Forensics of Fiber Analysis

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Forensics of Fiber Analysis
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Fibers

• A fiber is the smallest unit of a textile
  material that has a length many times greater
  than its diameter.
• Fibers can occur naturally as plant and animal
  fibers, but they can also be man-made.
• A fiber can be spun with other fibers to form a
  yarn that can be woven or knitted to form a
  fabric.
• The type and length of fiber used, the type of
  spinning method, and the type of fabric
  construction all affect the transfer of fibers
  and the significance of fiber associations.

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Fiber Evidence

• Fibers are gathered at a crime scene with
  tweezers, tape, or a vacuum. 
• They generally come from clothing, drapery, wigs,
  carpeting, furniture, and blankets. 
• For analysis, they are first determined to be
  natural, manufactured, or a mix of both.

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Fiber Number

• The number of fibers on the clothing of a victim
  identified as matching the clothing of a suspect
  is important in determining actual contact.
• The greater the number of fibers, the more
  likely that contact actually occurred between
  these individuals

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Forensics of Fiber Analysis

• Cross transfers of fiber often occur in cases in
  which there is person-to-person contact
• Investigators hope that fiber traceable back to
  the offender can be found at the crime scene, as
  well as vice versa.
• Success in solving crimes often hinge on the
  ability to narrow the sources for the type of
  fiber found, as the prosecution did with their
  probability theory on the fibers

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Forensic Fiber Analysis
Why would this information be valuable to a
forensic scientist?

• The world produced approximately 80 billion
  pounds of fabric in 1995, about half of which was
  cotton
• The other approximately 44 billion pounds of
  fiber were manufactured or synthetic.

Table 1.U.S. Annual Production for Manufactured
Fibers 1995(millions of pounds)
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Forensic Fiber Analysis

• It could be argued that the large volume of
  fibers produced reduces the significance of a
  fiber association discovered in a criminal case.
• Considering the volume of textiles produced
  worldwide each year, the number of textiles
  produced with any one fiber type and color is
  extremely small.
• The likelihood of two or more manufacturers
  exactly duplicating all of the aspects of the
  textile is extremely remote

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Fiber Evidence

• The problem with fiber evidence is that fibers
  are not unique. 
• Unlike fingerprints or DNA, they cannot pinpoint
  an offender in any definitive manner. 
• There must be other factors involved, such as
  evidence that the fibers can corroborate or
  something unique to the fibers that set them
  apart.

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Microscopic Examination

• A compound microscope uses light reflected from
  the surface of a fiber and magnified through a
  series of lenses,
• The comparison microscope (two compound
  microscopes joined by an optical bridge) is used
  for more precise identification. 
• The phase-contrast microscope, reveals some of
  the structure of a fiber.
• Electron microscopes either pass beams through
  samples to provide a highly magnified image, or
  reflect electrons off the sample's surface. 
• A scanning electron microscope converts the
  emitted electrons into a photographic image for
  display.  This affords high resolution and depth
  of focus.

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Spectrometer

• The spectrometer, which separates light into
  component wavelengths. 
• every organic element has a uniqueness to its
  constituent parts. 
• By passing light through something to produce a
  spectrum, the analyst can read the resulting
  lines, called "absorption lines." 
• That is, the specific wavelengths are
  characteristic of its component molecules of the
  substance. 

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Micro-spectrophotometer

• This microscope locates minute traces or shows
  how light interacts with the material under
  analysis. 
• Linking this to a computerized spectrophotometer
  increases the accuracy.
•   The scientist can get both a magnified visual
  and an infrared pattern at the same time, which
  increases the number of identifying
  characteristics of any given material.

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Natural Fibers vs Manufactured

• Natural fibers come from plants (cotton) or
  animals (wool). 
• Manufactured fibers are synthetics like rayon,
  acetate, and polyester, which are made from long
  chains of molecules called polymers. 

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Natural Fibers

• Many different natural fibers originating from
  plants and animals are used in the production of
  fabric.
• Cotton fibers are the plant fibers most commonly
  used in textile materials
• the type of cotton, fiber length, and degree of
  twist contributing to the diversity of these
  fibers.

