JS 113: Organic and Inorganic Analyses

1
JS 113 Organic and Inorganic Analyses

• Announcements
• Schedule and Assignments
• Return and Review Exam 1
• Learning Objectives- Organic Analyses
• Define- Elements vs.. Compounds
• Difference between solid, liquid or gas and
  define phase
• Distinguish Organic vs.. Inorganic compounds
• Distinguish between qualitative and quantitative
  analysis
• Explain equilibrium and Henrys law
• Describe chromatography, gas chromatography (GC)
  and retention time
• Define Rf and electrophoresis
• Review spectrophotometry
• Describe Mass Spec and GC-MS
• Learning Objectives- Inorganic Analyses
• Describe the usefulness of trace elements in
  comparisons of phys. evidence
• Distinguish continuous and line emission spectra
• Describe the following instruments/techniques and
  how they are used
• Emission spectrograph

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Announcements and Assignments

• Assignments
• Read chapters 5 and 6
• Read Chapters 9 and 10 on Drugs and Toxicology
• Study for the Quiz Chapters 5, 6, 9 and 10
• Guest Lectures
• Tom Abercrombie- 100807
• Sandra Sachs 101007
• Return and review exams

3
Elements and Compounds

• Element- simplest substances known providing
  building blocks for all matter
• 109 known elements 89 natural, others created
• Periodic Table- elements listed by name and
  symbol arranged in rows with similar chemical
  properties. e.g. carbon (C )
• Atom- smallest particle of an element that can
  exist and retain its identity
• Compound when 2 or more elements are combined to
  form a new substance different in physical and
  chemical properties from its elemental
  constituents e.g. CO2

4
The Periodic Table
5
Physical States

• Solid, liquid, and gas different forms or
  states of matter
• Solid- definite shape and volume
• Liquid- definite volume and takes shape of
  container
• Gas- neither definite shape nor volume
• Substances can change from one form to another
• Freezing- Water to Ice (0C) or Vaporizing- water
  to steam (100C)
• Sublimation - solid? gas
• No new chemical substance is being formed.
  Attractive forces change
• Phases- substances can be distinguished by a
  visible boundary
• For example- Oil and Vinegar or Sugar in Water

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Organic vs.. Inorganic substances

• Organic v. Inorganic
• Organic contains carbon ( C ) combined w/ H,
  O, N, S, P, Cl, Br
• Inorganic substance all other known (no C )
• Qualitative vs. Quantitative determinations
• Qualitative
• results in the identity of the material
• Requires determination of numerous properties
• For example- powder reveals presence of heroin
  and quinine
• Quantitative
• result in percentage combination of components of
  a mixture
• Precise measurement of a single property of the
  material
• For example powder contains 10 heroin and 90
  quinine
• Analytical techniques for identification of
  organic compounds
• Spectrophotometry- study of absorption of light
  by chemical substances usually requires material
  to be in pure states
• Chromatography- separating and identifying
  components of a mixture

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Chromatography Principles (1)

• Useful to separate mixtures into components
• William Henry (1803)
• Henrys Law - When a volatile chemical compound
  is dissolved in a liquid and is brought to
  equilibrium with air, there is a fixed ratio
  between the concentration of the volatile
  compound in air and its concentration in the
  liquid and this ratio remains constant for a
  given temperature
• Distribution or partitioning determined by
  solubility of the gas in the liquid. The higher
  the solubility the greater the tendency to remain
  in the liquid phase

8
Chromatography Principles (2)

• One phase moves continuously in one direction
• Air is forced to move continuously over the water
  and since B (clear) has greater in moving gas,
  its molecules will travel over the liquid faster
  than A (Dark)
• Race between chemical compounds.
• Substances are first mixed
• Materials with preference for moving phase slowly
  pull ahead
• At end, all substances separated crossing the
  finish line at different times
• Gas Chromatography- GC, High Performance Liquid
  chromatography- HPLC, Thin Layer chromatography-
  TLC.

