-point and side-by-side microscopic comparison
provides the most discriminating method of determining if two
or more fibers are consistent with originating from the same
source. These guidelines require specific pieces of instrumentation
outlined herein.
Back to the top
6.0. Sample Handling
6.1. Preparing Samples
Items of evidence
are visually inspected, and tweezers are used to remove fibers
of interest. Simple magnifiers and stereomicroscopes, with a
variety of illumination techniques, may also be employed. Other
methods such as tape lifting or gentle scraping are usually conducted
after a visual examination. Tape lifts should be placed on clear
plastic sheets, glass microscope slides, or another uncontaminated
substrate that eases the search and removal of selected fibers.
Do not overload the tapes. The tape lifts or any material recovered
from scraping should be examined with a stereomicroscope, and
fibers of interest should be isolated for further analysis. Fibers
on tape lifts are removed using tweezers, other microscopic tools,
and solvents (1-6). Tape should not be attached to paper or cardboard.
6.2. Avoiding Contamination
Take care to ensure contamination does not occur. This must be
accomplished by examining questioned and known items in separate
areas, at different times, or both. The work area and tools must
be thoroughly cleaned and inspected before examining items that
are to be compared.
Back to the top
7.0. Analysis
7.1. Microscopy
Fibers should be first examined with a stereomicroscope. 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. If the sample contains
yarns, threads, or sections of fabric, construction should be
recorded (7-9).
7.1.1. Side-by-Side Comparisons. If all of the characteristics are
the same under the stereoscope, the next step is to examine the
fibers with a comparison microscope. This side-by-side and point-by-point
examination is the best technique to discriminate between fibers,
especially those that appear to be similar. The physical characteristics
of the fibers (see subsection 7.3) must be compared visually
with the comparison microscope to determine if they are the same
in the known and questioned samples. Photography is recommended
to capture the salient features for later demonstration.
7.1.2. Illumination and
Magnification. Comparisons
should be made under the same illumination conditions at the
same magnifications. For comparison microscopes, this requires
color balancing the light sources. This is best achieved with
two fibers or fiber samples from the same source mounted on two
microscope slides, which are then compared. The visual responses
from the two samples must be approximately the same color, brightness,
and clarity. A balanced neutral background color is optimal.
7.2. Fiber Mounts
Many suitable media are available as temporary and permanent
fiber mounts. The choice of mountant depends on availability,
the particular application, and examiner preference. However,
the following certain criteria (5, 10-15) must be met:
7.2.1. Mounting Media. An examiner should be aware of the
possible deleterious effects that a mounting medium (especially
solvent-based media) has on textile fibers, particularly when
mounted for a long time. It is preferable that the mounted fibers
that were previously examined microscopically be used for chemical
analysis. If fibers must be removed for further testing, the
mounting medium should be removed with a solvent that will not
affect the structure or composition of the fiber.
7.2.2. Consistency of
Mountants. Fibers
that are to be compared microscopically must be mounted in a
mounting medium. The same mountant should be used for both questioned
and known fibers.
7.2.3. Indexing Mountants. If a solvent-based mounting medium
is used for refractive index (q.v.) determination, the index
of the mountant should be checked periodically against solid
refractive index standards and, if necessary, readjusted to its
proper value by the addition of solvent (16). Additionally, the
refractive index of the medium can be measured directly, and
the value can be recorded by the examiner. If such a medium is
used for permanent mounts, the examiner should be aware of the
different refractive indices for the fluid medium and the resin
after solvent evaporation.
7.2.4. Using Liquids. Liquids used for exact refractive
index determinations should be known to within +0.0005 refractive
index units at nD. To make appropriate temperature
corrections, values for the temperature coefficient (dn/dt) for
each liquid and a thermometer covering the range 20-30°C,
calibrated in tenths of a degree, should be available. High dispersion
liquids (V < 30) are desirable for dispersion staining and
the Becke line method (17). Cargille refractive index liquids
are suitable for this purpose and are recommended for refractive
index measurements of fibers.
