Trace Evidence

What does forensic analysis of glass consist of?

examination of two or more fragments to determine whether they have a common origin

How are common source glass fragments identified?

the exclusion of all other sources may only be done if they can physically be matched together

At what scenes are glass fragments common in?

motor vehicle accidents, car theft, and buglaries

What is one of the most common materials submitted for forensic trace evidence analysis?

Glass

What are window glass composed of?

• soda lime

• sodium carbonate

• calcium oxide

• silicon oxide

  • Si-O

What is automobile headlight/heat-resistant glass composed of?

• silicon oxide

• boron

What is the stability of glass fragments that remain on clothing?

• very stable

• don’t degrade like biological evidence, and don’t alter after time

How can glass be a submitted sample?

must be confirmed as glass

How is glass differentiated from plastic?

examining relative hardness, solubility, and observation with a polarized light microscope

What happens after the sample is confirmed as glass?

the examiner will perform an analysis of physical properties (colour, fluorescence, thickness, curvature, density), optical properties (Refractive Index) and chemical composition

Comparing glass fragments: Challenges

• too fragmentary for comparison

• search for individual properties may be fruitless

• no basis for glass individualization due to the chemical composition of various window glasses uniform among manufacturers

What causes differences in glass?

Different methods of production and variations in the composition of the raw materials

How is glass differentiated?

By using sensitive and highly discriminating techniques

What is used to differentiate glass particles apart?

• physical property of density

• optical property

• refractive index

True or False: Glass distinguishing properties are class characteristics, and cannot provide the sole criteria for individualizing glass to a common source

True

Is there a glass database?

FBI lab has collected density and Ri values from submitted glass, which correlates the values to their frequency of occurrence in the glass population of the USA

At what time of the year are paint and glass fragments more likely to stick to clothing?

Winter

What is paint trace evidence?

• paint chip, smear, etc.

• any paint, liquid or dry

• transfer from one object to another

What is the importance of transfer of paint fragments?

has the potential to provide conclusive evidence of association between suspects or items

What are the differences in automobile paint?

manufacturers apply variety of coatings to the body and may change paint compositions slightly overtime or depending on location

What is the composition of paint?

• pigments

• binders

• additives

What are the layers of paint?

1. E-Coat

2. Primer

3. Basecoat

4. Clearcoat

What does the E-Coat consist of?

• epoxy

• typically black or gray

What does the primer layer consist of?

• epoxy-based (polyester)

• urethane

• white or pigmented

What does the basecoat consist of?

• colour

• acrylic polymer

• binder → holds paint on top of primer

What does the clearcoat consist of?

• glossy

• uncoloured

• transparent

• acrylic polymer

• polyurethane polymer

How many layers are in a car paint?

4 or 5

How are paints differentiated?

• two or more paints are compared to establish their common origin

• a microscopic examination side by side comparing colour, surface texture, and colour layer sequence

What is the PDQ?

• Paint Data Query

• Year, make, and model of car paint

Is there information on the likelihood of a known paint source and an unknown paint chip possibly coming from the known source?

there is no information provided on the likelihood of either situation or the frequency of two paint chips with the same physical, microscopical, and chemical properties in a given population - plays a role in court

What should the courts know about paint evidence evaluation and expert opinion?

• the rarity of transferred paint

• whether the characteristics of the transferred paint are commensurate with the alleged activity

What is a trace evidence fibre?

• cross-transfer between clothing fibre

• assist in concluding contact that has occurred - how and when

Forensic Examination of Fibre: Goal

narrow down the origin to a limited number of sources or even to a single source

Forensic Examination of Fibre: Challenge

mass production of garments and fabrics - limited value of fibre evidence

Forensic Examination of Fibre: Special

under special circumstances, a discovery of fibres at a crime scene provides individual identification with a high degree of certainty

What are the classes of fibres?

• Natural

• Manufactured

What does the class of natural fibre consist of?

• hair → human, animal

• cotton → plant-based source

• wool → animal-based source

What does the class of manufactured fibre consist of?

• polyester

• acrylic

• nylon

• spandex

What is used to examine fibres?

optical microscopy

How are fibres compared?

• comparison - standard/reference samples

• colour, morphological characteristics, diameter, striations, pitting, particle presence, cross-sectional shapes

How is dye composition taken into account during the examination?

• optical microscope cannot differentiate fibres with the same colour

• looking for dye composition (identity and relative amount)

Chemical composition of fibre examination: Goal

confirm that two fibres have the same chemical composition

Chemical composition of fibre examination: What to look for

different chemical structures, polymeric structure

level 1: confirm that all fibres involved belong to the same broad generic class

level 2: confirm that all fibres belong to the same sub-classification within their generic class

What is the goal of impression evidence analysis of shoe prints and tire tracks?

to identify a specific source of the impression, and the analytical process that this follows generally is an accepted sequence

What are the individual characteristics of shoe prints and tire tracks?

• wear or tear - cuts, scratches, gouges, holes

• random inclusions that result from manufacturing

• exposure - rocks, chewing gum, papers, or twigs

Shoeprints and tire tracks expert language: identification

definite conclusion of identity or source

Shoeprints and tire tracks expert language: probably made

very high degree of association

Shoeprints and tire tracks expert language: could have made

significant association of multiple class

Shoeprints and tire tracks expert language: inconclusive

limited association of some characteristics

Shoeprints and tire tracks expert language: probably did not make

very high degree of nonassociation

Shoeprints and tire tracks expert language: elimination

definite exclusion

Shoeprints and tire tracks expert language: unsuitable

lacks sufficient detail for a meaningful comparison