9
Gas Chromatography (1)

• Separates mixtures stationary liquid and moving
  gas
• Stationary liquid is in columns
• Packed columns contain liquid fixed on particles
  are 2-6m in length and 3mm diameter
• Capillary columns composed of glass, 15-60 m and
  0.25 to 0.75mm diameter. Stationary liquid phase
  is a thin film on column inner wall.
• Carrier gas (N) flows thru column carrying
  components of a mixture. Those with a greater
  affinity for gas are faster
• Once traversing the column, emerge separated into
  its components

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Gas Chromatography (2)

• Sample injected into a heated port with a heated
  column ? sample in vapor state
• As components emerge they enter the detector
• Flame ionizes substance generating an electric
  signal
• Recorded on a strip chart recorder as a function
  of time chromatogram
• Recorder response v time
• Retention time-
• Time required for a component to emerge
• Provides a useful identifying characteristic of a
  material
• Not considered absolute ID as other materials may
  have similar RT

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Gas Chromatography (3)

• GC is extremely sensitive and quantitative (down
  to ng how small is that?)
• Amount of substance is proportional to the peak
  area recorded
• Pyrolysis GC
• Important extension of GC
• Many forms of physical evidence, paint,
  fibers,plastics, can be dissolved in a solvent by
  heating or pyrolysis to high temps (500-1000C)
  for injection into the GC
• Pyrolyzers permit the gaseous products to enter
  the carrier gas stream where they flow thru the
  GC column and the material produces a pyrogram
  fingerprint of the material with many points of
  comparison

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High-Performance Liquid Chromatography (HPLC)

• Moving phase is liquid and stationary phase are
  coated solid particles
• As liquid carries the sample, different
  components are slowed to different degrees
  depending on their interaction with the
  stationary phase
• Major advantage over GC is it takes place at room
  temperature
• GC- needs to heat material. Any temperature
  sensitive material may be destroyed. Explosives
  are generally heat sensitive and therefore are
  more readily separated by HPLC

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Thin Layer Chromatography (TLC -1)

• Moving liquid phase, solid stationary phase
• TLC Procedure
• Sample is dissolved in a solvent
• Spotted onto the lower edge of the plate
• The plate is placed into a closed chamber with
  liquid
• The liquid slowly rises up by capillary action.
  Separation occurs as the components with the
  greatest affinity for the moving phase migrate
  faster
• Visualized UV fluorescence or developed with a
  chemical reagent spray ? color spots

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TLC -2
Q K

• Questioned sample (Q) must be developed alongside
  a standard or known (K) sample. If Q and K
  travel the same distance up the plate from the
  origins then they can be tentatively identified
  as the same
• ID cannot be considered definitive as other
  materials may have similar migration
• Distance traveled up can be assigned an Rf value
  distance traveled by the component divided by
  the distance traveled by the liquid phase. For
  example if the moving phase travels 10cm and spot
  8cm then Rf 8cm/10cm 0.8
• Rapid and sensitive down to 100ug
• Principal application is detection and
  identification of components in a complex mixture

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Pen Ink TLC Hands on exercise

• Draw a straight line with pencil 1 inch from the
  bottom of your plate paper towel
• Spot at least 8 different inks across the plate
  at ½ inch intervals- Label your plate with team
  name and pen ink (eg. red expo marker)
• Pour your solvent in to approximately ¼ inch
  depth
• Slowly drop your plate into the solvent
• Permit the front to move up at least 3 inches
• Remove the plate and let air dry
• Answer the following
• 1) Are there differences in migration?
• 2) Do you see any evidence of separation of dyes?
• 3) Are there any inks that do not migrate?
• 4) Based on your observations, which inks have
  the most affinity for the mobile phase? For the
  stationary phase?

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Electrophoresis

• Separation of materials according to migration
  rates on a stationary solid phase
• Uses electric potential across the stationary
  medium
• Medium may include starch or agarose coated on a
  glass plate of polymer in a capillary
• Substances possessing an electric charge migrate.
  The speed depends on size and charge
• Principal applications are the separation of
  mixtures of proteins and DNA

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Spectrophotometry Review

• Theory of Light- White light ROYGBIV
• Light is a wave - wavelength is inversely
  proportional to frequency- Visible light is only
  a small part of the electromagnetic spectrum
• Color visual indication of an objects ability
  to absorb some and reflect visible light
  components
• Different materials have different absorptions
• Absorption of UV, visible and IR are particularly
  applicable for identification of organic
  substances. How much? - Beers Law- Akc , A
  absorption cconcentration kproportionality

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Spectrophotometer

• Instrument used to measure and record the
  absorption spectrum of a chemical substance
• Components- 1. Radiation source
• 1- Radiation source (UV, vis, IR)
• 2. Monochromator or frequency selector
• 3. Sample holder
• 4. Detection to convert electromagnetic radiation
  into an electric signal (digitizer)
• 5. Recorder

