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Introduction
Chapter 5
Inorganic Analysis:
Introduction
• Inorganic, or non-carbon containing substances,
will often be encountered as physical evidence.
• Forensic scientists analyze inorganics such as
tools, coins, weapons, explosives, poisons, and
metal scrapings as well as trace components in
paints and dyes.
• Many manufactured products and most natural
materials contain small quantities of elements,
known as trace elements, present in
concentrations of less than 1 percent.
Definitions
• Emission: light emitted from a source
and separated into its component colors or
frequencies
• Continuous spectrum: a type of
emission spectrum showing a continuous
band of colors all blending into one another
• Line spectrum: a type of emission
spectrum showing a series of lines
separated by black areas
• Proton: a positively charged particle that
is one of the basic structures in the nucleus
of an atom
• Chemists categorize all chemicals into two
classes: organic and inorganic.
• The instruments and methods used to
measure the properties of inorganic
materials differ from those used to
measure the properties of organic
materials.
Introduction
• The presence of trace elements is particularly
useful, because they provide “invisible”
markers that may establish the source of a
material
• The forensic scientist must perform tests that
will ultimately determine the specific chemical
identity of the suspect material to the
exclusion of all others.
• Electron: a negatively charged particle that
is one the fundamentals structural units of
the atom
• Neutron: a particle having not electrical
charge, which along with the proton is a
basic unit in the structure of the nucleus of
an atom
• Nucleus: the core of an atom containing
the protons and neutrons
• Atomic number: the number of protons in
the nucleus of an atom. Each element has
its own unique atomic number
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• Electron orbital: the pathway of electrons
as they move around the nuclei of atoms;
each orbital is associated with a particular
electronic energy level
• Excited state: the state in which an atom
is supplied energy and an electron is moved
from a lower to a higher energy level
• Atomic mass: the sum of the number of
protons and neutrons in the nucleus of an
atom
• Beta ray: a type of radiation emitted
by a radioactive element. The
radiation consists of electrons
• Gamma ray: a high-energy form of
electromagnetic radiation emitted by a
radioactive element
• X-ray diffraction: an analytical
technique for identifying crystalline
materials
• Isotope: an atom differing from another
atom of the same element in the number of
neutrons it has in its nucleus
• Radioactivity: the particle and/or gamma
ray radiation emitted by the unstable
nucleus of some isotopes
• Alpha ray: a type of radiation emitted by a
radioactive element. The radiation is
composed of helium atoms minus their
orbiting electrons
Analytical Techniques for Inorganic
Materials
• Qualitative analysis: confirms the presence of an
element or molecule of interest
• Quantitative analysis: determines the
compounds present and their concentrations
• Spectrophotometry
– A technique for identifying or measuring a substance
based on its absorption or emission of different
wavelengths of light
Examples of Atomic Spectroscopy: spectra of atoms in the gaseous state
Six Techniques
• Six main techniques are available to forensic scientists
for determining the elemental composition of materials:
– Emission spectroscopy
– Inductively coupled plasma
– Atomic absorption spectrophotometry
– Neutron activation analysis
– X-ray diffraction
– X-ray Fluorescence
AES
AAS
AFS
LINE
Spectra
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Atomic Emission Spectroscopy
• An emission spectrometer vaporizes and heats
samples to a high temperature so that the atoms
present in the material achieve an “excited” state.
• 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 than be matched line for line in
a comparison between samples.
Flame Tests
Inductively Coupled Plasma Atomic
Emission
• In inductively coupled plasma emission spectrometry
(ICP-OES), 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.
• Two areas of forensic casework where ICP has been
applied are the identification and characterization of
mutilated bullets and glass fragments.
INDUCTIVELY
Coupled Plasma
Torch
(ICP) 1964
Greenfield
For Atomic
Emission and MS
Tesla Coil
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Emission Spectrum
Atomic Spectroscopy
• ICP-OES
– The ICP-OES method most widely used today
measures the concentration of 10 elements in
glass fragments.
