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Isotopes and Forensic Science
University of Lincoln presentation
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Overview
1. The forensic perspective2. The atom3. Radioactive and stable isotopes –
properties and measurement4. Dating techniques5. Stable isotope applications6. Nuclear forensic science – atomic
detectives
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1. The Forensic Perspective
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Prof Edmond Locard (1877-1966)
“Wherever he steps, whatever he touches, whatever he leaves, even unconsciously, will serve as a silent witness against him. Not only his fingerprints or foot prints, but his hair, the fibres from his clothes, the glass he breaks, the tool mark he leaves, the paint he scratches, the blood or semen he deposits or collects. All of these and MORE, bear mute witness against him”
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The Locard Principle of Exchange:
“When objects come into contact there is a transfer of particles”…….
This is a Principle that Scientists have struggled to exploit
But Improvements in technology have brought new concerns…
“Physical evidence cannot be wrong……..only interpretation can err”
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Trace Analysis
• Modern analytical techniques can now identify compounds containing <1ng (1 x 10-
9g) of substance– Increasing potential for CONTAMINATION and
MIS-INTERPRETATION
Recent high profile cases where:• Evidence was mishandled• Amount of evidence caused concern• Results may have been misinterpreted
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Drug Contaminated Banknotes:
•Spanish Euros average 335μg cocaine per note!
•£15M of drug- contaminated notes are destroyed in UK each year
• Cotton / linen matrix of banknotes is ideal for trapping crystals
• 99% of London banknotes are contaminated with Cocaine
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2. The Atom
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Example: Neon-20• Positively charged nucleus consisting of protons (Z) and neutrons (N)
• Electrons (e) occupy distinct energy levels around the nucleus
•Atomic Mass = Z + N
•Atomic Number = Z
•For Neon-20, we have Z = N = 10, written as:
Nucleus
Electron
Ne2010
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Henri Becquerel
Henri BecquerelNobel Prize
(Physics) 1903
Discovered Radioactivity in 1896
Placed pitchblende on a photographic plate and observed….
UU
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Marie and Pierre Curie• 1896 discovery of
Radium and Polonium
• 1903 PhD and Nobel Prize for Physics
• 1903 isolation of Radium
• 1908 Nobel Prize for Chemistry
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Types of radioactive decay: alpha, α(Z > 83)
Th23190
Daughter
nucleus
Parent nucleus
Alpha particle(Helium nucleus)U235
9224
2α
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Types of radioactive decay: beta, β- (N/Z too
large)Converting N to P
Ca4020
Daughter
nucleus
Parent nucleus
Beta particle(Negatron)
K4019
01-
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Types of radioactive decay: beta, β+
(N/Z too small)Converting N to P
Mo9542
Daughter
nucleus
Parent nucleus
Beta particle(Positron)
Tc9543
01
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Types of radioactive decay: gamma, γ (excited nucleus)
Co6027
Parent nucleus
Daughter nucleus
Gamma rays
Ni6028
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Radioactive Decay
Proportion of parent atoms remaining
0
20
40
60
80
100
120
0 1 2 3 4 5 6
Time
% P
ropo
rtio
n of
par
ent a
tom
s re
mai
ning
Growth/Decay curves for radioactive elements
Proportion of daughter atoms remaining
0
20
40
60
80
100
120
0 1 2 3 4 5 6
Time
% P
ropo
rtio
n of
dau
ghte
r at
oms
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Half-Life
Parent Daughter / Radiation
Half-Life (Y)
3H 3He + 0β-1 12.314C 14N + 0β-1 5730
87Rb 87Sr + 0β-1 49 X 109
210Pb 206Pb + 4α+2 20.4
238U 234Th + 4α+2 + γ 4.5 X 109
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Element Uranium- 238 series Th- 232 series U-235 series
Uranium U-238
4.5*109 y
U-234
245500 y
U-235
7.0*108 y
Proactinium Pa-234
1.2 min
Pa-231
32800 y
Thorium Th-234
24.1 d
Th-230 75400 y
Th-232
1.4*1010 y
Th-228
1.91 y
Th-231
25.5 h
Th-227 18.7d
Actinium Ac-228
6.1 h
Ac-227 21.8 y
Radium Ra-226 1600 y
Ra-228
5.75 y
Ra-224 3.7 d
Ra-223
11.4d
Francium
Radon Rn-222
3.8 d
Astatine
Polonium Po-218 3.1 min
Po-214
0.00014 s
Po-210
138 d
Bismuth Bi-214
19.9 min
Bi-210
5.0 d
Lead Pb-214 26.8 min
Pb-210 22.3 y
Pb-206
stable
Pb-208 stable
Pb-207 stable
α-decayZ: -2N: -4
β-decayZ: +1N: +/-0
Decay series of short lived nuclides
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3. Isotopes
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Frederick Soddy
Evidence:• 3 decay series end in Lead• Inability to separate
‘elements’ in the 3 decay series• Atomic Mass values not
always integers (e.g. Ne = 20.2)
1913 Soddy proposed existence of ISOTOPES
Definition: Atoms of the same elements with different Atomic Mass
Frederick Soddy
Nobel Prize (Chemistry) 1921
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Detection of Isotopes
Ne2010
Existence of ISOTOPES confirmed by Aston using the first mass spectrometer in 1919.