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UV and Visible Spectrophotometry

• Measures the absorbance of UV and visible light
  as a function of wavelength or frequency
• UV spec of heroin has max absorption at 278nm
  providing materials probable identity
• Will not provide definitive result - other
  material may have a similar UV absorption

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IR Spectrum

• IR specs provide far more complex patterns
• Different materials always have distinctively
  different IR spectra
• Each IR spectra is equivalent to a fingerprint
  of that substance and no other
• Fourier transform infrared spectrophotometer
  FT-IR
• Considered specific in itself for identification

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Mass Spectrometry (1)

• GC coupled to a MS overcomes limitation of GC
  (cannot produce specific identification alone)
• Material emerging from GC, enters a vacuum where
  they are bombarded by high energy electrons
  causing them to lose electrons and acquire a
  positive charge (ions).
• These ions are unstable and fragment
• Fragments pass through an electric field where
  they are separated according to their masses.
• No two substances produce the same fragmentation
  pattern under carefully controlled conditions.
• Very sensitive one millionth of a gram

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Mass Spectrometry (2)
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Mass Spectrometry (3)

1. Sample first injected into a heated inlet port
   and carrier gas sweeps it into the GC column
2. GC separates the mixture into its components
3. Ion source filiment wire emits electrons striking
   the sample molecules causing them to fragment
   according to mass
4. Detector counts the fragments passing thru the
   quadrupole Signal is small and must be amplified.
5. Measures abundance of each fragment displaying
   the mass spectrum

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Summary 1

• Organic substances contain C. Inorganic ones
  comprise all others
• Choice of analytical techniques depends on
  substance category (organic vs inorganic) and the
  need for qualitative vs. quantitative
  determinations
• Qualitative relates just to the identity of the
  material whereas quantitative relates to the
  percent composition of components in a mixture
• Chromatography, spectrophotometry and mass spec
  are used by forensic scientists to identify or
  compare organic materials
• Chromatography is a means of separating and
  tentatively identifying the components of a
  mixture.
• Spectrophotometry is the study of the absorption
  of light by chemical substances
• Mass spectrometry characterizes by observing a
  substances fragmentation patterns after
  collision with high energy electrons

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Summary 2

• GC separates components of a mixture on the basis
  of their distribution between a moving (carrier)
  gas and a stationary phase which is a thin film
  of liquid contained in a column. The record of
  the separation is a chromatogram
• A direct connection between GC and MS allows
  components to flow into the MS (GC-MS).
  Fragmentation of each component produces a
  fingerprint pattern of the substance.
• HPLC separates compounds in a stationary phase
  and mobile liquid phase with temp sensitive
  compounds like explosives
• TLC uses a solid stationary phase and mobile
  liquid phase
• Electrophoresis uses electric potential to
  separate proteins and DNA of different size and
  charge on a gel-coated plate or polymer filled
  capillary
• Most labs use UV and IR spec to characterize
  chemical compounds. UV spec produces simple vs.
  IR complex spectra and distinctive spectra
  providing a fingerprint of the substance

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Most Abundant Elements

• 75 of the earths crust is compose of 2
  elements Oxygen and Silicon
• 99 made up of only 10 elements with carbon
  comprising less than 0.1!
• Expect non-carbon containing elements to be
  present in physical evidence- e.g. iron, steel,
  copper, aluminum- tools, coins, weapons, metal
  scrapings
• Examples include- inorganic chemicals such as
  pigments in paints and dyes and in explosives or
  poisons such as mercury, lead or arsenic

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Identification vs. Comparison Review

• Identification of inorganic evidence
• Examples Explosive formulation suspected of
  containing potassium chlorate or a powder
  suspected to contain arsenic
• Complete the tests ? results identical to tests
  previously recorded for knowns to be a valid
  conclusion as to the chemical identity of
  evidence
• Comparison to ascertain common origin-
• Example Brass pipe found on the suspect compared
  to a broken pipe at a crime scene
• Condition of the pipes may not allow fitting of
  broken edges
• Pipes are alike because they are brass (alloy of
  copper and zinc) but hundreds of thousands of
  brass pipes known to exist.
• Distinguishing these pipes requires comparison
  using chemical analyses on trace elements
  providing meaningful criterion to increase
  probability the two pipes originated from the
  same source

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Dirt is Good! Or Trace with trace elements!

