Fiber Analysis . Physical Aspects of Forensic Science - PowerPoint Presentation

Slide1

Fiber Analysis

.

Physical Aspects of Forensic ScienceSlide2

Textile Fiber Defined

Defined as the smallest part of a textile materialMany objects in our environment (clothing, ropes, rugs, blankets, etc.) are composed of yarns made of textile fibersSlide3

Textile Fiber Categories

Animal (hairs)Wool, cashmere, silkVegetable

Cotton, kapok, linenMineralAsbestosManmadeAcetate, rayon, nylon, acrylic, polyester, and olefinSlide4

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)Slide5

Importance of Fiber Evidence

Perpetrators of crimes are not always aware or able to control the fibers they have left behind or picked upSlide6

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 fiberSlide7

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 > FibersSlide8

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 > FibersSlide9

Fibers can be classified into three main categories:

Natural

(animal, plant, mineral)Manufactured

Synthetic

Trace > FibersSlide10

Natural Fibers:

Found in nature

Can be artificially colored or treated

Cotton

Wool

Hemp

Trace > Fibers > NaturalSlide11

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

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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 hairThe cuticle (outer covering) is made of flattened cells, commonly called scalesSlide13

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 woolOther 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 Slide14

Animal Fibers

Cattle and rabbit hair are found in the manufacture of certain kinds of feltsFelts 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 fibersSlide15

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 easilySlide16

Plant Fibers

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

Under microscope, fibers resemble twisted ribbon

Trace > Fibers > NaturalSlide17

Vegetable Fibers

Only cotton is found in any large extent in items of clothingApproximately 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 baggingsSlide18

Vegetable Fibers

Cotton fibers have a distinctive flattened, twisted microscopic appearance, which is quite characteristic

The fibers resemble a twisted ribbonIn 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 microscopeSlide19

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 DustSlide20

Linen - stem fiber from flax plant

Kapok - from seed hairs of kapok plant

Other fibers - Manila, hemp, sisal, jute

Other Plant Fibers:

Trace > Fibers > NaturalSlide21

Mineral Fibers

Asbestos - crystalline materialUsed to be used for insulation

Fractures into thin rods that can get into your lungs; can kill you

Not used much anymoreSlide22

Filament:

Long continuous fiber (like silk)

Staple:

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

Filament vs. StapleSlide23

Manade

FibersRepresent 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 Slide24

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 materialsThis is true before even considering differences in colorSlide25

Manufactured Fibers

Regenerated Fibers

Example:

Rayon

Cellulose is dissolved, then resolidified to form the polymer fiber

Can occur in filament or staple formSlide26

Examples:

Nylon and Polyester

Man made

Can also be filament or staple

Synthetic FibersSlide27

Acrylics

More common as evidence

Usually in staple formStaples spun together, similar to wool

Synthetic FibersSlide28

Begin by identifying and comparing

class characteristics for unknown sample (evidence) and known sample.

Unknown

Known

Trace > Fibers > AnalysisSlide29

Fibers from rug in a van.

Fibers found on victim.

Trace > Fibers > AnalysisSlide30

Class characteristics

Trace > Fibers > Analysis

Color: microscopic examination

Size: length and width can be measured

Shape: cross section is viewedSlide31

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 characteristicsSlide32

Chemical Composition: determined by advanced instrumentation

Class characteristicsSlide33

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. Slide34

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 cordsSlide35

Fiber

n

iso

n

ll

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 meltSlide36

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 hoursSlide37

Methods of Examination

In the recent past, the ID and comparison of fibers were at a relatively simple level which relied heavily on microscopySlide38

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

Generic class

Polymer compositionFinish--bright/dullCross-sectional shapeMelting pointRefractive IndicesBirefringenceColorFluorescenceAbsorption spectrumDye classDye ComponentsSlide39

Microscopy

Microscopic examination provides the quickest, most accurate, and least destructive means of determining the microscopic characteristics and polymer type of textile fibers.Slide40

Microscopic View

Acetate DacronSlide41

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.Slide42

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.Slide43

Comparison Microscopy

Side-by-side ComparisonBright Field AdjustmentSlide44

Comparison Microscopy

CharacterizationFluorescenceChemical factors

Environmental factorsSlide45

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.Slide46

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.Slide47

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.Slide48

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.Slide49

Microspectrophotometry

Microscope linked to a SpectrophotometerIR Absorption spectrumUV/VIS Absorption SpectrumSlide50

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 fibersSlide51

Microspectrophotometry

AdvantagesNondestructiveNot limited to sample size

DisadvantagesReactive dyesChemical compositionTentative identificationSlide52

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.Slide53

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.Slide54

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 rangeSlide55

TLC (continued)

TechniqueExtraction of dyesSolid stationary phase

Liquid moving phaseCapillary actionChromatogramSlide56

TLC (continued)

InterpretationRf (retention factor)

ColorProportionsScanning densitometerpeak height ratiosFluorescenceSlide57

TLC (continued)

Analysis of ChromatogramsPositive associationExclusionInconclusive