Analysis of Neon gas achieved separation of 3 stable isotopes:
90.9% 0.3% 8.8%
Ne2210Ne21
10
Ion Source
Detection system
Faraday cups
Magnetic sector
Isotope Ratio MS
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70 80 90 100
Number of protons, Z
Num
ber o
f neu
trons
, N
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Chart of the Nuclides
• A nuclide = an isotope
• Narrow band of stability
• For stability, N/P ratio rises with mass
• All nuclides outside the band and with Z > 83 are radioactive
• Elements can have from 0 – 10 stable Isotopes
β-
emission
β+ emission
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4. Dating Techniques
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The Clocks in the Rocks
In the beginning was the BIG BANG
Solar system / earth condensed from dust and gas
Radioactive elements decay to daughters (Parent : Daughter ratio changes with time)
With half-life can calculate the age of the earth
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Dating with Isotopes
Process:
• Select suitable isotope
• Clearly the 14C - 14N system is useless ….. beyond 40,000 years
• The 87Rb – 87Sr system is fine for 60M to 400G years!
Parent C-14
Daughter N-14
Parent Rb-87
Daughter Sr-87
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Process:• Ideally the crystals in
the rock should contain no Sr (eg: Mica in Granite)
• All subsequent Sr arises from Rb decay
• Earliest rocks on Earth are 4G years old
• On melt, the clock is reset when new igneous rock is formed
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Strontium Dating – the Forensic Application
• Sr has 4 stable isotopes• As a result of 87Rb
decay, 87Sr levels will rise with time
• 87Sr / 86Sr is highest in oldest rock
• Sr has similar chemistry to Calcium and ends up in BONE
88Sr 7%
87Sr 10%
86Sr 82%
84Sr 1%
87Sr / 86Sr ratios:
0.703 in young rock
0.750 in oldest rock
0.715 ----
0.730 ----
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Strontium Dating – Tracing Adam
• Sept 2001, torso of 5 yr old boy recovered from Thames
• Suspected ‘muti’ style ritual killing
• Gut contents suggested poisoning and UK as place of murder
• Bone 87Sr/86Sr was high – Pre-Cambrian levels
• Police search for relatives in rural area between Benin City and Ibadan, Nigeria…arrest made in Dec 2003
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Carbon Dating• 14C half life = 5730 yrs• Excellent for dating
organic material from 0-40,000 yrs*
Why is there any 14C left, & how does it work?
• Assume 14C is continually being produced at the same rate.
• Whilst alive, 14C levels remain constant. Only on death does the level start to drop
• However, calibration is needed to get accurate dates
Nitrogen-14 Carbon-14
Cosmic radiation
Neutron capture
14C is absorbed along with 12C and 13C into the tissue of living organisms in a fairly constant ratio
Soil
Beta decayCarbon-14 Nitrogen-14
When an organism dies 14C converts back to 14N by beta decay
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Carbon-14 and Tree Rings
• Bristlecone Pine allows calibration back 7000yrs …. …..HOW?
• Calibration shows that 14C production is variable…... WHY?
• Variation in Solar flux• Decreases caused by burning fossil fuels• Increases caused by A-bombs
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• Piltdown Man - a fake!
• Turin Shroud - mediaeval (1260 to 1390AD)
• Zoroastrian Mummy – post A-bomb era
Carbon Dating – the Forensic Application
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5. Stable Isotopes
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Lead in Human Teeth
• Pb has 4 stable isotopes
• Pb isotope ratios in our bodies reflect that of the environment:
–Enamel - Pb content set in
childhood
–Dentine - 1% of Pb
exchanges per annum
–Jaw bone - 10% exchanges
per annum
Isotope Origin
208Pb 232Th
207Pb 235U
206Pb 238U
204Pb Big Bang
Ratio 206Pb/204Pb in geology:
Varies according to origin / age …..
Data from Australian Citizens –Tooth Enamel
Origin 206Pb/204Pb
Australia 16.56
CIS 17.98
Balkans 18.23
UK 16.92
Gulson et al J. Forensic Science 42, 787-791.