• Raw materials originate from earths crust
• Purification is not 100 and cannot exclude all
  minor impurities
• Manufactured products and natural materials
  contain small quantities of elements in trace
  amounts (lt 1)
• Trace elements provide additional points of
  comparison
• See Table 6.2 for Brass example
• Soil, fibers, glass and metallic objects- Kennedy

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Brass Pipe Trace Elements
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Evidence in the Kennedy Assassination

• Did Lee Harvey Oswald act alone?
• Warren Commission concluded he was alone assassin
• Oswald fired 3 shots from behind in the Texas
  School Book Depository
• President hit by 2 bullets, 1 missing the limo
• 1 bullet hit the president in the back, exited
  his throat and then struck Governor Connelly then
  exited his chest, struck his right wrist and then
  lodged in his left thigh. Bullet later found in
  the governors stretcher
• Second bullet in the skull fatally wounded Kennedy

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Evidence in the Kennedy Assassination

• In the Texas book repository room, a 6.5mm
  Mannlicher Carcano military rifle was found with
  Oswalds palm print and3 spent 6.5mm Western
  Cartridge Co. Mannlicher-Carcano (WCC/MC)
  cartridge cases
• Oswald seen there in the am
• Critics of the Warren commission cite
• eyewitness accounts and acoustical data
  contending someone else fired from a region in
  front of the limo
• One bullet caused both president and Connelly's
  back wound? If so the bullet would be mutilated
  and deformed. Instead no deformity some
  flattening and only 1 weight loss

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Evidence in the Kennedy Assassination

• 1977 US House of Representative Select Committee
  on Assassinations requested the bullets and
  bullet fragments recovered from the car and
  various wound areas be examined for trace element
  levels.
• Lead alloys are used in manufacture of bullets.
  Antimony added to lead as a hardening agent
  copper, bismuth and silver commonly found.
  Antimony and Silver were compared Previous
  studies showed these have probative value for
  WCC/MC bullets. Ranges of antimony 20-1200ppm
  and AG 5-15 ppm

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Evidence in the Kennedy Assassination

• Results indicate Q1 and Q9 (bullet from
  Connelly's stretcher and Connelly's wrist) were
  indistinguishable
• Q2, Q4,5 and Q14, Large fragment from the car,
  fragments from Kennedys brain and small
  fragments found in the car were also
  indistinguishable.

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Evidence in the Kennedy Assassination

• Conclusions derived
• There is evidence of only two bullets- one of
  composition of 815 ppm antimony and 9.3 silver,
  the other of composition 622 ppm antimony and 8.1
  ppm silver
• Both bullets have a composition highly consistent
  with WCC/MC bullet lead although other sources
  cannot entirely be ruled out
• Bullet from Connelly stretcher also damaged
  Connelly's wrist. Absence of bullet fragments
  from the back wounds of Kennedy and Connelly
  prevented any effort at linking these wounds to
  the stretcher bullet
• None of these can totally verify the Warren
  Commissions reconstruction but results are
  consistent
• Analysis was conducted by Neutron Activation
  analysis

35
Emission Spectrum of Elements

• Elements selectively absorb and emit light
• Techniques used to determine elemental
  composition of materials
• Emission spectroscopy
• Inductively Coupled Plasma Emission Spectrometry
  and
• Atomic Absorption Spectrophotometry

36
Emission Spectra

• Light emitted from a bulb or any other light
  source is passed through a prism, separating it
  into component colors or frequencies Emission
  Spectrum- the resulting display of colors
• Example- sunlight passing through a prism
  yielding rainbow colors. This is called a
  continuous spectrum as all colors merge or blend
  into one another to form a continuous band

37
Continuous vs. Line Spectrum

• Unlike white light from the sun ? continuous
  spectrum, other light sources such as sodium,
  neon or mercury arc lamps when passed through a
  prism result in several individual colored lines
  separated by dark spaces. Each line represents a
  definite wavelength or frequency of light called
  a line spectrum

38
Continuous vs. Line Spectrum

• Solid or liquid heated ? continuous spectrum-not
  very indicative of composition
• Vaporized and excited by high temp each element ?
  light of select frequencies characteristic of
  this element
• Line spectra produced are in essence a
  fingerprint of an element and a practical method
  of identification.