7.3. Physical Characteristics of Manufactured
Fibers
7.3.1. Fiber Diameter. The diameter of circular fibers
can be measured using a calibrated eyepiece graticule. Noncircular
fibers require special considerations (18). If fiber diameters
are not uniform within a sample, a determination of the range
of diameters exhibited by the sample is recommended.
7.3.2. Fiber Color. Color can be uniform along the length
of a fiber, or it can vary. Variation in color between fibers
in a sample should be recorded. The examiner should be able to
distinguish between dyed, surface-dyed, and pigmented fibers.
7.3.3. Delustrant Particles. The presence or absence of delustrant
particles is a useful comparative feature. If present, the size,
shape, distribution, relative abundance, and general appearance
should be noted. Delustrant particles, although not indicative
of any particular generic fiber type, can be characteristic of
end-use properties needed by a manufacturer. Also, delustrants
serve to eliminate all but manufactured fibers.
7.3.4. Cross-Sectional
Shape. When viewed
longitudinally on glass slides in a suitable mountant, the apparent
cross-sectional shape of fibers can often be determined by slowly
focusing through the fiber (optical sectioning). Actual fiber
cross sections provide the best information on cross-sectional
shape. (See section 8.1)
7.3.5. Fiber Surface Characteristics. Record fiber surface characteristics
such as manufacturing striations, damage, and surface debris
(e.g., droplets, blood, or other foreign material). Surface striations
are more apparent in a mounting medium of refractive index significantly
different from those of the fiber (7).
7.4. Physical Characteristics of Natural
Fibers
7.4.1. Physical Features. Color, diameter, and miscellaneous
physical features described previously should be noted for natural
fibers. The following characteristics should also be noted.
7.4.2. Morphological Features
of Animal Hairs.
The principal morphological features of animal hairs are the
root, medulla, cortex, and cuticle. Shield size and subshield
strictures are also useful traits for species identification.
Medullary and cortical structures are best observed on hairs
mounted on a slide with a suitable mounting medium. Cuticular
scales are best observed on replicas cast in a transparent polymer
(scale casts). Scale counts (scales per 100 micrometers) can
help distinguish specialty fur fibers (19-22). Silk, a protein
fiber produced by caterpillars, has morphological features that
differ from animal hairs. Some features of silk include crossover
marks and a wedge to triangular cross section with rounded corners.
In textiles, silk is occasionally seen as paired fibers cemented
together, but it is most often found as single fibers (23).
7.4.3. Plant Fibers. Plant fibers can be encountered
as the technical fiber (cordage, sacks, and mats) or as individual
cells (fabrics and paper). The examination of technical fibers
should include a search for epidermal tissue and crystals and
the preparation of a cross section. Additionally, a chemical
test for lignin may be performed. Technical fibers should be
macerated, fabrics teased apart, and paper repulped for the examination
of individual cells. Relative thickness of cell walls and lumen,
cell length, and the presence, type, and distribution of dislocations
should be noted. The direction of twist of the cellulose in the
cell wall can also be determined (24). Other characteristic cells
should be noted and compared to authentic specimens (25-27).
7.5. Physical Characteristics of Inorganic
Fibers
7.5.1 Asbestos Minerals. Mineral fibers are commonly called
asbestos, which is a general term for many naturally occurring
fibrous hydrated silicate minerals. The asbestos minerals include
chrysotile, amosite, crocidolite, fibrous tremolite/actinolite,
and fibrous anthophyllite. Chrysotile belongs to the serpentine
group of minerals that are layer silicates. The other asbestos
minerals are amphiboles and are classified as chain silicates.
Asbestos fibers alone or mixed with other components occur in
building materials and insulation products. Chrysotile is the
only asbestos mineral that would be encountered as a woven fabric,
but any of the asbestos minerals are found in pressed sheets
such as gaskets. Take care when analyzing asbestos fibers because
they are considered a potential health hazard.
