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Inorganic Analysis. Inorganic versus Organic. ¾ of the weight of the earth’s crust is composed of SILICON and OXYGEN. What are some inorganic materials we would encounter in the forensic’s lab?.
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Inorganic versus Organic ¾ of the weight of the earth’s crust is composed of SILICON and OXYGEN. • What are some inorganic materials we would encounter in the forensic’s lab?
All tests must be designed to ultimately determine the specific chemical identity tothe “exclusion of all others”. May involve identification of trace elements. Trace elements (<1%) are invisible markers that may establish a source or additional points for comparison.
Identifying and quantifying trace elements. • 1. Emission spectroscopy - identifying inorganic elements. • 2. Atomic absorption spectrophotometry • - quantifying inorganic elements.
Emission Spectra of Elements. • Display of colors from an element is called emission spectra. • An Emission spectrograph can be used to produce the spectra which will identify the element.
Continuous versus Line spectra • Visible light continuous spectrum, where all the colors “merge” together. • Elements line spectra, where each line represents a definite wavelength (or frequency) that is characteristic of that element.
Line spectra “the cosmic barcodes” Argon Mercury Sodium Neon
ICP Emission Spectroscopy • Inductively Coupled Plasma • High voltage spark is applied to argon gas flowing through a plasma torch. • Reaches 7,000-10,000 degrees Celsius aerosol of sample • Uses: • mutilated bullets • glass fragments
Atomic Absorption Spectrophotometry/Spectroscopy • The filament of the radiation source must be the made of the same element as the one being analyzed. Specimen is inserted into an air-acetylene flame vaporized atoms are exposed to radiation emitted from a radiation source called a hollow cathode tube or discharge tube.
How do we know what elements are in the sample we are testing?
Hollow cathode tube (discharge lamp) emits only those frequencies/wavelengths of light that are present in the emission spectrum of the element. Atomic absorption spectroscopy measures the absorption of the specific frequencies of light emitted from the discharge lamp. From this we can determine the amount of a particular element.
Advantages and Disadvantages Advantages: • will detect trace elements, one-trillionth of a gram • Simple and low cost Disadvantages: Must select the discharge lamp to match the element under investigation.
Where do emission and absorption spectra come from? ATOMS !!! Protons about 2000x mass of electron Positive charge Neutron about 2000x mass of electron neutral charge Electron smallest subatomic particle. negative charge
Popular model shows electrons orbiting around a central nucleus (containing protons and neutrons). • Remember the atom has no net charge, therefore, #protons (+ charge) = #electrons (- charge)
Atoms are neutral Each element is a collection of atoms with the same # of protons (or electrons), ie, electrically neutral
The Atomic Number is the number of PROTONS (or electrons) The Atomic Symbol Na 11 The Atomic Mass is the number of PROTONS + number of NEUTRONS 22.99
Electrons move around the nucleus in a fixed path……….this is an energy orbital. • Each orbital has a definite amount of energy…………..this is the energy level.
Theory underlying A.A.S.(Atomic absorption spectrophotometry) • When elements absorb energy (eg. from heat or light) strongly they emit energy/light with a characteristic color, as the outer electrons can easily be raised to an excited state. • The spectral colors result from the fact that light is emitted in discrete quanta, the energy (wavelength or color) of which is determined by the structure of the respective electron shells.
If a photon of exactly the right energy "hits" an atom, it can be absorbed and cause an electron to jump to an outer, higher energy orbit. • A photon of the same energy is emitted when the electron falls back down to its original orbit.
A photon of light can interact with an electron causing it to jump to a higher orbit. E = hf Where, E = energy difference between the two orbitals h = Planks constant (6.626 x 10-34 J-sec) f = frequency of absorbed light.
What does this mean? • An element is selective in the frequency of light it will absorb. Thus, we can distinguish between elements based on the frequency of light they absorb (or transmit/release).
Neutron Activation Analysis (N.A.A.) • N.A.A. allows both identification and quantification of elements. • Uses neutrons to cause a sample to emit radiation which can be measured. What about neutrons? • Isotopes have same # of protons but different # of neutrons eg. Hydrogen, deuterium, tritium.
What is radioactivity? emission of radiation that accompanies the spontaneous disintegration of an unstable nucleus. Alpha (α) radiation Beta (β) radiation Gamma (γ) radiation
Back to Neutron Activation Analysis! • Basic technique involves: Source of Neutrons bombard specimen captured by the nucleus new “activated” isotope created decompose and emit radioactivity as gamma rays. • Resulting gamma radiation can be measured to identify the elements in the specimen.
Neutron Activation Analysis cont.. Advantages: • Nondestructive method for identifying and quantifying trace elements. • Detect one-billionth of a gram • Simultaneous analysis for 20-30 elements Disadvantages: • Expensive
X-ray Diffraction • Sometimes called “X-ray crystallography” • Can only be used with solid or crystalline materials. • X-rays are aimed at the crystal and how they interact with the atoms in the substance are recorded diffraction pattern. • Major disadvantage: lack of sensitivity.