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Isotope Fractionation• The Chemistry of
elements is largely determined by its electronic structure
• But differences in mass give rise to kinetic and equilibrium effects
Stable Carbon Isotopes:
12C – 98.89%
13C - 1.11%
Light isotope:
- forms weaker bonds
- is more reactive
- as CO2 will diffuse faster
RESULT:
In photosynthesis some plants discriminate AGAINST 13C
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Fractionation via Photosynthesis
• 2 major photosynthesis pathways: C3 and C4
plants
• C3 discriminate much more than C4 plants
RESULTS:
CO2 (air) = 1.11% 13C
C4 plants = 1.10% 13C
C3 plants = 1.08% 13C
Examples of C3
Plants:
Wheat, Barley, Rice, Oats, Sugar Beet …in fact most nutritionally important plants.
Examples of C4
Plants:
Sugar Cane and Maize.
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Measuring C-13 Levels• Use Isotope Ratio
Mass Spectrometry (IRMS)
• Very Precise (+/- 0.0005% 13C)
Key Features:- Measured as CO2
- Dual Inlet- Triple Collector- Measured v. Ref Gas
Species Mass
12C16O2 44
13C16O2 45
Ion Source
Detection system
Faraday cups
Magnetic sector
Isotope Ratio MS
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Stable Isotopes – Some Forensic Applications
Food / Drink adulteration
• Malt is expensive
• Sugar is cheap!
• Both ferment to produce alcohol
• Temptation to adulterate!
Fermentation:
Sugars C2H5OH
Sugar Source δ13C
Malt (UK) -25.9
Maize -13.0
Sugar Cane -12.0
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Adulteration: Alcoholic Beverages
Malt Maize/Cane
UKGerman
Malt Blended Scotch B’bon
Cz Bootleg
Wine Bootleg Port/Brandy
Δ13C per mil v. PDB-30 -20 -10
Wines
Beer
WhiskyWhisky
Sugars
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Adulteration: Maple SyrupSame approach has
been used for honey, fruit juice, olive oil and maple syrupBy combining 2H:1H ratios with 13C:12C ratios we can achieve greater DISCRIMINATION
-30
-20
-10
δ13C
110100902H ppm
‘Maple’
Maple
Beet
Cane
Adulterated syrup shown to contain 40% Beet sugar Martin et al (1996) J Agric Food Chem, 44,
3206.
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Nakamura et al (1992) Biomed Mass Spectrom, 8, 390.
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Travelling German Business Men
USA
Japan
δ13C
-18
-18
-20
-20
Weeks32 41
Nakamura et al Biomed Mass Spectrom, 8, 390.
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Sources of Heroin
It is important to establish:
• Geographical source
• Evidence of ‘batch’ synthesis of Heroin
Morphine
Heroin
OH
O
OH
N CH3
O
O
O
N CH3
CH3
CH3
O
O
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Geographical Source of Heroin
Drug source Heroin
δ13CMorphine (H-M)
Acetyl source
Lebanon -30.9 -28.7 2.2 Lab 1
Lebanon -31.4 -28.9 2.5 Lab 1
Thailand -32.2 -30.5 1.7 Lab 2
Turkey -31.4 -29.7 1.7 Lab 2
Turkey -32.7 -29.6 3.1 Lab 3
From: Besacier et al (1997)J Forensic Science, 42, 429
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6. Nuclear Forensic Science – the Atomic
Detectives
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Trafficking Nuclear Materials
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Man-made Radioactive Isotopes
• Smuggled Plutonium – can identify the reactor type in which the fuel was originally radiated and the type of plant where the material was subsequently reprocessed
In 1997, two pieces of stainless steel contaminated with alpha-emitters were found in a scrap metal yard in Germany.
Source was identified as a fast-breeder reactor in Obninsk, Russia
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Weapons-grade Plutonium
• The isotopic composition of plutonium can indicate INTENT
In 1994, a small lead cylinder discovered in a garage in Tengen on the Swiss-German border was found to contain plutonium metal, isotopically enriched to 99.7%
Weapons-grade Pu-239
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Radioactive Fingerprints
• Preserving the conventional chain of evidence whilst dealing with radioactive samples can be problematic
For example – lifting fingerprints and swiping for radioactive contamination cannot both be carried out
The first ever radioactive fingerprint has recently been identified on an object contaminated with alpha-emitting isotopes
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Conclusions
• ‘Crime is a complex interaction of people and things at different points in time and space’ (RE Stockdale, Science Against Crime).
• Isotopes provide a powerful and new approach for investigating diverse crimes.
• Need for careful standardisation / calibration
• Enormous research and development opportunities.
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Acknowledgements
• JISC• HEA• Centre for Educational Research and
Development• School of natural and applied sciences• School of Journalism• SirenFM• http://tango.freedesktop.org