Trace evidence ii: metals, paint, and soil

Green river killer Why was gary ridgeway initially considered a suspect? What evidence was collected at ridgeway’s residence with the aid of the search warrant? What forensic advancement led to the conviction of Gary Ridgeway? What additional trace evidence helped to connect ridgeway to the murders?

Trace elements How can trace elements be useful if they look the same? Provide markers that may establish the source of a material or at least provide additional points for comparison of two items of physical evidence

Atomic structure Proton, electron, neutron Atomic number Atomic weight

isotope

alpha, beta, gamma

Neutron activation analysis Bombarding trace elements with neutrons to measure the energy of the gamma rays emitted.

Advantages/disadvantages Provides a nondestructive method for identifying and quantitating trace elements Expensive and due to radiation there are regulatory requirements

paint Encountered most frequently in hit and run and burglary cases Purpose is to find composition for comparison or origin 33,000 to one that there is a false match

paint 4 organic coatings: Electrocoat primer – first layer applied to a steel body of a car. Black to gray Primer surface – corrosion control. Before base coat and is highly pigmented Basecoat – provides color and asthetics. Is resistant to weather, uv, acid rain Clearcoat – unpigmented coat to improve gloss, durability, and appearance

paint Color is most distinctive forensic characteristic followed by surface texture and color layer sequence Pyrolysis gas chromatography used to break apart chain of polymers that make up paint binder. Decomposed products sent through gas chromatograph to record patterns and find chemical makeup

Inductively coupled plasma emission spectrometry (Icp) Atoms are excited by placing the sample in a hot plasma torch Sample in aerosol form is collided with energetic argon electrons and charged particles are generated. The ions emit light of characteristic wavelengths that correspond to elements in the sample

collection Picked up with forceps or scooped with piece of paper If hit and run is the case, standard/reference paint must be collected from undamaged area of car No attempt to collect paint from any impression or dent from an object so as to not damage the evidence of the impression itself

soil Definition: any disintegrated surface material, natural or artificial, that lies on or near the earth’s surface

Soil evidence Value: a geologist may be able to examine soil found on a victim or suspect and direct police to an area or areas where soil originally was picked up Can also be used to connect crime scene to suspect

Soil examination Side by side visual comparison using color and texture charts Low power microscopes reveal presence of plant and animal materials High power microscopes reveal minerals and rocks