– It permits the classification of glass fragments
as either sheet glass or container glass.
Atomic Absorption
Spectroscopy
Atomic Absorption Spectrometer design
• Sample heated to vaporize atoms while leaving a
substantial number of atoms in an unexcited state.
• Vaporized atoms exposed to radiation emitted from a
discharge tube
• Atoms absorb radiation & become excited
• The amount of radiation absorbed is recorded
• The amount absorbed is determined using Beer’s Law
(calibration curve)
• Can quantitate the amount of metal ion present in a
sample
– gunshot residue (particularly Pb)
Atomic Absorption (AA)
Atomic Spectroscopy
• X-ray fluorescence spectrometry (XRF)
– Is a nondestructive method
– Is based on the detection of X-ray radiation
emitted from the sample being analyzed
– Measures the photon energy to identify which
element is present and measures the intensity
of that photon to quantify the amount of the
element in the sample
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Atomic Spectroscopy
• XRF
– Has an elemental range limited to elements
larger than beryllium
– Is used for bulk analysis
– Is a surface technique—reports the elements
present on the sample’s surface
X-Ray Diffraction
Neutron Activation
• For chemists, nuclear chemistry provides a new tool for
identifying and quantizing the elements.
• A nuclear reactor is 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.
X-ray Diffraction
• X-ray diffraction is applied to the study of solid,
crystalline materials.
• As the X-rays penetrate the crystal, a portion of the
beam is reflected by each of the atomic planes.
• As the reflected beams leave the crystal’s planes, they
combine with one another to form a series of light and
dark bands known as a diffraction pattern.
• Every compound is known to produce its own unique
diffraction pattern, thus giving analysts a means for
“fingerprinting” inorganic compounds.
X-Ray Diffraction Data
for a single atom
Neutron Activation
• Neutron activation analysis measures the gamma-ray
frequencies of specimens that have been bombarded
with neutrons.
• This method is highly sensitive and nondestructive
analysis for simultaneously identifying and
quantitating 20 to 30 trace elements.
• Forensic analysis has employed neutron activation
on find trace elements in metals, drugs, paint, soil,
gunpowder residue, and hair.
• Quantitative & qualitative multi-element analysis of
major, minor & trace elements
• Quantitation in parts per billion
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Neutron Activation Analysis
Neutron Activation Data
Spectra of short-lived elements in a pottery sample
Neutron Activation Data
Neutron Activation Data
Shows medium- and long lived elements
Continuation of medium- & long-lived elements
Forensic Determination of Metals
• Iron, aluminum, and copper: the three
metals most commonly used for fabricating
everyday objects
• Arsenic, mercury, lead, and cadmium:
metals that are sometimes encountered in
industrial workplaces and occasionally as
poisons
Forensic Determination of
Metals
• Alloys
– Are mixtures of metals designed to have
properties that are more desirable than those
of their components
– Are made by mixing metals in different ratios
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Gunpowder Residues
• When a gun is fired, the primer undergoes
a chemical reaction that leads to the
detonation of the smokeless powder in the
cartridge.
• This reaction does not always consume all
of the primer and powder.
Gunpowder Residues
• The most popular GSR detection
techniques require collection of samples
from skin of victim at the scene.
• The atomic absorption test is the most
commonly used GSR test.
• Scanning electron microscopy gives the
most conclusive results.
Gunshot Residues
• Scanning electron microscopy (SEM)
– GSR is collected with adhesive tape.
– Alternatively, GSR may be collected with
polyvinyl alcohol (useful if skin is partially
covered in blood).
– SEM can reveal the finest details of
structure.
Gunpowder Residues
• Gunshot residue (GSR): the materials
remaining behind after the firing reaction
and the products of the combustion
reaction
– Can be used to detect a fired cartridge
– Can be spread on nearby objects, including
the victim and the perpetrator
Gunpowder Residues
• Atomic absorption spectroscopy (AAS)
– GSR is collected with swabs.