All asbestos minerals can
be easily identified by their optical properties using polarized
light microscopy. Although not considered essential, the dispersion
staining technique is extremely helpful (28-29). Scanning electron
microscopy with energy dispersive spectrometry can also be used
to characterize the asbestos minerals. Nonmicroscopical techniques
for asbestos identification include X-ray diffraction and infrared
spectroscopy.
Glass fibers are often encountered
in building materials and insulation products. Glass fibers are
also called manmade vitreous fibers (30). On the basis of the
starting materials used to produce glass fibers, they can be
placed into three categories: fiberglass (continuous and noncontinuous),
mineral wool (rock wool and slag wool), and refractory ceramic
fibers (glass ceramic fibers). Single crystal and polycrystalline
refractory fibers such as aluminum oxide, silicon carbide, zirconium
oxide, and carbon are not included because they are not considered
glass fibers.
Light microscopy is used,
together with classical immersion methods, to determine the refractive
index for the classification and comparison of glass fibers.
The dispersion staining technique is used when determining the
refractive index and variation of the refractive index within
a sample. Determination and comparison of the refractive index
of noncontinuous (fiberglass) wool, rock wool, and slag wool
can also be accomplished by annealing the fibers and using the
double variation method (31-33). Solubility tests using 10 percent
HCl should be conducted and the results noted. A binder resin
that fluoresces under UV light may also be present on some glass
wool products.
Scanning electron microscopy
with energy dispersive spectrometry is used to provide elemental
composition. Elemental ratios are used for comparison purposes.
It is necessary to eliminate any absorption effects when acquiring
the energy dispersive spectrum. Otherwise, artificial variation
in the elemental composition is introduced (34).
7.6. Optical Characteristics
Detailed discussions of optical
characteristics are provided by McCrone (35-38), McCrone, McCrone,
and Delly (18), Bloss (39), Chamot and Mason (40), Hartshorne
and Stuart (41), and Stoiber and Morse (42).
7.6.1. Refractive Index. The refractive index, n, of a transparent
material is n = (speed of light in a vacuum)/(speed of light
in the material). All transparent fibers other than glass display
two principle refractive indices, one for light polarized parallel
to the long axis of the fiber (nll) and one for light polarized perpendicular
to the long axis of the fiber (n^). For fibers examined in unpolarized
light, a third quantity, niso (defined as 1/3[2 n^
+ nll]), may also be estimated. Because
refractive index varies with wavelength and temperature, a standard
refractive index (n), is defined for all transparent materials
as the refractive index at a wavelength of 589 nm (the D line
of sodium) at 25°C.
The refractive indices of
a fiber are determined by several methods. Whatever the method
used, determination of nll and n^ should be made using plane polarized
light with the fiber aligned parallel and perpendicular to the
privileged direction of the polarizer, respectively. The vibration
direction of the polarizer should coincide with the horizontal
line of the eyepiece graticule.
Refractive index measurements
are either relative or exact. A relative refractive index measurement
involves (a) determining whether an immersed object is higher
or lower in refractive index than the immersion medium and (b)
estimating the approximate refractive index on the basis of amount
of contrast between the fiber and the medium. The contrast shows
the amount of difference between the fiber and the medium. Exact
numerical values for nll and n^ of a fiber (at 589 nm at 25°C)
can be determined by the Becke line method or by dispersion staining.
Measurements using these methods have a precision of ±0.001
(18).
For a fiber displaying two
refractive indices, birefringence is defined as |nll - n^|. Birefringence is determined measuring
nll and n^ and using the previous formula or
by determining the retardation with the corresponding thickness
of the fiber and calculated with the following formula:
Retardation (nm)/Birefringence = 1,000 × Thickness (mm)
The retardation can be estimated
by observing the interference color displayed at the point where
the thickness of the fiber is measured and by comparing it to
the Michel-Lévy chart. Take care when interpreting results
from deeply dyed fibers, as the dye can obscure the interference
colors. A wedge slice through the fiber; the use of various compensators
such as the Sénarmont, quartz wedge, and tilting (Berek);
or both can be used to make a more accurate determination of
retardation (43). When measuring retardation of a fiber using
a tilting compensator or quartz wedge, one must assure no error
has been introduced because of differences in dispersion of birefringence
between the compensator and the fiber (44). This is of special
concern with the examination of fibers with high birefringence.
