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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

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fiber analysis

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
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

Textile Fiber Categories
animal fibers
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
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 continued6
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 continued7
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
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
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 continued10
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
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
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 continued13
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
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
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
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 evidence17
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
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
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 m m 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

From Less than 1 cm of a 20 mm Diameter Fiber It is Possible to Determine:
microscopy
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
Microscopic View

Acetate Dacron

stereomicroscope
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
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
Comparison Microscopy
  • Side-by-side Comparison
  • Bright Field Adjustment
comparison microscopy26
Comparison Microscopy
  • Characterization
  • Fluorescence
    • Chemical factors
    • Environmental factors
comparison microscope27
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
Fluorescence Microscopy
  • The sample is illuminated by ultraviolet light, causing some phases to fluoresce so they can be observed, counted, sized and mapped.

Kevlar fibers in complex composite material strongly fluoresce.

polarized light microscope
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
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.
microspectrophotometry31
Microspectrophotometry
  • Microscope linked to a Spectrophotometer
    • IR Absorption spectrum
    • UV/VIS Absorption Spectrum
microspectrophotometry32
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
microspectrophotometry33
Microspectrophotometry
  • Advantages
    • Nondestructive
    • Not limited to sample size
  • Disadvantages
    • Reactive dyes
    • Chemical composition
    • Tentative identification
scanning electron microscopy
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
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
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 continued37
TLC (continued)
  • Technique
    • Extraction of dyes
    • Solid stationary phase
    • Liquid moving phase
    • Capillary action
    • Chromatogram
tlc continued38
TLC (continued)
  • Interpretation
    • Rf (retention factor)
    • Color
    • Proportions
    • Scanning densitometer
      • peak height ratios
    • Fluorescence
tlc continued39
TLC (continued)
  • Analysis of Chromatograms
    • Positive association
    • Exclusion
    • Inconclusive
tlc continued40
TLC (continued)
  • Advantages
    • Highly discriminatory
    • Inexpensive
    • Dye batch variation
    • Reactive dyes
    • Optically isomeric pair
tlc continued41
TLC (continued)
  • Disadvantages
    • Destructive
    • Pale fibers
    • Tentative identification
pyrolysis gas chromatography
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
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
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
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
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 continued47
Case Study (continued)
  • Eleven brown uniform and multicolor acrylic fibers were found in the combed hair samples
  • Similar fibers were searched from the tape-lifted samples taken from the clothes--about 200 fibers were found in the clothes
case study continued48
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 continued49
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 continued50
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 continued51
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
Evidential Value
  • 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
Conclusions of Fiber Analysis
  • The unknown fibers could have originated from the known.
  • The unknown fibers could not have originated from the known.