Fiber Analysis

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Physical Aspects of Forensic Science

Fiber Evidence

A fiber is the smallest unit of a textile material that has a length many times greater than its diameter. 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. This becomes very important when there is a possibility of fiber transfer between a suspect and a victim during the commission of a crime.

http://www.fbi.gov/hq/lab/fsc/backissu/july2000/deedric3.htm#Fiber%20Evidence

Matching unique fibers on the clothing of a victim to fibers on a suspect’s clothing can be very helpful to an investigation, whereas the matching of common fibers such as white cotton or blue denim fibers would be less helpful.

The discovery of cross transfers and multiple fiber transfers between the suspect's clothing and the victim's clothing dramatically increases the likelihood that these two individuals had physical contact.

Many different natural fibers that come from plants and animals are used in the production of fabric.

http://www.fireflydiapers.com/articles/diaperarticle_naturalfibersabsorb.htm

Cotton fibers are the plant fibers most commonly used in textile materials

The animal fiber most frequently used in the production of textile materials is wool, and the most common wool fibers originate from sheep.

Natural Fibers

Images: http://www.trashforteaching.org/phpstore/product_images/YarnWS.JPG http://www.fbi.gov/hq/lab/fsc/backissu/july2000/deedric3.htm#Fiber%20Evidence

http://www.jivepuppi.com/images/fiber_evidence.jpg

More than half of all fibers used in the production of textile materials are synthetic or man-made.

Nylon, rayon, and polyester are all examples of synthetic fibers.

Cross-section of a man-made fiber

Synthetic Fibers

Fibers under a microscope

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

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

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

Natural Fibers: Found in nature Can be artificially colored or treated

Cotton Wool Hemp

Trace > Fibers > Natural

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

Plant Fibers

Cotton - seed hairs of cotton plantby far most common fiber (find almost everywhere)

Under microscope, fibers resemble twisted ribbon

Trace > Fibers > Natural

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

Linen - stem fiber from flax plantKapok - from seed hairs of kapok plantOther fibers - Manila, hemp, sisal, jute

Other Plant Fibers:

Trace > Fibers > Natural

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: Long continuous fiber (like silk)

Staple: Filament is cut into smaller pieces; staples are spun together to form thread (like cotton)

Filament vs. Staple

Manade 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

• 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

Example: Rayon

Cellulose is dissolved, then resolidified to form the polymer fiber

Can occur in filament or staple form

Examples: Nylon and Polyester

Man made Can also be filament or

staple

Synthetic Fibers

Acrylics More common as

evidence Usually in staple form Staples spun together,

similar to wool

Synthetic Fibers

Begin by identifying and comparing class characteristics for unknown sample (evidence) and known sample.

Unknown Known

Trace > Fibers > Analysis

Fibers from rug in a van.

Fibers found on victim.

Trace > Fibers > Analysis

Class characteristics

Trace > Fibers > Analysis

Color: microscopic examinationSize: length and width can be measuredShape: cross section is viewed

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

Class characteristics

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

At each step, the number of cords can be counted.

At each step, the twist direction is either “S” or “Z”

Small cords or fibers twisted together to form larger cords

Fiber niso nll n Biref MP (ºC)

K1 1.518 to 1.528 1.544 to 1.551 1.505 to 1.516 0.035 to 0.039 Does not melt

K2 1.777 to 1.877 2.050 to 2.350 1.641 to 1.646 0.200 to 0.710 Does not melt

K3 1.512 to 1.521 1.510 to 1.520 1.512 to 1.525 -0.001 to -0.005

Does not melt

K4 1.538 to 1.539 1.530 to 1.539 1.538 to 1.539 -0.000 to -0.002

192 – 210

K5 1.533 to 1.545 1.568 to 1.583 1.515 to 1.526 0.049 to 0.061 210 – 230

K6 1.540 to 1.541 1.577 to 1.582 1.515 to 1.526 0.056 to 0.063 250 – 264

K7 1.522 1.553 1.507 0.046 182 – 186

K8 1.535 to 1.539 1.568 to 1.574 1.518 to 1.522 0.050 to 0.052 133 – 138

K9 1.567 to 1.575 1.632 to 1.642 1.534 to 1.542 0.098 to 0.102 282 – 290

K10 1.474 to 1.478 1.474 to 1.479 1.473 to 1.477 0.002 to 0.005 245 – 260

Q 1.520 1.515 1.513 -0.003 Does not melt

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

From Less than 1 cm of a 20 mm 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

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 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

• 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