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Chapter 11
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
11-1
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Many manufactured products and even
most natural materials contain small
quantities of elements, known as trace
elements, present in concentrations of less
than 1 percent.
For the criminalist, the presence of these
trace elements is particularly useful,
because they provide “invisible” markers
that may establish the source of a material
or at least provide additional points for
comparison.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
11-2
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For chemists, nuclear chemistry provides a
new tool for identifying and quantitating the
elements.
A nuclear reactor is simply a source of
neutrons that can be used for bombarding
atoms, causing some neutrons to be captured
to produce radioactive isotopes (atoms with
the same number of protons but a different
number of neutrons).
To identify the radioactive isotope, it is
necessary to measure the energy of the
gamma rays emitted as radioactivity.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
11-3
Figure 11–4 The neutron activation process requires the capture of a
neutron by the nucleus of an atom. The new atom is now radioactive and
emits gamma rays. A detector permits identification of the radioactive atoms
present by measuring the energies and intensities of the gamma rays emitted.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
11-4
Figure 11-3 Isotopes of hydrogen.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
11-5
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Neutron activation analysis measures the
gamma-ray frequencies of specimens that
have been bombarded with neutrons.
This method provides a highly sensitive and
nondestructive analysis for simultaneously
identifying and quantitating 20 to 30 trace
elements.
Forensic analysis has employed neutron
activation to find trace elements in metals,
drugs, paint, soil, gunpowder residue, and
hair.
Since this technique requires access to a
nuclear reactor, however, it has limited value
to forensic analysis.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
11-6
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Paint spread onto a surface will dry into a hard
film that can best be described as consisting of
pigments and additives suspended in the binder.
One of the most common types of paint
examined in the crime laboratory involves
finishes emanating from automobiles.
Automobile manufacturers normally apply a
variety of coatings to the body of an automobile.
These coatings may include electrocoat primer,
primer surfacer, basecoat, and clearcoat.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
11-7
The wide diversity of automotive paint
contributes to the forensic significance of an
automobile paint comparison.
 Questioned and known specimens are best
compared side by side under a stereoscopic
microscope for color, surface texture, and color
layer sequence.
 Pyrolysis gas chromatography and infrared
spectrophotometry are invaluable techniques for
distinguishing most paint binder formulations,
adding further significance to a forensic paint
comparison.
 Solid materials, such as paint, may be heated or
pyrolyzed to high temperatures so that they will
decompose into numerous gaseous products
to
PRENTICE HALL
FORENSIC SCIENCE
©2008 Pearson Education, Inc.
An Introduction
flow through the GC column.
Upper Saddle River, NJ 07458
By Richard Saferstein
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11-8
Figure 11–7 Schematic diagram of pyrolysis
gas chromatography.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
11-9
Figure 11-8 Paint pyrograms of acrylic enamel paints.
(a) Paint from a Ford model and (b) paint from a Chrysler
model. Courtesy Varian Inc., Palo Alto, Calif.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
11-10
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The elements that are contained within paint
pigments can be identified by emission
spectroscopy or X-ray spectroscopy.
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
(Paint Data Query) database.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
11-11
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An atom is composed of a nucleus containing
protons and neutrons, with electrons found
outside the nucleus in electron orbitals.
The orbitals are associated with a definite amount
of energy called an energy level.
Each element has its own set of characteristic
energy levels at varying distances from the
nucleus.
Because energy levels have fixed values, an atom
will absorb only a definite value of energy, which
may come from heat or light.
This absorbed energy pushes the electrons into
higher energy level orbitals and the atom is now
considered in an excited state.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
11-12
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Normally, the electrons will not stay in this excited
state for long, and they will quickly fall back to their
original energy level, releasing energy in the form of
light emission.
Emission spectroscopy collects and measures the
various light energies given off by the atom.
The specific frequency of light absorbed or emitted
can be determined by the relationship E=hf, where E
is the energy difference between two orbitals, f is
frequency, and h is a universal constant called
Planck’s constant.
Because each element has its own characteristic set of
energy levels, each will emit a unique set of
frequencies.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
11-13
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An emission spectrograph vaporizes and
heats samples to a high temperature so that
the atoms present in the material achieve
an “excited” state.
Under these circumstances, the excited
atoms will emit light. If the light is
separated into its components, one
observes a line spectrum. Each element
present in the spectrum can be identified
by its characteristic line frequencies.
Emission spectra can then be matched line
for line in a comparison between samples.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
11-14
Figure 11–9 Some characteristic emission spectra.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
11-15
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In inductively coupled plasma emission
spectrometry (ICP), the sample, in the form
of an aerosol, is introduced into a hot
plasma, creating charged particles that emit
light of characteristic wavelengths
corresponding to the identity of the
elements present.
An area of forensic casework where ICP
has been applied are the identification and
characterization of mutilated bullets.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
11-16
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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.
Tools used to gain entry into buildings or
safes often contain traces of paint, requiring
the tool be collected, along with reference
paint samples.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
11-17
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The value of soil as evidence rests with its
prevalence at crime scenes and its
transferability between the scene and the
criminal.
Most soils can be differentiated by their gross
appearance.
A side-by-side visual comparison of the color
and texture of soil specimens is easy to
perform and provides a sensitive property for
distinguishing soils that originate from
different locations.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
11-18
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In many forensic laboratories, forensic
geologists will characterize and compare the
mineral content of soils.
Some crime laboratories utilize densitygradient tubes to compare soils.
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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.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
11-19
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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.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
11-20

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