The birefringence of noncircular fibers is estimated by measuring
both retardation and thickness at two points along the fiber
that represent their highest and lowest values (45).
7.6.2. Birefringence. For a birefringent fiber, the sign
of elongation is positive (+) if nll > n^ and negative (-) if nll < n^. It should be noted that all common
manufactured fibers with a birefringence higher than 0.010 have
a positive sign of elongation. Full- or quarter-wave compensators
are commonly used to make this determination for fibers with
low birefringence (5, 39).
7.6.3. Pleochroism. Pleochroism (or dichroism) is the
differential absorption of light by an object when viewed at
different orientations relative to the vibration direction of
plane polarized light. Certain dyed fibers and some mineral fibers
may exhibit pleochroism.
7.6.4. Fluorescence. Fluorescence is the emission of
light of a certain wavelength by an object when excited by light
of a shorter wavelength (higher energy). Fluorescence may arise
from fibers themselves or from dyes and other additives. Fibers
should be mounted in a low- to-nonfluorescent medium to observe
fluorescence. Examination using various combinations of excitation
and barrier filters is desirable. At each excitation wavelength,
the color and intensity or absence of fluorescence emission should
be noted (5, 7, 46-50).
7.7. Miscellaneous Techniques
7.7.1. Preparing Cross
Sections. Physical
cross sections from fibers as short as 1 mm can be prepared.
Manufactured and vegetable fibers may be sectioned anywhere along
their length (54-59). Animal hairs may be sectioned to yield
additional identifying characteristics (60-61). When observing
manufactured fiber cross sections, the general shape and distribution
of delustrant, pigment particles, or both; the presence and size
of spherulites or voids; depth of dye penetration; and surface
treatments should be recorded when present. The fiber dimensions
measured from a cross section can be used for the calculation
of birefringence and the determination of the modification ratio
of multilobed fibers.
7.7.2. Solubility Testing. Solubility is a destructive method.
Solubility testing can, however, provide supplemental information
to nondestructive methods. Possible reactions of fibers to solvents
include partial and complete solubility, swelling, shrinking,
gelling, and color change. If solubility tests are used as part
of an identification scheme, appropriate controls should be run
following the laboratory's quality assurance and control guidelines
for a lot or batch of reagents or solvents. It is desirable to
view known and questioned fibers simultaneously when comparing
their solubilities (5, 62-64).
7.7.3. Heat Effects. A polarized light microscope equipped
with a hot stage is recommended for observations of the effect
of heat on thermoplastic fibers. Using slightly uncrossed polars,
one may observe droplet formation, contraction, softening, charring,
and melting of fibers over a range of temperatures. These observations,
including melting temperature or temperatures, should be recorded.
Because manufactured fibers are composed of mixtures of chemical
compounds rather than pure polymers and are a combination of
crystalline and amorphous regions, changes are observed over
a temperature range rather than at a single melting point (5,
7, 65-69). Fibers should be mounted in an inert, heat-resistant
medium, such as a high-temperature-stable silicone oil, to ensure
reproducible melting behavior (70-71). Accurate and reproducible
results are best obtained using a heating rate of no greater
than 1-2°C/minute when near the initial melting temperature.
The hot stage should be calibrated using appropriate standards,
following established guidelines. The recommended melting point
apparatus should be adjustable for temperatures from ambient
to at least 300°C, in increments of 0.1°C, and should
allow a heating rate of as low as 1°C/minute (72-79).
