Trace Evidence ll: Metals, Paint, and Soil

11-1

PRENTICE HALL ©2008 Pearson Education, Inc. Upper Saddle River, NJ 07458

FORENSIC SCIENCE An IntroductionBy Richard Saferstein

Trace Evidence ll: Metals, Paint, and Soil

Chapter 11

11-2



Introduction • Manufactured products and even most natural

materials contain small quantities of elements, or trace elements, in total concentrations of less than 1 percent.

• The presence of these trace elements is useful, because they:

provide “invisible” markers that may establish the source of a material or additional points for comparison

11-3



Neutron Activation• Neutron activation analysis measures the

gamma-ray frequencies of specimens that have been bombarded with neutrons.

• Highly sensitive/nondestructive analysis identify and quantify up to 20-30 trace elements.

• Forensic analysis uses neutron activation to find trace elements in metals, drugs, paint, soil, gunpowder residue, and hair.

• Requires access to a nuclear reactor so limited use.

11-4



Paint• Paint spread on most surfaces will dry into a hard film.

• Paint contains pigments and additives suspended in a binding agent.

• Paint examined in the crime laboratory usually involves automobiles.

• Automobile manufacturers normally apply a minimum of 4 coatings to the steel parts of an automobile.

• These coatings include:

electrocoat primer

primer surfacer

basecoat

clearcoat.

11-5



Methods for Paint Comparison The huge supply of automotive paint helps to make-up the

PDQ (automobile paint comparison database).• Questioned and known paint specimens are best

compared side by side under a stereoscopic microscope.• Look for color, surface texture, and color layer

sequence.• Pyrolysis gas chromatography and infrared

spectrophotometry can identify formula of paint binders.– Solid materials, such as paint, may be heated until they are

pryolyzed (decompose) at very high temperatures– The gaseous products are measured as they flow through the

GC column.

11-6



Methods for Paint Comparison• The elements that are contained within paint

pigments can also be identified by emission spectroscopy or X-ray spectroscopy.

11-7



PAINT IDENTIFICATION• Crime laboratories

are often asked to identify the make and model of a car from a small amount of paint and will make use of color charts for automobile finishes or the PDQ database.

11-8



Origin of Spectra• An atom is composed of a nucleus

– containing protons and neutrons

Electrons move around the nucleus in electron orbitals.

• Orbitals have a finite amount of energy called an energy level.

Carbon Atom

11-9



Absorption Spectra Analysis• Each element has its own set

of energy levels at different distances from the nucleus.

• Energy levels have fixed values so an atom will only absorb a set amount of heat or light energy.

• The absorbed energy pushes the electrons into higher energy level orbitals and the atom is now considered in an excited state.

11-10



Where does Spectra Come From?

• Electrons will not stay in this excited state for long, and quickly fall back to the original energy level, releasing energy in the form of light.

• Emission spectroscopy collects and measures the various light energies given off by the atom.

• Because each element has its own characteristic set of energy levels, each will emit a unique set of frequencies.

Emission Spectrophotometer

11-11



Emission Spectrograph• Vaporizes and heats samples a very high

temperature – the atoms in the material achieve move around a lot (an “excited” state).

• The excited atoms emit light, separated into its components, produces a line spectrum. Each element present in the spectrum is identified by the light frequency (remember Ch 4?).

Emission spectra for sodium

11-12



Inductively Coupled Plasma (ICP)

• The sample in aerosol form is introduced into a hot plasma, creating charged particles that emit measurable light wavelengths that can be matched to known elements.

• ICP is used to identify and quantify the composition of mutilated bullets.

JFK bullet fragments

11-13



Collection and Preservation • Paint chips are most likely found on or near

persons or objects involved in hit-and-run incidents.Paper druggist folds and glass or plastic vials make excellent containers for paint.

• Paint smeared or embedded in garments or objects require the whole item to be packaged and sent to the laboratory.Uncontaminated standard/reference paint must always be collected.

Smears on cars and other large objects should be scraped down to the metal to preserve all the layers

11-14



Evidence Collection• Tools used to gain entry into buildings or

safes often contain traces of paint

The tool is collected where possible, along with reference paint samples.

• All evidence should be recorded in place before collecting (notes/pix)

• All containers should be labeled and sealed

11-15



Soil• The value of soil as evidence depends on:

-its prevalence at crime scenes -its transferability between the scene and the criminal

• Most soils can be differentiated by their appearance.

11-16



Soil Analysis• A side-by-side visual comparison under a

microscope of the color and texture of soil specimens is easy to perform and provides a way of distinguishing soils that originate from different locations.

11-17



Soil• In many forensic laboratories, forensic

geologists will characterize and compare the mineral content of soils.

• Some crime laboratories utilize density-gradient tubes to compare soils. – These tubes are typically filled with layers of

liquids that have different density values.– When soil is added to the density-gradient

tube, its particles will sink to the portion of the tube that has a density of equal value.

11-18



Collection of Soil• Standard /reference soils are to be

collected at various intervals within a 100-yard radius of the crime scene, as well as the site of the crime, for comparison to the questioned soil.

• Soil found on the suspect, such as adhering to a shoe or garments, must not be removed.

• Instead, each object should be individually wrapped in paper, and transmitted to the laboratory.

Trace Evidence ll: Metals, Paint, and Soil

Documents

Transcript of Trace Evidence ll: Metals, Paint, and Soil