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Introduction Chapter 5 Inorganic Analysis: Introduction • Inorganic, or non-carbon containing substances, will often be encountered as physical evidence. • Forensic scientists analyze inorganics such as tools, coins, weapons, explosives, poisons, and metal scrapings as well as trace components in paints and dyes. • Many manufactured products and most natural materials contain small quantities of elements, known as trace elements, present in concentrations of less than 1 percent. Definitions • Emission: light emitted from a source and separated into its component colors or frequencies • Continuous spectrum: a type of emission spectrum showing a continuous band of colors all blending into one another • Line spectrum: a type of emission spectrum showing a series of lines separated by black areas • Proton: a positively charged particle that is one of the basic structures in the nucleus of an atom • 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. Introduction • The presence of trace elements is particularly useful, because they provide “invisible” markers that may establish the source of a material • The forensic scientist must perform tests that will ultimately determine the specific chemical identity of the suspect material to the exclusion of all others. • Electron: a negatively charged particle that is one the fundamentals structural units of the atom • Neutron: a particle having not electrical charge, which along with the proton is a basic unit in the structure of the nucleus of an atom • Nucleus: the core of an atom containing the protons and neutrons • Atomic number: the number of protons in the nucleus of an atom. Each element has its own unique atomic number 1 • Electron orbital: the pathway of electrons as they move around the nuclei of atoms; each orbital is associated with a particular electronic energy level • Excited state: the state in which an atom is supplied energy and an electron is moved from a lower to a higher energy level • Atomic mass: the sum of the number of protons and neutrons in the nucleus of an atom • Beta ray: a type of radiation emitted by a radioactive element. The radiation consists of electrons • Gamma ray: a high-energy form of electromagnetic radiation emitted by a radioactive element • X-ray diffraction: an analytical technique for identifying crystalline materials • Isotope: an atom differing from another atom of the same element in the number of neutrons it has in its nucleus • Radioactivity: the particle and/or gamma ray radiation emitted by the unstable nucleus of some isotopes • Alpha ray: a type of radiation emitted by a radioactive element. The radiation is composed of helium atoms minus their orbiting electrons Analytical Techniques for Inorganic Materials • Qualitative analysis: confirms the presence of an element or molecule of interest • Quantitative analysis: determines the compounds present and their concentrations • Spectrophotometry – A technique for identifying or measuring a substance based on its absorption or emission of different wavelengths of light Examples of Atomic Spectroscopy: spectra of atoms in the gaseous state Six Techniques • Six main techniques are available to forensic scientists for determining the elemental composition of materials: – Emission spectroscopy – Inductively coupled plasma – Atomic absorption spectrophotometry – Neutron activation analysis – X-ray diffraction – X-ray Fluorescence AES AAS AFS LINE Spectra 2 Atomic Emission Spectroscopy • An emission spectrometer vaporizes and heats samples to a high temperature so that the atoms present in the material achieve an “excited” state. • The excited atoms will emit light. • If the light is separated into its components, one observes a line spectrum. • Each element present in the spectrum can be identified by its characteristic line frequencies. • Emission spectra can than be matched line for line in a comparison between samples. Flame Tests Inductively Coupled Plasma Atomic Emission • In inductively coupled plasma emission spectrometry (ICP-OES), the sample, in the form of an aerosol, is introduced into a hot plasma, creating charged particles that emit light of characteristic wavelengths corresponding to the identity of the elements present. • Two areas of forensic casework where ICP has been applied are the identification and characterization of mutilated bullets and glass fragments. INDUCTIVELY Coupled Plasma Torch (ICP) 1964 Greenfield For Atomic Emission and MS Tesla Coil 3 Emission Spectrum Atomic Spectroscopy • 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 Absorption Spectroscopy Atomic Absorption Spectrometer design • Sample heated to vaporize atoms while leaving a substantial number of atoms in an unexcited state. • Vaporized atoms exposed to radiation emitted from a discharge tube • Atoms absorb radiation & become excited • The amount of radiation absorbed is recorded • The amount absorbed is determined using Beer’s Law (calibration curve) • Can quantitate the amount of metal ion present in a sample – gunshot residue (particularly Pb) Atomic Absorption (AA) Atomic Spectroscopy • 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 4 Atomic Spectroscopy • 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 X-Ray Diffraction Neutron Activation • For chemists, nuclear chemistry provides a new tool for identifying and