14) Forensic Trace Evidence: Fibers

Filament textile fiber

• Unlimited/infinite length

• Continuous Strands

• Measured in Yards/Meters

• Measured in Inches/Centimeters

• Examples: Silk (natural), Nylon (synthetic)

Staple textile fibers

• Limited/Finite length

• Short fibers

• Examples: Cotton, Wote (natural), Viscose Rayon, Polyester. Polyester (man-made)

Why are Fibers important evidence in incidents involving personal contact. (i.e. homicide, assault, sexual offence)

A cross transfer of fiber may occur between the clothing of the suspect in the victim.

Mass production of garments and fabrics has _ value in terms of fiber evidence

limited

Ways to classify fibers

• Origin

• Chemical Composition

• Physical Properties

• Other Classification Systems

• Natural Fibers

• Synthetic Fibers

By origin

Derived from plants or animals. Cotton, wool, Silk, linen, natural minerals

Natural Fibers

Man-made fibers produced from petrochemicals. Polyester, Nylon, acrylic, spandex.

Synthetic fibers

• Cellulosic Fiber

• Protein Fibers

By chemical Composition

Made up of cellulose, a polysaccharide found in plant cell walls. Cotton, linen, and hemp

Cellulosic Fibers

Contain biopolymers made up of amino acids. Examples are wool and silk

Protein Fibers

Strength, elasticity, and durability

Physical properties

Other classification systems

• Their end use, such as textile and industrial fibers

• Manufacturing processes, like staple fibers and continuous filament fibers,etc.

Forensic fiber tests (and microscopic examinations) are used to…

identify the type of fiber and to distinguish between natural and synthetic fibers.

Two categories of fiber tests

• Non-Technical Tests: Burning and Feeling/Texture tests.

• Technical Tests: Melting point tests, microscopic tests, and solubility tests

Texture of Fibers

• Cotton: soft and smooth

• Wool: coarse and wiry

• Mylon: stiff and slippery

• Polyester: stiff and crisp

• Acrylic: soft and fluffy

• Acetate: silky and smooth

• Rayon: soft and drapey

Microscopic Examination Tools:

• Microscopic examination of fibers can be conducted using a variety of microscopes, including optical microscopes (compound and simple), and electron microscopes (SEM and TEM).

Optical Microscopes:

• These are the most commonly used type for fiber analysis due to their accessibility and ease of use.

Electron Microscopes (SEM and TEM):

• These can be used to obtain more detailed images of fibers, but they are more expensive and require more specialized training to operate.

Microscopic Characteristics

• Cotton: spiral twist, lumen

• Wool: Scaly appearance, medulla

• Silk: Smooth surface, triangular cross-section

• Linen: Long and narrow, smooth surface

• Polyester: Smoother surface, round or elliptical cross-section

• Nylon: Smooth surface, serrated cross-section

• Acrylic: Smooth surface, bean-shaped cross-section

• Spandex: Smooth surface, round or oval cross-section

A melting point test can…

distinguish between natural and synthetic fibers by determining the temperature at which a fiber melts.

Melting point test on natural fibers

• Generally do not melt; instead, they tend to charge or decompose when exposed to high temperatures

• Example: Cotton, being a natural fiber, will not melt but will burn and eventually turn to ash at high temperatures.

Melting point test on synthetic fibers

• typically have a specific melting point at which they melt and may even shrink from a flame.

• Example: When Polyester is subjected to heat, it will melt at a specific temperature (typically around 260-290 degree C)

What is the burn and flame test?

The burn test is a rudimentary test and is based on how fibers (as evidence) react to flame and the type of smoke they produce when burned.

Burn and flame test on natural fibers….

• When subjected to flame, natural fibers usually burn and may continue to glow after the flame is removed, producing a characteristic odor.

• Example: Wool will burn and may produce a characteristic burnt hair odor (because of keratin), leaving behind a black, crushable ash.

Burn and flame test on synthetic fibers

• tend to melt and shrink away from the flame, and they often extinguish once the flame is removed, producing a different kind of smoke and odor compared to natural fibers.

• Example: Nylon will melt and shrink away from the flame, often extinguishing once the flame is removed, and may produce a celery-like odor and black smoke, leaving behind a hard, bead-like residue.

Burn test with different fiber types

• Cotton: Burns with a flame and produces a white ash.

• Wool: Bruns with a flame and produces a black ash with a characteristic burnt hair odor.

• Nylon: Melts and produces black smoke.

• Polyester: Melts and produces black smoke.

• Acrylic: Melts and produces white smoke with a characteristic plastic odor.

• Acetate: Melts and produces black smoke with a characteristic winegar odor.

• Rayon: Burns with a flame and produces a white ash but will extinguish itself when the flame is removed.

What is the Solubility Test?

The solubility test is another rudimentary test used to distinguish between natural and synthetic fibers.

Procedure for the Solubility Test…

A small sample of the fiber is placed in a solvent, such as acetone or sodium hydroxide.

Natural fibers in solubility test

typically soluble in certain solvents

Synthetic fibers in Solubility test

typical not soluble.

Is the Solubility Test more definitive than the Burn test?

This test can be more definitive than the burn test. Still, it is important to use other tests to confirm the identity of a fiber due to some natural and synthetic fibers having similar solubility properties.

What is the dye test in forensics

The dye test can be used to identify the type of dye applied to a fiber. It helps in determining the possible source of origin of a fiber, albeit not conclusively.

Procedure for the dye test

1. Take a small sample of the fiber.

2. Place the fiber sample in a water solution or ethanol.

3. Heat the solution with the fiber sample in it.

4. After heating, examine the fiber under a microscope and the amount of dye absorbed cross-sectionally.

5. Observe any color changes in the fiber.

6. Document any color changes and the type of dye absorbed by the fiber for further analysis.

Observation: If the fiber absorbs any of the dyes, the color of the fiber will change, and the amount of soaking ability is much higher in natural fibers than in synthetic.

What is spectroscopy

It involves studying the interactions between matter and electromagnetic radiation. Techniques are ATR-FTIR (Attenuated Total Reflectance Fourier Transform Infrared) and FT-Raman

How does Spectroscopy help forensic examiners?

This helps the forensic examiner to extract detailed information about the molecular composition, chemical structure, and physical properties of fibers.

Dye and Pigment Analysis Using UV-Visible Spectroscopy

• Application: For analyzing dyes and pigments in fibers.

Identification of Chemical Composition Using IR

• Application: Allows for the differentiation between various synthetic and natural fibers through the detection of specific functional groups like carbonyl or amine groups.

Structural Analysis using Raman Spectroscopy

• Application: Provides insights into the molecular vibrations within the fiber, enabling the identification of molecular structures and polymorphs.

Microspectrofluorimetry

• Application: Allows for the detailed analysis of the color and optical properties of fibers (especially useful when examining dyed fibers).

Trace Evidence Analysis using Mass Spectrometry (MS):

• Application: Provides detailed information about the molecular weight and sequence of polymer units in fibers and their dye compositions.

Comparative Analysis with Database Integration

• Application: Allows forensic analysts to compare the spectroscopic profiles of unknown fibers with known samples.