Chapter 9 Inorganic Analysis: Forensic Determination of Metals and Gunshot Residue
Objectives (1 of 2) • Students should gain an understanding of: • The names and symbols that chemists use for elements and compounds • Properties of electrons, protons, and neutrons, the fundamental particles of all matter • The organization of the periodic table based on the arrangement of electrons in the elements
Objectives (2 of 2) • Students should gain an understanding of: • The use of inductively coupled plasma optical emission spectroscopy (ICP-OES) to elucidate the composition of metals • The use of the ICP-OES and X-ray fluorescence spectrometry to identify the presence of trace metals • The detection of gunshot residue by atomic absorption spectroscopy and by scanning electron microscopy with energy-dispersive X-ray
Introduction • 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.
Elements and Compounds (1 of 2) • All substances can be classified as either elements or compounds. • Elements: the building blocks from which all matter is constructed • Compounds: substances composed of two or more elements combined chemically in fixed proportions
Elements and Compounds (2 of 2) • There are 92 naturally occurring elements and 17 human-made elements. • Every compound is made up of some combination of these 109 elements.
Elements and Molecules • Very few elements exist in nature in their elemental form as single atoms. • Most matter is composed of molecules. • A molecule is a combination of two or more atoms. • A molecule is the smallest unit of a pure substance that can exist and still retain its physical and chemical properties. • A molecule may be composed of two or more identical atoms or two or more different kinds of atoms.
Physical Properties of Inorganic Substances (1 of 4) • Physical properties of a substance: properties that can be observed without changing the substance into another substance • Color – Odor • Taste – Hardness • Density – Solubility • Melting point – Boiling point
Physical Properties of Inorganic Substances (2 of 4) • Chemical reaction: a reaction in which a substance changes its molecular composition • Chemical properties of a substance: properties that can be observed when the substance undergoes a change in its molecular composition
Physical Properties of Inorganic Substances (3 of 4) • Matter can be classified as being in one of three states: • Solid • Liquid • Gas
Physical Properties of Inorganic Substances (4 of 4) • Factors affecting state: • Temperature • Surrounding pressure • Strength of the forces holding the substance’s internal components together • Many substances can be changed from one state to another only by extremes of temperature or pressure, or both.
Elements and Subatomic Particles (1 of 2) • Electron: a negatively charged particle; the smallest of the subatomic particles • Proton: a positively charged particle, opposite, but equal in charge, to an electron • Neutron: a neutral particle (no electrical charge)
Elements and Subatomic Particles (2 of 2) • Nucleus: center of the atom • Moving electrons are found in the area surrounding the nucleus • The nucleus contains the vast majority of the atom’s mass
Atomic Number • All atoms of a particular element have the same number of protons in their nuclei; this number determines the identity of the element. • Atomic number = # of protons = # of electrons • Mass number = # of protons + # of neutrons • Both a proton and a neutron have mass = 1.
Periodic Table (1 of 2) • Elements are arranged in order of atomic number. • The periodic table is divided into 7 horizontal rows (periods) and 18 vertical columns (groups). • Elements within a group have similar physical and chemical properties.
Periodic Table (2 of 2) • Metals are good conductors of heat and electricity, have a characteristic lustrous appearance, are ductile, and are malleable. • Nonmetals do not conduct heat or electricity to any significant extent, have little or no luster, and are neither ductile nor malleable. • Semimetals conduct electricity well only at high temperatures. • Semiconductors conduct electricity better than nonmetals but not as well as metals.
Analytical Techniques for the Analysis of Inorganic Materials (1 of 2) • Qualitative analysis: confirms the presence of an element or molecule of interest • Quantitative analysis: determines the compounds present and their concentrations
Analytical Techniques for the Analysis of Inorganic Materials (2 of 2) • Spectrophotometry • A technique for identifying or measuring a substance based on its absorption or emission of different wavelengths of light
Electromagnetic Radiation and Spectra (1 of 3) • Electromagnetic radiation (EMR): energy we encounter daily • Electromagnetic waves travel outward in all directions. • Waves are slowed very slightly when they travel through air or any medium where they encounter atoms or molecules.
Electromagnetic Radiation and Spectra (2 of 3) • Line spectra • If the light produced from putting an element into a flame is passed through a prism and focused onto a photographic film, a series of lines separated by black spaces appears. • This pattern of lines is unique for each element and can be used to identify that element.
Electromagnetic Radiation and Spectra (3 of 3) • Electrons orbit around the nucleus of an atom. Each orbit is associated with a definite energy level. • The lowest energy level is the ground state. • A quantum of energy is needed to excite an electron so that it will jump to a higher energy level (excited state). • Transitions that produce energy in the form of visible light are seen as lines in the spectrum.
Electron Configuration and the Periodic Table • Electron configuration is the basis for the arrangement of the elements in the periodic table and explains why elements with similar properties recur at periodic intervals in the table. • The arrangement of electrons can be used to identify which elements are present in a questioned sample.
Forensic Determination of Metals (1 of 2) • 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 (2 of 2) • 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
Atomic Spectroscopy (1 of 5) • Inductively coupled plasma optical emission spectroscopy (ICP-OES) • Argon ions are accelerated by an oscillating radiofrequency and form a closed annular “torch” that reaches temperatures as high as 10,000 °C. • Samples are carried into the torch by argon flowing through the device’s central quartz tube. • As an excited metal atom leaves the tube, it cools down, relaxes to the ground state, and releases a photon of light (optical emission). • Measurement of the wavelength of the optimal emission can identify the elements present in the sample.
Atomic Spectroscopy (2 of 5) • ICP-OES • A nebulizer is the most commonly used device for sample injection. • A simultaneous multichannel instrument may be used as the spectrometer. • ICP-OES accepts only liquid samples.
Atomic Spectroscopy (3 of 5) • 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 Spectroscopy (4 of 5) • 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
Atomic Spectroscopy (5 of 5) • 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
Gunpowder Residues (1 of 4) • 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 (2 of 4) • 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 (3 of 4) • 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.
Gunpowder Residues (4 of 4) • 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 (1 of 3) • 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.
Gunshot Residues (2 of 3) • Energy-dispersive X-ray 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.
Gunshot Residues (3 of 3) • 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