quantizing the elements. • A nuclear reactor is a source of neutrons that can be used for bombarding atoms, causing some neutrons to be captured to produce radioactive isotopes (atoms with the same number of protons but a different number of neutrons). • To identify the radioactive isotope, it is necessary to measure the energy of the gamma rays emitted as radioactivity. X-ray Diffraction • X-ray diffraction is applied to the study of solid, crystalline materials. • As the X-rays penetrate the crystal, a portion of the beam is reflected by each of the atomic planes. • As the reflected beams leave the crystal’s planes, they combine 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” inorganic compounds. X-Ray Diffraction Data for a single atom Neutron Activation • Neutron activation analysis measures the gamma-ray frequencies of specimens that have been bombarded with neutrons. • This method is highly sensitive and nondestructive analysis for simultaneously identifying and quantitating 20 to 30 trace elements. • Forensic analysis has employed neutron activation on find trace elements in metals, drugs, paint, soil, gunpowder residue, and hair. • Quantitative & qualitative multi-element analysis of major, minor & trace elements • Quantitation in parts per billion 5 Neutron Activation Analysis Neutron Activation Data Spectra of short-lived elements in a pottery sample Neutron Activation Data Neutron Activation Data Shows medium- and long lived elements Continuation of medium- & long-lived elements Forensic Determination of Metals • 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 • 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 6 Gunpowder Residues • 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 • 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. Gunshot Residues • 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. Gunpowder Residues • 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 • 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 • Energy-dispersive X-ray fluorescence 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. 7 Gunshot Residues • 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 Example of Bullet Analysis: JFK Assassination Warren Commission Conclusions • Oswald fired three shots from behind the president from the Texas School Book Depository Building • JFK struck by two bullets – one bullet missed the limousine Warren Commission Conclusions • One bullet entered JFK’s back, exited his throat, entered Connally’s back, exited his chest, hit his wrist, lodged in thigh – bullet later fell onto the stretcher Evidence • 6.5 mm Mannlicher-Carcano rifle found in Texas School Book Depository Building – Oswald’s palm print • One bullet entered JFK’s skull causing the fatal wound • three spent 6.5 mm Western Cartridge Co/ Mannlicher-Carcano (WCC/MC) cartridge cases • Bullets from victims FBI Emission Spectroscopy Neutron Activation Analysis • Compared the fragments from Connally’s wrist to the bullet found on the stretcher – technique only semiquantitative for such tiny fragments • “similar in composition” • “no significant differences were found within the sensitivity of the spectrographic method” – those elements quantified had order of magnitude uncertainties • Analyst Dr. Vincent P. Guinn – leading practitioner of forensic analysis of bullets & fragments by NAA • had examined about 165 different brands & production lots of bullets • Guinn’s studies had shown that quantities of anitmony, silver & copper could be used to distinguish bullets 8 Neutron Activation Analysis • Antimony is most important because it varies the most – used as a hardening agent • hardened bullets contain 0.5%-4% Sb (5,00040,000 ppm) • unhardened bullets contain 10-1,000 ppm Sb • virgin lead contains <10 ppm Sb The Analysis • • Guinn analyzed five types of samples – the stretcher bullet – two metal fragments from Connally’s wrist – a fragment from the front seat of the limosine – two fragments from JFK’s head – three small fragments from the rear floorboard carpet The same samples originally analyzed The Interpretation • Dependent on the two groups being analytically separate – requires their difference in concentration (~30%) to be much grater than the uncertainty of each group (~1% for each of the fragments) Neutron Activation Analysis • Compositions of most bullets of a given type are extremely reproducible within a box or within a production lot • Mannlicher-Carcano bullet compositions vary from bullet to bullet – Do not find a wide variation in composition within an individual bullet – can “usually be distinguished from one another” The Results • Antimony Data – samples fell into two groups • two samples with concentrations ~820 ppm • three samples with concentrations ~620 ppm • Silver analysis showed the same two groups with smaller differences The Interpretation • The stretcher bullet & wrist fragments fall into group #1 • The fragments from the JFK’s head, the front seat, & rear floor fit into group #2 • The fragments were found to be clearly distinguishable 9 Conclusions • Evidence of only two bullets • Both bullets have compositions consistent with WCC/MC bullet lead – other sources cannot be exclusively ruled out • It is highly probable that the stretcher bullet also caused the wrist injury – Absence of bullet fragments from back wounds prevents linking them to the stretcher bullet 10