7.7.4. Fiber Surface Morphology.
Scanning electron
microscopy with energy dispersive spectroscopy (SEM-EDS) is used
as an imaging and microanalytical tool in the characterization
of fibers. Fiber surface morphology can be examined with great
depth of field at continually variable magnifications. Fibers,
prepared cross sections, or both are mounted to a specimen stub
and may be conductively coated to prevent possible electron beam
charging. The use of a suitable calibration standard is recommended
for the accurate measurement of fiber cross sections.
Applications of SEM-EDS to
fiber analysis include the characterization of fiber cross sections,
identification of pigments and delustrants by elemental analysis,
fiber damage due to cuts and tears (51-53, 80-82), trace debris
on fibers, surface feature modifications such as washer/dryer
abrasion (83), and acid-washed treatment of denim garments (84).
Authors have examined fiber bonding in nonwoven fabrics and shrink-proofing
treatment of wool (85). Surface imaging using the SEM-EDS as
an aid in the identification of animal hair-scale structure has
been reported (86).
Back to the top
8.0. Report
Documentation
The examiner's analytical
notes should reflect the particular characteristics used in the
microscopic comparison, especially any calculated values, descriptions,
diagrams, or photographs. A positive association is when the
questioned and known fibers exhibit the same microscopic characteristics
and optical properties in all tested parameters and are therefore
consistent with originating from the same source. A negative
association is when the questioned and known fibers are different
in some significant aspect and are therefore from separate sources.
An inconclusive result indicates that no conclusion could be
reached, and some explanation is required as to why a definitive
conclusion was not possible.
Back to the top
9.0. References
(1) Grieve, M. C. and Garger,
E. F. An improved method for rapid and accurate scanning of fibers
on tape, Journal of Forensic Sciences (1981) 26:560-563.
(2) Choudhry, M. Y. A novel
technique for the collection and recovery of foreign fibers in
forensic science case work, Journal of Forensic Sciences
(1988) 33:249-253.
(3) Wickenheiser, R. A. Fiber
concentration by membrane vacuum filtration in preparation for
rapid microscopic comparison, Canadian Society of Forensic
Science Journal (1992) 25(31):177-181.
(4) Chable, J., Roux, C.,
and Lennard, C. Collection of fiber evidence using water-soluble
cellophane tape, Journal of Forensic Sciences (1994) 39(6):1520-1527.
(5) Gaudette, B. The forensic
aspects of textile fiber examination. In: Forensic Science
Handbook (Vol. 2). Ed., R. Saferstein. Prentice-Hall, Englewood
Cliffs, New Jersey, 1988.
(6) Robertson, J. The forensic
examination of fibers: Protocols and approaches—An overview.
In: Forensic Examination of Fibers. Ed., J. Robertson.
Ellis Horwood, Chichester, United Kingdom, 1992.
(7) Carroll, G. R. Forensic
fibre microscopy. In: Forensic Examination of Fibers.
Ed., J. Robertson. Ellis Horwood, Chichester, United Kingdom,
1992.
(8) Corbman, B. P. Textiles:
Fiber To Fabric. 6th ed. McGraw-Hill, New York, 1983, pp.
68-142.
(9) Grayson, M., ed. Encyclopedia
of Textiles, Fibers, and Nonwoven Fabrics. John Wiley and
Sons, New York, 1984.
(10) Cook, R. and Norton,
D. An evaluation of mounting media for use in forensic textile
fibre examination, Journal of the Forensic Science Society
(1982) 22:57-63.
(11) Roe, G. M., Cook, R.,
and North, C. An evaluation of mountants for use in forensic
hair examination, Journal of the Forensic Science Society
(1991) 31:59-65.
(12) McCrone, W. C. Use of
aroclors in microscopy, Microscope (1984) 32:277-288.
(13) Sacher, R. New Materials
for the microscopist, Microscope (1985) 33:241-246.
(14) Grieve, M. and Deck,
S. A new mounting medium for the forensic microscopy of textile
fibers, Science and Justice (1995) 35(2) 109-112.
(15) Loveland, R. P. and
Centifano, Y. M. Mounting media for microscopy, Microscope
(1986) 34:181-242.
