Lecture 5 Stable Isotopes

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Lecture 5 Stable Isotopes
Isotopes of Elements Chart of the Nuclides Delta
Notation Isotope Fractionation
Equilibrium Kinetic Raleigh
See E H Chpt. 5
When the universe was formed 15 billion years ago
(the Big Bang) light elements of H (99), He
(1) and trace amounts of Li were
formed. Subsequent reactions during star
formation created the remaining elements,
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Isotopes of Elements The chemical characteristic
of an element is determined by the number of
protons in its nucleus. Atomic Number
Protons define the chemistry Different
elements can have different numbers of neutrons
and thus atomic weights (the sum of protons plus
neutrons). Atomic Weight protons neutrons
referred to as isotopes
There are 92 naturally occurring elements Some
are stable some are radioactive
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The chart of the nuclides (protons versus
neutrons) for elements 1 (Hydrogen) through 12
(Magnesium).
Valley of Stability
Most elements have more than one stable isotope.
b decay
11 line
X
X
a decay
Number of neutrons tends to be greater than the
number of protons
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Full Chart of the Nuclides
11 line
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Examples for H, C, N and O Atomic Protons Neut
rons Abundance Weight (Atomic Number)
(approximate) Hydrogen H 1P
0N 99.99 D 1P
1N 0.01 Carbon 12C 6P 6N 98.89 13C
6P 7N 1.11 14C 6P
8N 10-10 Nitrogen 14N 7P 7N 99.6 15N
7P 8N 0.4 Oxygen 16O 8P
8N 99.76 17O 8P 9N 0.024 18O 8P
10N 0.20
?1/2 5730 yr
All Isotopes of a given element have the same
chemical properties, yet there are small
differences due to the fact that heavier isotopes
typically form stronger bonds and diffuse
slightly slower Abundance is for the average
Earths crust, ocean and atmosphere
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Mass Spectrometer Basic Schematics
1. Input as gases 2. Gases Ionized 3. Gases
accelerated 4. Gases Bent by magnetic
field 5. Gases detected
Gases ionized
Gases accelerated
high vacuum
Magnetic field deflects ion beam
Detectors
Isotopes are measured as ratios of two isotopes
by various kinds of detectors. Standards are run
frequently to correct for instrument stability
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Nomenclature Report Stable Isotope Abundance as
ratio to Most Abundant Isotope (e.g.
13C/12C) -Why? The Ratio of Isotopes is What is
Measured Using a Mass Spectrometer The Ratio
Can Be Measured Very Precisely. The isotope
ratio of a sample is reported relative to a
standard using d (del) notation usually
with units of because the differences are
typically small.
Define H heavy L light
d (in ) (Rsample - Rstandard) / R
standard x 1000 or R / Rstd
if d is in
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Example d13C (in o) (13C/12C)sample /
(13C/12C) standard 1 x 1000
Example If (13C/12C) sample 1.020 (13C/12C) std
d 13C 1.020 (13C/12C) std / (13C/12C) std -
1 x 1000
0.020 x 1000 20 o
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Standards Vary
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Isotopic Fractionation
The state of unequal stable isotope composition
within different materials linked by a reaction
or process is called isotope fractionation
Fractionation Factor ? ?A-B
RA / RB where R ratio of two isotopes in
materials A or B often ? Rproducts /
Rreactants
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• Two kinds of Isotope Fractionation Processes
• Equilibrium Isotope effects
• Equilibrium isotope fractionation is the partial
  separation of isotopes between two
• or more substances in chemical equilibrium.
• Usually applies to inorganic species. Usually not
  in organic compounds
• Due to slightly different free energies for atoms
  of different atomic weight
• Vibrational energy is the source of the
  fractionation. Equilibrium fractionation results
  from the reduction in vibrational energy when a
  more massive isotope is substituted for a less
  massive one. This leads to higher concentrations
  of the heavier isotope in substances where the
  vibrational energy is most sensitive to isotope
  fractionation (e.g., those with the highest bond
  force constants)
• If molecules are able to spontaneous exchange
  isotopes they will exhibit slightly
• different isotope abundances at thermodynamic
  equilibrium (their lowest energy state)

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For example exchange reactions between light
Al, Bl and heavy Ah, Bh aA1 bBh ?
aAh bB1 The heavier isotope winds up in the
compound in which it is bound more
strongly. Heavier isotopes form stronger bonds
(e.g. think of like springs). If a 1 the
isotopes are distributed evenly between the
phases. Example equilibrium fractionation of
oxygen isotopes in liquid water (l) relative to
water vapor (g). H216O(l) H218O(g) ?
