Paint Trace Evidence K Culbertson Paint Physical evidence

Paint Trace Evidence K. Culbertson

Paint • Physical evidence • One of the most prevalent types of evidence to be analyzed • Hit-and-run cases – Color, make, model of car • Burglary cases

Composition of Paint • Mixture of pigments and additives suspended in a binder – Pigments give color – Binder is the support medium • The components of paint are dissolved in a suitable solvent

Automobile Paint • Numerous coatings of paint – Adds significant diversity to automobile paint • Coatings for a steel body – Electrocoat primer – Primer surfacer – Basecoat – Clearcoat

Electrocoat Primer • Black to gray in color • Applied to steel body • Provides corrosion resistance

Primer Surfacer • Smoothes out and hides any seems or imperfections • Light gray for pastel color coats • Red for dark-colored color coats

Basecoat or Color coat • Provides color and aesthetics of the finish • Must resist environmental elements – Weather – UV radiation – Acid rain • May also have pearl luster or mica pigments

Clearcoat • Un-pigmented • Improves gloss, durability, and appearance

Forensic Examination • The stereomicroscope is the most important instrument for locating and comparing paint specimens – Color – Surface texture – Color layer sequence

Chemical Analysis • Chemical analysis of paint pigments or its binder composition, or both – Pyrolysis gas chromatography – Infrared spectroscopy - pigments – Raman spectroscopy - binders

IR vs. Raman • IR spectra on the left and Raman spectra on the right for ten analyzed automotive topcoats

Raman

Raman

Raman

Raman • Raman spectra of basecoat cross sections. All paint samples were the same color blue, but were composed of different pigments

Raman • Raman spectra (top) of the red paint observed on the damaged car bumper, and (bottom) of the red standard/reference paint from the suspected vehicle (bottom)

IR • FTIR spectra (a) of the red standard/reference paint from the suspected vehicle; and (b) of the red paint observed on the damaged car bumper

Can paint incriminate? - Yes • Subtle differences in binder formulations • Paint chips were taken from 260 vehicles in a wreck yard. Color, layer structure, and spectroscopies were compared. All were distinguishable except for one pair. • The odds against the crime-scene paint originating from another randomly chosen vehicle are approximately 1 in 33, 000 • Make and model can be identified from paint chip which is used in the search for an unknown car involved in a hit-and-run – Since 1974 model year, NIST collects and disseminates to crime labs auto pain color samples from U. S. domestic passenger cars. US also has access to Canada’s Paint Data Query (PDQ).

Can paint incriminate? - No • Each car plant can use a different supplier which can also vary between models in a given year, batch formulas can vary • Most paint chips will not have a layer structure complex enough to allow them to be individualized to a single source • Paint chips typically cannot be physically fit together to prove common origin • In a test, paint chips were taken from 260 vehicles in a wreck yard – compared color, layer structure, and spectroscopy. All were distinguishable except for one pair • The odds against the crime-scene paint originating from another randomly chosen vehicle are approximately 33, 000 to 1

Collection & Preservation • Collection must be done with the utmost care to keep the paint chip intact • Packaged in pharmacy folds or plastic vials • If paint is smeared or embedded, it is best to send the whole item to the lab • Uncontaminated reference paint should always be collected from the undamaged are of the vehicle for comparison – Other portions of the car may have faded or been repainted