(16) Allen, R. M. Practical
Refractometry by Means of the Microscope. 2nd ed. R. P. Cargille
Laboratories, 1985.
(17) McCrone, W. C., McCrone
L. B., and Delly, J. G. Polarized Light Microscopy. Ann
Arbor Science Publishers, Ann Arbor, Michigan, 1978.
(18) Metropolitan Police
Forensic Science Laboratory. Biology Methods Manual. Commissioner
of Police of the Metropolis, London, 1978, Section 6, pp. 16-17.
(19) David, S. K. and Pailthorpe,
M. T. Classification of textile fibers: Production, structure
and properties. In: Forensic Examination of Fibers. Ed.,
J. Robertson. Elllis Horwood, Chichester, United Kingdom, 1992,
pp. 3-10.
(20) Appleyard, H. M. Guide to the Identification of Animal
Fibers. 2nd. ed. Wool Industries Research Association, Leeds,
United Kingdom, 1978.
(21) Petraco, N. A. Microscopical
method to aid in the identification of animal hair, Microscope
(1987) 35:83-92.
(22) Robertson, J. The forensic
examination of fibers: Protocols and approaches—An overview.
In: Forensic Examination of Fibers. Ed., J. Robertson.
Ellis Horwood, Chichester, United Kingdom, 1992, pp. 79-87.
(23) Mauersberger, H. R.
Matthew's Textile Fibers. 6th ed., John Wiley and Sons,
Inc., New York, 1954.
(24) Valaskovic, G. A. Polarized
light in multiple birefringent domains: A study of the Herzog
Effect, Microscope (1991) 39:269-286.
(25) David, S. K. and Pailthorpe,
M. T. Classification of textile fibers: Production, structure
and properties. In: Forensic Examination of Fibers. Ed.,
J. Robertson. Ellis Horwood, Chichester, United Kingdom, 1992,
pp. 10-18.
(26) Catling, D. L. and Grayson,
J. Identification of Vegetable Fibers. Chapman and Hall,
London, 1982.
(27) Luniak, B. Identification
of Textile Fibers, Qualitative and Quantitative Analysis of Fibre
Blends. 2nd ed. Pitman, London, 1953.
(28) McCrone, W. C. The
Asbestos Particle Atlas. Ann Arbor Science, Ann Arbor, Michigan,
1980.
(29) McCrone, W. C. Asbestos
Identification. McCrone Research Institute, Chicago, Illinois,
1987.
(30) Man-Made Vitreous
Fiber: Nomenclature, Chemistry, and Physical Properties.
Nomenclature Committee of TIMA, Inc., 1993.
(31) Miller, E. T. Practical
method for the comparison of mineral wool insulations in forensic
laboratories, Journal of the AOAC (1975) 58(5):865-870.
(32) Miller, E. T. Comparison
of mineral wool insulations in forensic laboratories, Journal
of the AOAC (1977) 60:772-777.
(33) Miller, E. T. Comparison
of mineral wool insulations in forensic laboratories: Second
collaborative study, Journal of the AOAC (1979) 62(4):792-798.
(34) Goldstein, J. I., Newbury,
D. E., and Echlin, P. Scanning Electron Microscopy and X-ray
Microanalysis: A Text for Biologists, Materials Scientists, and
Geologists. Plenum Press, New York, 1992.
(35) McCrone, W. C. Particle
characterization by PLM: Part I. No polar, Microscope
(1982) 30(3):185-196.
(36) McCrone, W. C. Particle
characterization by PLM: Part II. Single polar, Microscope
(1982) 30(4):315-331.
(37) McCrone, W. C. Particle
characterization by PLM: Part III. Crossed polars, Microscope
(1983) 31(2):187-206.
(38) McCrone, W. C. Light
microscopy. In: Physical Methods of Chemistry. Vol. 4,
eds., B. W. Rossiter and J. F. Hamiton. John Wiley and Sons,
New York, 1991.
(39) Bloss, F. D. An Introduction
to the Methods of Optical Crystallography. Holt, Rinehart,
and Winston, Inc., New York, 1961.