– An atomic absorption spectrometer with a flameless
graphite furnace atomizer determines the elements
present in the sample:
• Measures one element at a time
• Can determine the concentration of lead, antimony,
and barium released from the primer
• Can determine presence of gunshot residue
Gunshot Residues
• Energy-dispersive X-ray fluorescence spectroscopy (EDX)
– An EDX detector that measures the energy of the X-ray
photons is attached to the SEM.
– Its elemental range is limited to elements larger than
beryllium.
– SEM can be used to search the surface of the tape for
individual particles of residue.
– SEM-EDX permits the finest details of the sample to be
photographed.
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Gunshot Residues
• Greiss Test
– Is performed first on any evidence because it
will not interfere with later tests for lead
residues
– Determines the muzzle-to-garment distance
– Detects both organic nitrites and nitrates
Example of Bullet Analysis:
JFK Assassination
Warren Commission Conclusions
• Oswald fired three shots from behind
the president from the Texas School
Book Depository Building
• JFK struck by two bullets
– one bullet missed the limousine
Warren Commission
Conclusions
• One bullet entered JFK’s back, exited his
throat, entered Connally’s back, exited his
chest, hit his wrist, lodged in thigh
– bullet later fell onto the stretcher
Evidence
• 6.5 mm Mannlicher-Carcano rifle found in
Texas School Book Depository Building
– Oswald’s palm print
• One bullet entered JFK’s skull causing the
fatal wound
• three spent 6.5 mm Western Cartridge Co/
Mannlicher-Carcano (WCC/MC) cartridge
cases
• Bullets from victims
FBI Emission Spectroscopy
Neutron Activation Analysis
• Compared the fragments from Connally’s
wrist to the bullet found on the stretcher
– technique only semiquantitative for such tiny
fragments
• “similar in composition”
• “no significant differences were found within the
sensitivity of the spectrographic method”
– those elements quantified had order of
magnitude uncertainties
• Analyst Dr. Vincent P. Guinn
– leading practitioner of forensic analysis of
bullets & fragments by NAA
• had examined about 165 different brands &
production lots of bullets
• Guinn’s studies had shown that quantities
of anitmony, silver & copper could be used
to distinguish bullets
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Neutron Activation Analysis
• Antimony is most important because it
varies the most
– used as a hardening agent
• hardened bullets contain 0.5%-4% Sb (5,00040,000 ppm)
• unhardened bullets contain 10-1,000 ppm Sb
• virgin lead contains <10 ppm Sb
The Analysis
•
•
Guinn analyzed five types of samples
– the stretcher bullet
– two metal fragments from Connally’s wrist
– a fragment from the front seat of the limosine
– two fragments from JFK’s head
– three small fragments from the rear floorboard carpet
The same samples originally analyzed
The Interpretation
• Dependent on the two groups being
analytically separate
– requires their difference in concentration
(~30%) to be much grater than the uncertainty
of each group (~1% for each of the fragments)
Neutron Activation Analysis
• Compositions of most bullets of a given type are
extremely reproducible within a box or within a
production lot
• Mannlicher-Carcano bullet compositions vary from bullet
to bullet
– Do not find a wide variation in composition within an
individual bullet
– can “usually be distinguished from one another”
The Results
• Antimony Data
– samples fell into two groups
• two samples with concentrations ~820 ppm
• three samples with concentrations ~620 ppm
• Silver analysis showed the same two
groups with smaller differences
The Interpretation
• The stretcher bullet & wrist fragments fall
into group #1
• The fragments from the JFK’s head, the
front seat, & rear floor fit into group #2
• The fragments were found to be clearly
distinguishable
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Conclusions
• Evidence of only two bullets
• Both bullets have compositions consistent with WCC/MC
bullet lead
– other sources cannot be exclusively ruled out
• It is highly probable that the stretcher bullet also caused
the wrist injury
– Absence of bullet fragments from back wounds
prevents linking them to the stretcher bullet
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