6. Fiber examination and its forensic importance.pdf

Forensic botony

1. Forensic Aspects of
FiberExamination
Dr. Nisha K K
All Saints’ College, Tvm

2. Forensic Importance
•Fiber has an important evidentiary value in the field of forensic
science
•It provides a variety of the clue materials -individual fibers, threads,
a piece of cloth, a clothing, a cord string, ropes or their pieces, hair
from different sources, woolen fibers, furs etc..
•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 naturalor man made
Dr.Nisha K K
All Saints’ College, Tvm

3. Fiber examination
•Typically, fiber examinations involve a comparison of samples from
known and questioned sources to determine whether they are
consistent with having originated from the same source (e.g., carpet
from a suspect's car compared with foreign fibers removed from the
victim's clothing)
•Textile fibers can be exchanged between individuals, between
individuals and objects, and between objects. When fibers are
associated with a specific source, such as fabric from the victim,
suspect, or scene, a value is placed on that association
•Have to determine if the questioned fiber exhibits the same chemical,
microscopic and optical properties to that of the known source as
fiber
Dr.Nisha K K
All Saints’ College, Tvm

4. FiberColor
•Color may be uniform or may vary along the length of the fiber
•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
Dr.Nisha K K
All Saints’ College, Tvm

5. Fluorescence
•Fluorescence isthe ability of certain chemicals to
absorb light at a particular wavelength and then
emit light at a longer wavelength
•At different excitation wavelengths (330-380 nm,
380-420 nm, 450-480 nm and 510-560 nm) the
color and intensity or the absence of fluorescent
emission will be noted
•Some fibers will fluoresce when exposed to UV
light
A nylon fibre under
ultraviolet excitation-
magnification 400x.
Same fibreunder blue
light excitation
Dr.Nisha K K
All Saints’ College, Tvm

6. Optical Properties
•Four basic optical properties-studied with the help of a polarizing
microscope
1.Refractive Index
2.Isotropic Refractive Index
3.Birefringence
4.Sign of Elongation
Dr.Nisha K K
All Saints’ College, Tvm

7. Refractive Index
•Is the bending of light as it passes from air into a solid or liquid
•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 have two principle refractive indices, one for light
polarized parallel to the long axis of the fiber (n
ll) and one for light
polarized perpendicular to the long axis of the fiber (n
┴)
•For fibers examined in unpolarizedlight, a third quantity, isotropic
refractive index, n
iso(defined as 1/3[2 n
┴+ n
ll]), 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 Dr.Nisha K K
All Saints’ College, Tvm

8. Find Refractive Index
•BeckeLine: The bright halo near the boundary of a fiber that moves
with respect to that boundary as the microscope is focused through
best focus
•BeckeLine Method: A method for determining the refractive index of
a fiber relative to its mountantby noting the direction in which the
Beckeline moves when the focus is changed. The Beckeline 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.
Dr.Nisha K K
All Saints’ College, Tvm

9. Birefringence
•A property of certain materials that causes light to split into two paths
when it passes through them
•This happens because the material has different refractive indices
depending on the direction of the light's polarization
•Birefringence is also known as double-refraction
•It can be obtained by subtraction after determining the refractive
indices -i.e. n
ll–n
┴
•A quick and cheap test for fiber analysis that can be performed is the
measurement of its birefringence using the Michel-Lévychart.
•Birefringence can provide a relationship between textiles in clothes
for forensic applications, such as inidentification of suspects through
matching of fiber strands of the victim and the suspect
Dr.Nisha K K
All Saints’ College, Tvm

10. Michel-Levy Birefringence Chart
Protocol for the birefringence analysis of a material under a polarized light microscope
Dr.Nisha K K
All Saints’ College, Tvm

11. Dye analysis
•Dyes-Soluble substances that add color to textiles-natural or
synthetic substance used to fasten a color or modify the color of
something
•In forensic casework, dyes hold an important value in detection and
evaluation of fibers
•Dye Analysis can be performed by a variety of methods like TLC, HPLC,
and Capillary electrophoresis after extraction of dye from fiber
•Extraction of dye from fiber is done by solvent and detection by TLC
•Extracted dye is identified accurately by high performance liquid
chromatography (HPLC)
Dr.Nisha K K
All Saints’ College, Tvm

12. Case study
•Dr. Jeffrey MacDonald, Army captain and Green Beret accused of
murdering his pregnant wife and two children in 1970, he was made
famous in the book and subsequent made for TV movie “Fatal Vision”
•The investigators were suspicious of the doctor because of the nature
of his wounds –suggesting that they were not defensive and perhaps
even self inflicted
•Blue threads that matched the doctor’s pajama top were found under
the body of his wife as well as in the bedroom of both murdered
children and even under the fingernails of one of his daughters
•Dr. MacDonald is currently in prison and still maintains his innocence.