(40) Chamot, E. M. and Mason,
C. W. Handbook of Chemical Microscopy. Vol. 1, 3rd ed.,
John Wiley and Sons, Inc., New York, 1958.
(41) Hartshorne, N. H. and
Stuart, A. Crystals and the Polarizing Microscope. 4th
ed. American Elsevier Publishing Company, New York, 1970.
(42) Stoiber, R. E. and Morse,
S. A. Crystal Identification with the Polarizing Microscope.
Chapman and Hall, New York, 1994.
(43) Feirabend, J. Problems
involved in the double refraction determination of stretched
polyethylene terephthalate fibers, Melliand Text (English
ed., Feb. 1976), pp. 149-156.
(44) Sieminski, M. A. A note
on the measurement of birefringence in fibers, Microscope
(1975) 23:35-36.
(46) Lloyd, J. B. F. Forensic
significance of fluorescent brighteners: Their qualitative TLC
characterisation in small quantities of fibre and detergents,
Journal of the Forensic Science Society (1977) 17:145-152.
(47) Kubic, T. A., King,
J. B., and DuBey, I. S. Forensic analysis of colorless textile
fibers by fluorescence microscopy, Microscope (1983) 31:213-222.
(48) Hartshorne, A. W. and
Laing, D. K. Microspectrofluorimetry of fluorescent dyes and
brighteners on single textile fibres, Part 1: Fluorescence emission
spectra, Forensic Science International (1991) 51:203-220.
(49) Hartshorne, A. W. and
Laing, D. K. Microspectrofluorimetry of fluorescent dyes and
brighteners on single textile fibres: Part 2. Colour measurements,
Forensic Science International (1991) 51:221-237.
(50) Hartshorne, A. W. and
Laing, D. K. Microspectrofluorimetry of fluorescent dyes and
brighteners on single textile fibres: Part 3. Fluorescence decay
phenomena, Forensic Science International (1991) 51:239-250.
(51) Rochow, Rochow. An
Introduction to Microscopy By Means of Light, Electrons, X-Rays,
or Ultrasound. Plenum Press, New York, 1978, pp. 211-220.
(52) Heuse, O. and Adolf,
F. P. Non-destructive identification of textile fibres by interference
microscopy, Journal of the Forensic Science Society (1982)
22:103- 122.
(53) Hemsley, D. A., ed.
Applied Polymer Light Microscopy. Elsevier Applied Science,
New York, 1989.
(54) Hall, D. Practical
Fiber Identification. Auburn University, Auburn, Alabama,
1982.
(55) Palenik, S. and Fitzsimons,
C. Fiber cross sections: Part I, Microscope (1990) 38:187-195.
(56) Palenik, S. and Fitzsimons,
C. Fiber cross sections: Part II, Microscope (1990) 38:313-320.
(57) Barna, C. E. and Stoeffler,
S. F. A new method for cross sectioning single fibers, Journal
of Forensic Sciences (1987) 32:761-767.
(58) Grieve, M. C. and Kotowski,
T. M. An improved method of preparing fiber cross sections, Journal
of the Forensic Science Society (1986) 26:29-34.
(59) Craven, B. J. Cross-sectional
measurement of cellulose acetate fibers using scanning electron
microscopy and image analysis, Microscope (1993) 41:115-117.
(60) Brunner, H. and Coman,
B. The Identification of Mammalian Hair. Inkata Press,
Melbourne, Australia, 1974.
(61) Mathiak, H. A. A key
to the hairs of mammals of southern Michigan, Journal of Wildlife
Management (1993) 2:251-268.
(62) Praeger, S. S. Selective
solvents for analyzing textile fiber mixtures, American Dyestuff
Report (1957) 46:497-498.
(63) Merkel, R. S. A scheme
for fiber identification with emphasis on new polyacrylic, polyamide,
polyester and cellulosic fibers, American Dyestuff Report
(1960) 49:13-25.
