Honors Forensic Science
INORGANIC ANALYSIS

Introduction
 Organic
substances constitute a substantial portion
of physical evidence submitted to crime labs
 Carbon does not appear among earth’s most
abundant elements
 Inorganics
are also encountered as physical
evidence at crime scenes
 Examples
include
Metals in tools,
coins, weapons,
scrapings
 Pigments in
paints and dyes
 Explosive
formulations
 Poisons


Most inorganic analysis is for the identification
and comparison of physical evidence

The Emission Spectrum of Elements
 Elements
selectively absorb and emit light
 When the light emitted from a bulb or from any
other light source is passed through a prism, it is
separated into its component colors or frequencies
or emission spectrum
 Types
of emission spectrums
 Continuous
spectrum
A type of emission spectrum showing a continuous band of
colors all blending into one another
 Examples – sunlight or light from incandescent bulb passes
through a prism

 Line
spectrum
A type of emission spectrum showing a series of lines separated
by black areas.
 Each line represents a definite wavelength or frequency
 Examples – light from a sodium lamp or mercury arc lamp

 An
emission spectrograph is an instrument used to
obtain and record the line spectra of elements
 Requires
a means for vaporizing and exciting the atoms of elements so
that they emit light
 a means for separating light into its component frequencies
 a means for recording the resultant spectrum

 Uses
in Forensics
 Rapid
comparison of the elemental composition of two or
more specimens
 Inductively
coupled plasma (ICP) emission
spectrometry
 Identifies
and measures elements through light emitted
by excited atoms
 Uses hot plasma torch instead of electrical arc to excite
atoms
 Has been applied in the area of identification and
characterization of mutilated bullets and glass fragments

Atomic Absorption Spectrophotometer
 When
an atom is vaporized, it will absorb many of
the same frequencies of light that it emits in an
excited state
 In this technique, the specimen is heated to a
temperature that is hot enough to vaporize its
atoms while leaving a substantial number of atoms
in an unexcited state
 It
has its most useful application in providing an
accurate determination of an element’s
concentration in a sample
 Is useful in detecting trace amounts of elements
 Drawback – analyst can determine only one
element at a time, each time having to select the
proper lamp to match the particular element under
investigation

The Origin of Emission and Absorption Spectra
 Subatomic
 Proton
particles
– positive electrical charge; found in nucleus
 Neutron – neutral particle; found in nucleus
 Electron – negative charge; outside the nucleus
 Number of protons is equal to the number to electrons to
yield a neutral charge for entire atom
 Differences
among atoms of elements originate in
the number of subatomic particles, such as the
number of protons. An element is a collection of
atoms, all having the same number of protons
 Electrons move around the nucleus and are
confined to specific electron orbitals or energy
levels
 An
atom is in its most stable state when all of its
electrons are positioned in their lowest possible
energy orbitals in the atom but when the atom
absorbs energy, such as heat and light, its
electrons are pushed into higher energy orbitals =
excited state
 Only a definite amount of energy can be absorbed
in moving an electron from one level to another
•
Elements are selective in the frequency of light it will
absorb and this selectivity is determined by the
electron energy levels each element possesses.
Similarly if atoms are exposed to intense heat, enough
energy will be generated to push electrons into higher
unoccupied energy levels
 Normally,
electrons do not remain in this excited
state for long but will fall back to its original energy
level and as it does, it releases energy in the form
of light
 Because each element has its own characteristic
set of energy levels each will emit a unique set of
frequency values providing a “picture” of the energy
levels that surround the nucleus of each element

Neutron Activation Analysis
 Atoms
of the same element must have the same
number of protons but do not have to have the
same number of neutrons.
 Atomic mass = the sum of the number of protons
and neutrons in the nucleus of an atom
 Atoms
having the same number of protons but
differing solely in the number of neutrons are called
isotopes
 Most elements have two or more isotopes and most
are stable
 Isotopes that are unstable and decompose are
considered to be radioactive
 Radioactivity
is the emission of radiation that
accompanies the spontaneous disintegration of
unstable nuclei
 Types
Alpha rays – composed of a helium atom minus electrons; is
positively charged
 Beta rays – electrons; have a negative charge
 Gamma rays – high energy form of electromagnetic radiation
emitted by a radioactive element


To identify the activated isotope, it is necessary to measure the
energy of the gamma rays emitted as radioactivity
 Neutron
activation analysis is the technique of
bombarding specimens with neutrons and
measuring the resulting gamma-ray radioactivity
 Advantage
– provides a non-destructive method for
identifying and quantitating trace elements
 Has been employed for characterizing the trace elements
present in metals, drugs, paint, soil, gunpowder residues
and hair

X-Ray Diffraction
 Is
a technique for identifying crystalline materials
 As x-rays penetrate the crystal, a portion of the
beam is reflected by each of the atomic planes.
They interact 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” compounds
 Drawback – not very sensitive and often fails to
detect the presence of substances comprising less
than 5% of a mixture