39
Emission Spectrometer

• Main components
• Vaporizes and excites atoms to emit light
• Separate light into component frequencies
• Record resultant spectra

40
Emission spectra of evidence

• Contains numerous elements hence numerous lines
• Identification by comparison to a standard chart
  showing position of principal spectral lines of
  all elements
• More commonly in forensic analysis is the simple
  comparison of two or more specimen line-by-line

41
Inductively Coupled Plasma Emission Spectrometry
(ICP)

• Identifies and measures elements through light
  energy emitted by excited atoms
• Inductively Coupled Plasma is caused by a chain
  reaction of colliding electrons
• high voltage spark releases electrons from argon
  gas
• Acceleration in magnetic field more collisions
  and more release
• Discharge sustained by RF energy

42
Inductively Coupled Plasma Emission Spectrometry
(ICP) is Hot, very hot

• ICP discharge acts like a very intense continuous
  flame-7000-10,000 oC
• Sample introduced into hot plasma collides with
  argon electrons ? charged particles (ions) emit
  light of characteristic wavelengths corresponding
  to identity of elements
• Applications- mutilated bullets and glass
  fragments.
• Bullets not suitable for comparison to test fired
  bullets.
• copper, arsenic, silver, antimony, bismuth,
  cadmium and tin
• Class characteristic as currently no way (no
  database) of providing statistical significance
• Accepted in NJ Supreme Court State v Noel.
  1999

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Atomic Absorption Spectrophotometry

• When atoms are vaporized they absorb the same
  frequencies of light that are emitted when
  excited.
• First the sample is partially vaporized
  (acetylene flame) leaving a substantial number
  unexcited.
• Second it is exposed to radiation source
• This source, the discharge lamp is chosen to emit
  only frequencies of light putatively present in
  the emission spectrum of the element in question

• For example if one wanted to determine the
  presence
• of antimony, the discharge lamp would be
  constructed
• with antimony. The sample will absorb light only
  when it contains antimony

44
Atomic Absorption Spectrophotometry

• Application is the accurate determination of an
  elements concentration in a sample
• Concentration of absorbing element will be
  directly proportional to the quantity of light
  absorbed.
• Sensitive to trace levels
• Limitation is that only one element at a time can
  be measured
• Modification by substituting heated graphic
  furnace or heated strip of metal (tantalum) for
  the flame ? more efficient volatilization
  resulting in 1 trillionth of a gram sensitivity!
• How does this work at the atomic level?

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Fundamentals of the Atom

• Subatomic particles proton, electron and
  neutron- basic structural units of the atom

46
Fundamentals of the Atom

• Electrons (-) orbiting around a central nucleus
  analogous to the solar system where the planets
  revolve around the sun
• Nucleus contains protons () and neutrons
  (neutral)
• Atoms have no net electrical charge therefore
  protons electrons

47
Atomic structure of elements

• Behavior of elements is related to the
  differences in the atomic structure
• Each element contains a different number of
  protons Atomic number
• The periodic table represent the atomic number
  number(s) of protons
• Element is a collection of atoms all having the
  same number of protons.

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The Periodic Table
49
Atomic structure of elements

• Electrons move around the nucleus confined to a
  path of flight electron orbital
• Each orbital is associated with a definite amount
  of energy energy level
• Each element has its own characteristic energy
  levels located at varying distances from the
  nucleus- some are full, some empty

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Excitation at the Atomic Level

• Atoms in stable states have electrons positioned
  in their lowest possible orbitals
• When an atom absorbs energy or light its
  electrons are pushed into higher energy orbitals
  excited state
• Because energy levels have fixed values only
  definite amounts can be absorbed

51
Excitation at the Atomic LevelAtomic Absorption
Spectrophotometry

• Elements are selective in the frequencies of
  light they absorb
• Selectivity is determined by the electron energy
  levels in each element
• Atomic Absorption Spectrophotometer, a photon of
  light will interact with an electron causing it
  to jump into a higher orbital
• Energy must correspond to the energy difference
  between the two orbitals
• Ehf Eenergy difference, h-frequency of absorbed
  light and h Planks constant
• Any energy value more or less will not affect the
  transition
• Like playing pool too little force you wont
  make it in, too much it might bounce out!