(64) Stratmann, M. The solubility
characteristics of fibres, Textile Industries (1970) 72:13-19.
(65) Bruschweiler, W. and
Schoch, H. Thermal microscopy of fibres—A methodological
addition to fibre analysis, Archiv fuer Kriminologie (1982)
169:89-98.
(66) Grieve, M. C. The use
of melting point and refractive index determination to compare
colourless polyester fibres, Forensic Science International
(1983) 22:31-48.
(67) Hartshorne, A., Wild,
F. M., and Babb, N. L. The discrimination of cellulose di- and
triacetate fibres by solvent test and melting point determination,
Journal of the Forensic Science Society (1991) 31:457-461.
(68) 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.
(69) Smith, S. G. Identification
of unknown synthetic fibers: Part III. Revision and application
of micro fusion, American Dye Report (1959) 48(26):23-26.
(70) Grabar, D. G. and Haessly,
R. Identification of synthetic fibers by micro fusion methods,
Analytical Chemistry (1956) 28:1586-1589.
(71) Petraco, N., DeForest,
P. R., and Harris, H. A new approach to the microscopical examination
and comparison of synthetic fibers encountered in forensic cases,
Journal of Forensic Sciences (1980) 25:571-582.
(72) Julian, Y. and McCrone,
W. C. Accurate use of hot stages, Microscope (1971) 19:225-234.
(73) Cassidy, F. H. A simple
thermistor device for checking the calibration of the Mettler
hot stage, Journal of the Forensic Science Society (1986)
26:409-415.
(74) Woodard, G. D. Calibration
of the Mettler FP2 hot stage, Microscope (1970) 18:105-108.
(75) Skirius, S. A poor microscopist's
hot stage, Microscope (1984) 32:100-102.
(76) Moran, B. R. and Moran,
J. F. An inexpensive digital temperature monitoring device for
the poor microscopist's hot stage, Microscope (1987) 35:291-301.
(77) Wilson, L. and McGee,
W. W. Construction and calibration of a demountable poor microscopist's
hot stage, Microscope (1988) 36:125-132.
(78) Valaskovic, G. A simple
control unit for the EC slide hot stage, Microscope (1991)
39:53-55.
(79) McCrone, W. C. Calibration of the EC slide hot stage, Microscope
(1991) 39:43-52.
(80) Pelton, W. R. Distinguishing
the cause of textile fiber damage using the scanning electron
microscope (SEM), Journal of Forensic Sciences (1995)
40(5):874-882.
(81) Ukpabi, P. and Pelton,
W. R. Using the scanning electron microscope to identify the
cause of fibre damage, Part I: A review of related literature,
Journal of the Canadian Society of Forensic Science (1995)
28(3):181-187.
(82) Pelton, W. R. and Ukpabi,
P. Using the scanning electron microscope to identify the cause
of fibre damage, Part II: An exploratory study, Journal of
the Canadian Society of Forensic Science (1995) 28(3):189-200.
(83) Goynes, W. R. A scanning
electron microscope study of washer-dryer abrasion in textile
fibers. In: Proceedings of the 28th Annual EMSA Meeting,
1971, pp. 346- 347.
(84) Hecking, L. T. and Erikson,
C. Characterization of acid washed denim garments and accessories
using the stereomicroscope and the scanning electron microscope,
Customs Laboratory Bulletin (1993) 5(3).
(85) Greaves, P. H. and Saville,
B. P. Scanning electron microscopy. In: Microscopy of Textile
Fibres. Bios Scientific Publishers LTD, Oxford, United Kingdom,
1995, pp. 51-67.
(86) Robson, D. Fibre surface
imaging, Journal of the Forensic Science Society (1994)
34(3):187-191.
(87) Wiggins, K. Microfibers:
A forensic perspective, Journal of Forensic Sciences (1997)
42:842-845.
Back to the top
Back
to chapter listing.
FORENSIC SCIENCE COMMUNICATIONS APRIL 1999 VOLUME 1 NUMBER 1 |
