Fiber Analysis

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Fiber Analysis. Physical Aspects of Forensic Science. . 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. Animal (hairs) Wool, cashmere, silk
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Fiber Analysis Physical Aspects of Forensic Science . 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
  • Animal (hairs) Wool, cashmere, silk Vegetable Cotton, kapok, linen Mineral Asbestos Manmade Acetate, rayon, nylon, acrylic, polyester, and olefin Textile Fiber Categories Fibers Fibers are very useful as trace evidence:
  • Vary widely in class characteristics color, shape, chemical composition, etc.
  • Easily transferred from one source to another (carpets, clothes, etc.)
  • Significant persistence (won’t degrade)
  • 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
  • Trace > Fibers How are fibers used as evidence?
  • As with other trace evidence, fibers can be transferred to/from a person or objects linking them to one another.
  • Trace > Fibers How long do fibers persist? Most fiber evidence is lost (fall off) a short time after the transfer occurs. The fibers that do remain will be persistent. Trace > Fibers Fibers can be classified into three main categories:
  • Natural (animal, plant, mineral)
  • Manufactured
  • Synthetic
  • Trace > Fibers > Natural Natural Fibers:
  • Found in nature
  • Can be artificially colored or treated
  • Cotton Wool Hemp Animal Fibers Wool - Hairs from sheep
  • Most common of animal fibers
  • Hairs are spun to form thread
  • Silk - comes from silkworm
  • Spun as double filament (separated before use)
  • Because of length, doesn’t shed easily
  • Other Hairs from Animals 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
  • 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
  • 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
  • Trace > Fibers > Natural Plant Fibers Cotton - seed hairs of cotton plant
  • by far most common fiber(find almost everywhere)
  • Under microscope, fibers resemble twisted ribbon 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
  • 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
  • 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 Trace > Fibers > Natural Other Plant Fibers: Linen - stem fiber from flax plant Kapok - from seed hairs of kapok plant Other fibers - Manila, hemp, sisal, jute Mineral Fibers Asbestos - crystalline material
  • Used to be used for insulation
  • Fractures into thin rods that can get into your lungs; can kill you
  • Not used much anymore
  • Filament vs. Staple Filament: Long continuous fiber (like silk) Staple: Filament is cut into smaller pieces; staples are spun together to form thread (like cotton) ManadeFibers
  • 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
  • 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
  • Manufactured Fibers Regenerated Fibers
  • Cellulose is dissolved, then resolidified to form the polymer fiber
  • Can occur in filament or staple form
  • Example:Rayon Synthetic Fibers
  • Man made
  • Can also be filament or staple
  • Examples: Nylon and Polyester Synthetic Fibers Acrylics
  • More common as evidence
  • Usually in staple form
  • Staples spun together, similar to wool
  • Trace > Fibers > Analysis Begin by identifying and comparing class characteristics for unknown sample (evidence) and known sample. Known Unknown Trace > Fibers > Analysis Fibers from rug in a van. Fibers found on victim. Trace > Fibers > Analysis Class characteristics Color: microscopic examination Size: length and width can be measured Shape: cross section is viewed Class characteristics Refractive Index – n. The ratio of the speed of light in air or in a vacuum to the speed of light in another medium. Other microscopic properties (PLM) Class characteristics Chemical Composition: determined by advanced instrumentation Threads, Yarn, Rope, Cordage Smallest component is fibers (staple) twisted together to form thread or is a filament. This thread can then be twisted with other threads to form a thicker thread (string, etc.) This thicker cord can then be twisted with other thicker cords, etc. Threads, Yarn, Rope, Cordage Small cords or fibers twisted together to form larger cords
  • At each step, the number of cords can be counted.
  • At each step, the twist direction is either “S” or “Z”
  • 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
  • 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
  • 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
  • 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
  • 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 ElectronMicroscopy
  • 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
  • Extraction of dyes
  • Solid stationary phase
  • Liquid moving phase
  • Capillary action
  • Chromatogram
  • TLC (continued)
  • Interpretation
  • Rf (retention factor)
  • Color
  • Proportions
  • Scanning densitometer
  • peak height ratios
  • Fluorescence
  • TLC (continued)
  • Analysis of Chromatograms
  • Positive association
  • Exclusion
  • Inconclusive
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