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Isotopes and Forensic Science

University of Lincoln presentation

http://creativecommons.org/licenses/by-nc-sa/2.0/uk/

http://chemistryfm.blogs.lincoln.ac.uk/


Overview

1. The forensic perspective2. The atom3. Radioactive and stable isotopes –

properties and measurement4. Dating techniques5. Stable isotope applications6. Nuclear forensic science – atomic

detectives



1. The Forensic Perspective



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”



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”



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



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



2. The Atom



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



Henri Becquerel

Henri BecquerelNobel Prize

(Physics) 1903

Discovered Radioactivity in 1896

Placed pitchblende on a photographic plate and observed….

UU



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



Types of radioactive decay: alpha, α(Z > 83)

Th23190

Daughter

nucleus

Parent nucleus

Alpha particle(Helium nucleus)U235

9224

2α



Types of radioactive decay: beta, β- (N/Z too

large)Converting N to P

Ca4020

Daughter

nucleus

Parent nucleus

Beta particle(Negatron)

K4019

01-



Types of radioactive decay: beta, β+

(N/Z too small)Converting N to P

Mo9542

Daughter

nucleus

Parent nucleus

Beta particle(Positron)

Tc9543

01



Types of radioactive decay: gamma, γ (excited nucleus)

Co6027

Parent nucleus

Daughter nucleus

Gamma rays

Ni6028



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



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



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



3. Isotopes



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



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


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



4. Dating Techniques



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



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



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



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 ----



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



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



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



• Piltdown Man - a fake!

• Turin Shroud - mediaeval (1260 to 1390AD)

• Zoroastrian Mummy – post A-bomb era

Carbon Dating – the Forensic Application



5. Stable Isotopes



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.



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



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.



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



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



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



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.



Nakamura et al (1992) Biomed Mass Spectrom, 8, 390.



Travelling German Business Men

USA

Japan

δ13C

-18

-18

-20

-20

Weeks32 41

Nakamura et al Biomed Mass Spectrom, 8, 390.



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



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



6. Nuclear Forensic Science – the Atomic

Detectives



Trafficking Nuclear Materials


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


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


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


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.


Acknowledgements

• JISC• HEA• Centre for Educational Research and

Development• School of natural and applied sciences• School of Journalism• SirenFM• http://tango.freedesktop.org

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