Week 2 - Spectroscopy - Structure of Atoms PDF

FORENSIC SCIENCE SPECTROSCOPY AND THE STRUCTURE OF ATOMS Learning Objectives explain the concept of chemical elements from an understanding of atomic structure explain how emission and absorption spectroscopy work flame tests, AAS, SEM-EDX, NAA Cases September 2001: 1963: how...

FORENSIC SCIENCE SPECTROSCOPY AND THE STRUCTURE OF ATOMS Learning Objectives explain the concept of chemical elements from an understanding of atomic structure explain how emission and absorption spectroscopy work flame tests, AAS, SEM-EDX, NAA Cases September 2001: 1963: how 1821: Death of part of the body of a many people Napoleon boy found in the shot JFK? Thames  is a substance that cannot - be transformed into another substance Elements and Compounds & consists of multiple elements combined tyt Elements are the basic materials: 93 natural, more artificial H, O, C, Fe, Zn, U, Ir, Sm, Gd, Kr, Tl…. Gunturtiny elements ELEMENTS CAN NEITHER BE CREATED NOR DESTROYED Robert Boyle 1627-1691  Paper made of burn Carbon dioxide CO2 Cellulose = C, H, O Water H2O paper elements can neither CO2 & Still created nordevea in Human Body H2O they just appear N oxides different compounds C, H, O, N, P, S, Ca, Na, K, Fe…. burn P oxides body S oxides Au or Hg in teeth Ca oxides or hydroxides (fillings in teeth …….. Au Hg oxides Elements The elements may be organised according to chemistry: ~ chemicties a The Periodic Table (Mendeleev, Chemist, Russia) Or musically (Tom Lehrer, Mathematician, Harvard) http://www.privatehand.com/flash/elements.html Analysis Which elements are present, and how much? ex. suspected lead or mercury poisoning Are these metals present in the body? regardless of their chemical form Toxicology ex. FUN analysis how much of these elements were present in the Piltdown bones? Time of Death  What is it made of? How can we identify the elements present in an object? Bulk composition (what is most it of it made of?) e.g. bullets – lead Trace impurities (what is a very small part of it made of?) e.g. bullets - silver and antimony if multiple elements are present, I results you get for one might Chemical tests – large amounts mask results that - destructive no from another - subject to interference - fast - easy to do - simple equipment - not sensitive  Trace impurities: How small is small? S.I. prefixes 1 Megagram 1 Mg = 1,000,000 g is called a tonne biggest than 1 Kilogram (1 Kg) = 1,000 g 19 1 gram (1 g) 1 milligram (1 mg) = 0.001 gram (1/1000th) [se) smaller than 1 microgram (1 g) = 0.000001 gram (1/1000,000th) I can't see) 19 1 nanogram (1 ng) = 0.000000001 gram (1/billionth) ~ 1 picogram (1 pg) = 0.000000000001 gram (1/trillionth) Even a picogram contains about 100,000,000,000 atoms How small is small? trace elements are often measured in ppm = part per million 1 in 1,000,000 5 people in all of Singapore or even in ppb = parts per billion 1 in 1,000,000,000 7 people in the whole world The Spectrum White light consists of all colours. These can be separated by passing light through a prism. ~ light from I sun is split into The same effect results in rainbows. its constituent colours White light is split when Early investigators such passing through a prism as Newton, used sunlight Spectroscopy Early 19th Century: black bands observed in the solar spectrum: ~ sunlight Fraunhofer lines - found to be due to hydrogen black consists caused Other lines led to the discovery of helium bylines part of are light from absorbed sun being by H Norman Lockyer & proposed - existence I helium  the whole spectrum Different spectroscopic techniques use different frequencies of light. The different frequencies of light interact differently with the molecules. shortwavelength, high free : longwarelea nt (inverse uls) Structure of the Atom Tiny, dense, positively charged core called a nucleus in which nearly all the mass is concentrated, around which the light, negative constituents, called electrons circulate at some distance: “planetary model” (not e best model) Ernest Rutherford 1871-1937 Atomic structure Why don’t atoms destroy themselves due to electrostatic attraction? Niels Bohr – electrons are restricted to specific energy levels so they can't drop down to e nucleus as there aren't any energy his ready for them Atomic structure Electrons