INORGANIC ANALYSIS – CHAPTER 6
HONORS FORENSIC SCIENCE
I.
II.
Introduction
a. Organic substances constitute a substantial portion of physical evidence submitted to
crime labs
b. Carbon does not appear among earth’s most abundant elements
c. Inorganics are also encountered as physical evidence at crime scenes
i. Examples include
1. Metals in tools, coins, weapons, scrapings
2. Pigments in paints and dyes
3. Explosive formulations
4. Poisons
d. Most inorganic analysis is for the identification and comparison of physical evidence
The Emission Spectrum of Elements
a. Elements selectively absorb and emit light
b. 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
c. Types of emission spectrums
i. Continuous spectrum
1. A type of emission spectrum showing a continuous band of colors all
blending into one another
2. Examples – sunlight or light from incandescent bulb passes through a
prism
ii. Line spectrum
1. A type of emission spectrum showing a series of lines separated by
black areas.
2. Each line represents a definite wavelength or frequency
3. Examples – light from a sodium lamp or mercury arc lamp
d. An emission spectrograph is an instrument used to obtain and record the line spectra of
elements
i. Requires
1. a means for vaporizing and exciting the atoms of elements so that they
emit light
2. a means for separating light into its component frequencies
3. a means for recording the resultant spectrum
e. Uses in Forensics
i. Rapid comparison of the elemental composition of two or more specimens
f. Inductively coupled plasma (ICP) emission spectrometry
i. Identifies and measures elements through light emitted by excited atoms
ii. Uses hot plasma torch instead of electrical arc to excite atoms
III.
IV.
V.
iii. Has been applied in the area of identification and characterization of mutilated
bullets and glass fragments
Atomic Absorption Spectrophotometer
a. When an atom is vaporized, it will absorb many of the same frequencies of light that it
emits in an excited state
b. 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
c. It has its most useful application in providing an accurate determination of an element’s
concentration in a sample
d. Is useful in detecting trace amounts of elements
e. 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
a. Subatomic particles
i. Proton – positive electrical charge; found in nucleus
ii. Neutron – neutral particle; found in nucleus
iii. Electron – negative charge; outside the nucleus
iv. Number of protons is equal to the number to electrons to yield a neutral charge
for entire atom
b. 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
c. Electrons move around the nucleus and are confined to specific electron orbitals or
energy levels
d. 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
e. Only a definite amount of energy can be absorbed in moving an electron from one level
to another
f. 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
g. 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
h. 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
a. Atoms of the same element must have the same number of protons but do not have to
have the same number of neutrons.
VI.
b. Atomic mass = the sum of the number of protons and neutrons in the nucleus of an
atom
c. Atoms having the same number of protons but differing solely in the number of
neutrons are called isotopes
d. Most elements have two or more isotopes and most are stable
e. Isotopes that are unstable and decompose are considered to be radioactive
f. Radioactivity is the emission of radiation that accompanies the spontaneous
disintegration of unstable nuclei
i. Types
1. Alpha rays – composed of a helium atom minus electrons; is positively
charged
2. Beta rays – electrons; have a negative charge
3. Gamma rays – high energy form of electromagnetic radiation emitted
by a radioactive element
ii. To identify the activated isotope, it is necessary to measure the energy of the
gamma rays emitted as radioactivity
g. Neutron activation analysis is the technique of bombarding specimens with neutrons
and measuring the resulting gamma-ray radioactivity
i. Advantage – provides a non-destructive method for identifying and quantitating
trace elements
ii. Has been employed for characterizing the trace elements present in metals,
drugs, paint, soil, gunpowder residues and hair
X-Ray Diffraction
a. Is a technique for identifying crystalline materials
b. 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
c. Every compound is known to produce its own unique diffraction pattern, thus giving
analysts a means for “fingerprinting” compounds
d. Drawback – not very sensitive and often fails to detect the presence of substances
comprising less than 5% of a mixture