INORGANIC ANALYSISHonors Forensic Science

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 particles

Proton – 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