release AE energy when they drop to a lower level Ground state Excited State Energy is released as electromagnetic le- in higher radiation 11 energy The frequency of the electromagnetic radiation emitted or absorbed is proportional to E (the energy drop) de Broglie eqn: E = h & freq Allowed Energy Levels the rungs of a ladder are allowed levels in an atom , luls are energy released energy not equally that matches gap to spaced like next rung on this ladder Energy emitted Atomic structure add every Electrons jump to a higher level when they absorb energy Ground State Excited State Energy is absorbed as electromagnetic radiation The frequency of the electromagnetic radiation emitted or absorbed is proportional to E (the energy drop) ~ i fregt of electromagnetic radiate must be right de Broglie eqn: E = h fregt to match AE Allowed Energy Levels the rungs of a ladder are allowed levels add energy that matches gap to next rung Energy absorbed & in i form of electromagnetic radiate Energy Levels Black lives e in spectrum 19th century are in early due to electromagnetic energy being a ri/he absorbed by or other atoms atoms have for ove Multiple energy levels - multiple lines in the spectra afor The pattern is characteristic for each element  absorb electromagnetic radiati e specific only of atoms of e ~ match i AE that free elements present Absorption Spectroscopy er promoted to add E higher 1 energy absorb electromagnetic radiata Ground State Excited State E and therefore frequency characteristic of the element can determine presence of element from spectrum  Emission Spectroscopy > energy given out fregt of > radiate given out matches AE yeth L lvls supplied energy energy Excited given State out E aselectromagnetic E and therefore Ground State frequency characteristic if we art measure of radiati of the element absorbed/ emitted we can of element , in I couc measure assume am e sample as we can , absorbed/ emitted will be proportional to e 2) of I element  emission spectrum Using a prism, the spectrum of the element can be obtained used in this experiment electricity , is to excite H to their excited state, they drop back to then their ground states & give off light & I beam of & to light is passed through slits e prism. I prism breaks I light into its components & diff bands of lighe diff colors (signifying diff fregt) are observed I gas bands no emiss ? diff wavelength/ free = bed it that I emits does not match AE in H afor Flame tests for metals lemiss) If a metal salt is introduced into a flame, a characteristic colour is produced Cheat) energy of the flame excites electrons in the sodium atoms & fo higher sodium energy l (orange) emit orange light as they drop back to the ground state At matches I freg of orange light : flame turns orange Barium Calcium Lithium Copper Potassium (green) (red) (red) (green) (lilac) Flame tests for metals qualitative quantitative Y now which Flame photometer Cemiss1) element is present (light of particular fregt of interest a is separated & I intensity of that light is measured to measure. cous  replace flame photometer ~ Atomic Absorption Spectrometer (quantitative) labsorpt) not flame detector loss of intensity is related to amount of the atom of interest other hollow monochromator in the flame because the atoms cathode lamp & sample becomes in the flame absorb e diff frog & not e are provided bye lamp atomised Advantages: fast specific gas sensitive acetylene/ oxygen display or small sample size or computer acetylene/ NO Disadvantages: destructive dissolve in acid ~ sample dissolved light from the element by element in acid Chas to be in solute form to pass lamp at frequency flume) for the element in of interest  samplea beam of e bombarding - EDX: Energy Dispersive X-ray Fluorescence lemiss) non destructive analysis looking at the core electrons Use an electron beam to eject core electrons fire electron beam & core electron electromagnetic radiat is? knocked out emitted HE , will be quite big fregt will , bequitehia b when a higher energy electron X-ray ejecting a core electron drops to fill the vacancy, a creates a vacancy characteristic X-ray is emitted  EDX diff elements correspond to diff X-ray freat Intensity S correlated to 17 of elements present energy of emitted X-Ray /frog- Microscopy technique microscope