Cotton fibers
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Other plant fibers

• Flax (linen), ramie, sisal, jute, hemp, kapok,
  and coir.
• The identification of less common plant fibers at
  a crime scene or on the clothing of a suspect or
  victim would have increased significance.

Flax fibers viewed withpolarized light
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Animal Fiber Wool

• Wool is the most frequently used in the
  production of textile materials
• and the most common wool fibers originate from
  sheep.
• Finer woolen fibers are used in the production of
  clothing
• coarser fibers are found in carpet.
• Fiber diameter and degree of scale protrusion of
  the fibers are other important characteristics.

Wool fibers
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Other Animal Fibers

• Although sheep's wool is most common, woolen
  fibers from other animals may also be found.
• These include camel, alpaca, cashmere, mohair,
  and others.
• The identification of less common animal fibers
  at a crime scene or on the clothing of a suspect
  or victim would have increased significance.

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Man-Made Fibers

• More than half of all fibers used in the
  production of textile materials are man-made.
• Some man-made fibers originate from natural
  materials such as cotton or wood
• others originate from synthetic materials.
• Polyester and nylon fibers are the most commonly
  encountered man-made fibers, followed by
  acrylics, rayons, and acetates.
• There are also many other less common man-made
  fibers.
• The amount of production of a particular man-made
  fiber and its end use influence the degree of
  rarity of a given fiber.

Cross section of man-made fibers
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Federal Trade Commission Rules and Regulations
Under the Textile Products Identification Act,
1954Generic names for manufactured fibers

• Acetate
• Acrylic
• Anidex
• Aramid
• Glass
• Lyocel
• Nylon
• Metallic
• Modacrylic

• Novoloid
• Olefin
• Polyester
• Rayon
• Saran
• Spandex
• Vinal
• Vinyon

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Man-Made Fibers

• The cross section of a man-made fiber can be
  manufacturer-specific
• Some cross sections are more common than others,
  and some shapes may only be produced for a short
  period of time.
• Unusual cross sections encountered through
  examination can add increased significance to a
  fiber association.

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Fiber Forensics

• Generally, the analyst gets only a limited number
  of fibers to work withsometimes only one.
• Whatever has been gathered from the crime scene
  is then compared against fibers from a suspect
  source, such as a car or home
• Fibers are laid side by side for visual
  inspection through a microscope.

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Important Considerations

• An is the length of time between the actual
  physical contact and the collection of clothing
  items from the suspect or victim.
• If the victim is immobile, very little fiber loss
  will take place, whereas the suspect's clothing
  will lose transferred fibers quickly.
• The likelihood of finding transferred fibers on
  the clothing of the suspect a day after the
  alleged contact may be remote, depending on the
  subsequent use or handling of that clothing.

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Steps of Fiber Analysis

• The first step in fiber analysis is to compare
  color and diameter. 
• Dyes can also be further analyzed with
  chromatography, which uses solvents to separate
  the dye's chemical constituents. 

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Fiber Color

• Color influences the value given to a particular
  fiber identification.
• Often several dyes are used to give a fiber a
  desired color.
• Individual fibers can be colored prior to being
  spun into yarns.
• Yarns can be dyed, and fabrics made from them can
  be dyed.
• Color can also be applied to the surface of
  fabric, as found in printed fabrics.
• How color is applied and absorbed along the
  length of the fiber are important comparison
  characteristics.
• Color-fading and discoloration can also lend
  increased value to a fiber association.

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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

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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.
• A balanced neutral background color is optimal.

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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 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.

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Summary of Steps of Fiber Analysis

• In short, the fiber analyst compares shape, dye
  content, size, chemical composition, and
  microscopic appearances, yet all of this is still
  about "class evidence.
•  Even if fibers from two separate places can be
  matched via comparison, that does not mean they
  derive from the same source, and there is no
  fiber database that provides a probability of
  origin.

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• Fiber Location
• Where fibers are found also affects the value
  placed on a particular fiber association. The
  location of fibers on different areas of the body
  or on specific items at the crime scene
  influences the significance of the fiber
  association.

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