52
Emission at the Atomic LevelEnergy is a two way
street

• Electrons will not remain in high energy state
  and quickly fall back to its original energy
  level
• As it falls back it releases energy
• Emission spectrum - energy loss comes about in
  the form of light emission
• Each element has its own unique set of energy
  levels each emits a unique set of frequencies
• Emission spectrum is a picture of the energy
  levels surrounding the nucleus of each element

• Atomic Absorption spectroscopy measures the
  value and amount of light energy going into the
  atom
• Emission spectroscopy collects and measures the
  various light energies given off.
• Either method- Atom are identified by the
  existence of characteristic energy levels

53
Neutron Activation Analysis

• Changing the number of subatomic particles?
  nuclear energy
• New tool for identifying and quantitating
  elements
• Atoms of single elements must have protons
  electrons. Not so with neutrons
• Total number of protons and neutrons atomic
  mass
• Isotopes are atoms having the same no. of protons
  but different numbers of neutrons

54
Neutron Activation Analysis

• Most elements have many isotopes. Some are
  stable others are not and decompose with time by
  radioactive decay
• Radioactivity is the emission of radiation
  accompanying decay of unstable nuclei
• Alpha helium atoms minus electrons
• Beta electrons
• Gamma high energy form of electromagnetic
  radiation emitted by a radioactive element

55
Neutron Activation Analysis

• Neutron Activation Analysis is the technique of
  bombarding specimen with neutrons and measuring
  the resultant gamma-ray radioactivity.
• Scientists create radioactive isotopes by
  bombarding atoms with neutrons
• When a neutron is captured by the nucleus of an
  atom a new isotope is formed? activated and many
  decompose by emitting radioactivity
• To identify the activated isotope one measures
  the gamma irradiation. Gamma rays of each
  element is associated with characteristic energy
  values. Once identified the amount can be
  measured by the intensity of the gamma ray
  radiation

56
Neutron Activation Analysis

• Advantage non destructive method for
  identifying and quantifying trace elements
• Sensitive to one-billionth of a gram (1ng)
• Multiplex capable- simultaneously analyze 20-30
  elements
• Limitation is cost
• Metals, drugs, paint, soil, gunpowder residue and
  hair
• Example from NAA comparison of stolen copper
  telegraphic wires 4 wires at scene of theft
  compared to B seized at a scrap yard and
  suspected of being stolenA1 and B matched

57
X ray Diffraction

• ES, AA and NAA reveal presence of elements not
  how combined into compounds
• Focusing a beam of X-rays at a crystal and
  studying how the atoms in the substance interact
  is called X ray diffraction
• 95 of all inorganic compounds are crystalline in
  nature
• Limitation- lack of sensitivity- fails to detect
  those present at 5 level in mixtures (ES, AA and
  NAA more sensitive)

58
Summary 1

• Inorganic substances are present in tools,
  explosives, poisons and metals as well as in
  paints and dyes. Trace elements are useful
  because they provide invisible markers that may
  be used to establish source of material or for
  additional points of comparison
• Emission spectroscopy, inductively coupled plasma
  and atomic absorption spectrophotometry are
  techniques used by forensic scientists to
  determine elemental composition of materials
• In ES a sample is vaporized and atoms achieve an
  excited state. Excited atoms emit light
  separated into its components in a line spectrum.
  Each element can be identified by its
  characteristic line frequencies.
• In ISP, the sample in the form of an aerosol is
  introduced in a hot plasma creating charged
  particle that emit light of characteristic
  wavelengths corresponding to identity of the
  elements

59
Summary 2

• In AAS, the sample is partially vaporized
  (acetylene flame). Second it is exposed to
  radiation source This source, the discharge lamp
  is chosen to emit only frequencies of light
  putatively present in the emission spectrum of
  the element in question. Finally if the element
  is present a portion of the light will be
  absorbed. Many elements can be detected at the
  level of one-trillionth of a gram.
• NAA measures the gamma-ray frequencies of
  specimens that have been bombarded with neutrons.
  Highly sensitive and non-destructive method for
  simultaneously identifying and quantifying 20-30
  trace elements. However, it requires a nuclear
  reactor and is expensive.
• X-ray diffraction is used to study crystalline
  materials. As X-rays penetrate crystals a
  portion of the beam is reflected and the
  reflected beams from the crystals planes combine
  to form a series of light and dark bands known as
  a diffraction pattern. Each compound is known to
  produce its own unique diffraction pattern giving
  a means for fingerprinting inorganic compounds.
  This works on organic compounds too (e.g. DNA)