basedonbombardin are ↑ ofe Scanning Electron ~ fr Microscope imaging (SEM) Luses a beam of electrons instead of light a particle of gunshot residue diameter about 5 m called combine SEM & EDX to give a technique SEM-EDX SEM-EDX Lemissa) in a SEM, there is an electron gun that fires a beam of electron at the object of interest and hit the object and bounce and are detected. The signals from the detector are used to generate the image Besides the electrons hitting and bouncing off the sample, they can also hit the atom and cause a core electron to be ejected and a higher energy level electron will drop down to fill the vacancy. The drop is from a higher energy level to a much lower energy level and the radiation given out is high energy and corresponds to X-rays which are characteristic of the element  uses atomic nucleus rather - than electrons Neutron Activation Analysis lemiss ) = git neutron absorbed by nucleus giving non-destructive Lexcited State) & forms of electromagnetic radiate trei - ve; but needs a nuclear reactor Characteristic energy sample may be radioactive afterwards Neutron Activation Analysis Gamma-ray spectrum from 0 to 800 keV showing medium- and long- lived elements measured in a sample of pottery irradiated for 24 hours, decayed for smaller peaks in - much elements 9 days, and present smalle quantities vice versa counted for 30 minutes Qualitative and quantitative multi-element analysis 70% of elements can be studied Who Killed Napoleon? Napoleon’s Career: Artillery officer, General, Dictator of France, Emperor of (most of) Europe Defeated, Waterloo, 1815 Exiled to St Helena; 1821 dies Who Killed Napoleon? 1952 Analysis of hair samples (neutron activation analysis) shows arsenic the British? the French? Who Killed Napoleon? 1980 Dr David Jones (Newcastle): what colour was Napoleon’s wallpaper? ↓ ~ non-destructive SEM-EDX shows presence of arsenic in his wallpaper copper arsenite or Scheele’s Green green in I wallpaper certain species would of metabolised ~ mold argenic CuAsO3H AsH3 (gas) forming dre to St Helena's subtropical warm humid climate Who Killed Napoleon? autopsy? eye-witnesses? His doctors – using mercury chloride & toxic drug i Drs prescribed to him Kennedy Assassination 22nd Nov. 1963 Lee Harvey Oswald (Shooter) Oswald fired three times (One complete miss) If there was a second gunman, how many types of bullet would there be?  theyfly throne air ,theybecome extremea ~ Then fragments Analysis of Bullets - not pure lead! into now to tell which fragment belongs Neutron activation analysis to which bullet ? - analysis of trace elements of bullets bullet fragment silver (Ag)/ppm antimony (Sb)/ppm bullet will Q1 8.8 833 each have its unique Q9 9.8 797 amount of these elements Q2 8.1 602 e indicating Q4,5 7.9 621 presence f 2 bullets Q14 8.2 642 Bullets of this type had Ag 5 to 15 ppm; Sb 20 to 1200 ppm Most likely to be a single gunman  Adam 2001: part of a body of a boy found in the Thames forsoo limbsa head missing cuts made with very sharp knife blood drained - ethnic : African still wearing a pair of orange shorts shorts from China, sold in West Germany stomach contained food, pollen (N.European), clay, calabar bean, gold particles & toxic plant native to West these 3 are associated in West African Africa black magic , he is murdered as part of he was West African black magic ceremony , a human sacrifice You Are What You Eat Bone consists of calcium phosphate in a protein matrix. Calcium is derived from the diet. Other metals may also be incorporated if they are present in the diet. you grow up where may be present in e chemisty of your bones in more who live (particularly true for people where they Relative abundance of trace elements in bone can indicate traditional societies eat what is grown locally & geographical origin: strontium, copper, lead. not were as true we eat for advance societies what is imported from all sorts of places around e world strontium copper lead he was abducted Small soil & samples in Nigeria & people taking area bore of Nigeria trafficked to I to match trace element west Germany profile ofI soil to that If & transported to bones London where his he was used as a human sacrifice  Elemental Analysis Spectroscopic methods: emission (flame photometry) absorption (AAS) EDX: X-Ray combine with SEM NAA