H218O(l) H216O(g) At 20ºC, the equilibrium
fractionation factor for this reaction is a
(18O/16O)l / 18O/16O)g 1.0098
Q
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Example The carbonate buffer system involving
gaseous CO2(g), aqueous CO2 (aq), aqueous
bicarbonate HCO3- and carbonate CO32-. An
important system that can exhibit equilibrium
isotope effects for both carbon and oxygen
isotopes
13CO2(g) H12CO3- ? 12CO2(g)
H13CO3- The heavier isotope (13C) is
preferentially concentrated in the chemical
compound with the strongest bonds. In this case
13C will be concentrated in HCO3- as opposed to
CO2(g). For this reaction ? has the form ?H/L
(H/L)product / (H/L)reactants (H13CO3- /
H12CO3-) / (13CO2 / 12CO2) ?H/L 1.0092 at 0ºC
and 1.0068 at 30ºC
Q
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Example Estimation of temperature in ancient
ocean environments CaCO3(s) H218O ? CaC18OO2
H2O The exchange of 18O between CaCO3 and
H2O The distribution is Temperature dependent
last interglacial
last glacial
Holocene
d18O of planktonic and benthic foraminifera from
piston core V28-238 (160ºE 1ºN) Planktonic and
Benthic differ due to differences in water
temperature where they grow.
Assumptions 1. Organism ppted CaCO3 in isotopic
equilibrium with dissolved CO32- 2. The d18O
of the original water is known 3. The d18O of the
shell has remained unchanged
Planktonic forams measure sea surface T Benthic
forams measure benthic T
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d18O in CaCO3 varies with Temperature
from lab experiments
E H Fig 5.3
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Complication Changes in ice volume also
influence d18O More ice, thus higher salinity
more d18O left in the ocean
d18O increases with salinity
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2. Kinetic Fractionation Non-equilibrium during
irreversible reactions like photosynthesis Occurs
when the rate of chemical reaction is sensitive
to atomic mass Results from either differential
rates of bond breaking or diffusion
rates Compounds move at different rates due to
unequal masses. Light are always faster. For
kinetic fractionation, the breaking of the
chemical bonds is the rate limiting step.
Essentially all isotopic effects involved with
formation / destruction of organic matter are
kinetic There is always a preferential
enrichment for the lighter isotope in the
products. 12CO2 mw 44 These must
have the same kinetic energy (Ek 1/2mv2) 13CO2
mw 45 so 12CO2 travels 12 faster
than 13CO2. All isotope effects involving
organic matter are kinetic Example 12CO2 H2O
12CH2O O2 faster 13CO2 H2O 13CH2O
O2 slower Thus organic matter gets enriched
in 12C during photosynthesis (d13C becomes
negative)
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Carbon Carbon has only two stable isotopes with
the following natural abundances 12C
98.89 13C 1.11
Below are some typical d13C values on the PDB
scale in . Standard (CaCO3 PDB)
0 Atmospheric CO2 -8 (was -6, getting
lighter due to new CO2) Ocean SCO2 2
(surface) 0 (deep) Plankton CaCO3 0
(same as seawater) Plankton organic carbon
-20 Trees -26 Atmospheric CH4 -47 Coal and
Oil -26
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d13C in different reservoirs
E H Fig. 5.6
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d13C of atmospheric CO2 versus time
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Raleigh Fractionation - A combination of both
equilibrium and kinetic isotope effects Kinetic
when water molecules evaporate from sea
surface Equilibrium effect when water molecules
condense from vapor to liquid form
Any isotope reaction carried out so that
products are isolated immediately from the
reactants will show a characteristic trend in
isotopic composition.
Example Evaporation Condensation
Processes d18O in cloud vapor and condensate
(rain) plotted versus the fraction of remaining
vapor for a Raleigh process. The isotopic
composition of the residual vapor is a function
of the fractionation factor between vapor and
water droplets. The drops are rich in 18O. The
vapor is progressively depleted.
Where Rvapor / R liquid f (?-1)
where f fraction of residual vapor
? Rl/Rv
Fractionation increases with decreasing
temperature
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Distillation of meteoric water large kinetic
fractionation occurs between ocean and vapor.
Then rain forming in clouds is in equilibrium
with vapor and is heavier that the vapor. Vapor
becomes progressively lighter. dD and d18O get
lower with distance from source.
Water evaporation is a kinetic effect. Vapor is
lighter than liquid. At 20ºC the difference is 9
(see Raleigh plot). The BP of H218O is higher
than for H216O
Air masses transported to higher latitudes where
it is cooler. water lost due to rain raindrops
are rich in 18O relative to cloud. Cloud gets
lighter
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d18O variation with time in Camp Century ice
core. d18O was lower in Greenland snow during
last ice age
Effect of temperature Effect of ocean salinity
15,000 years ago d18O -40 10,000 to present
d18O -29 Reflects 1. d18O of
precipitation 2. History of airmass cumulative
depletion of d18O
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d13C in important geological materials
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Influence of carbon source and kinetic
fractionation on the average isotopic composition
of marine and terrestrial plants.
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Vertical profiles of SCO2, d13C in DIC, O2 and
d18O in O2
North Atlantic data
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d18O in average rain versus temperature
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Meteoric Water Line
linear correlation between dD and d18O in waters
of meteoric origin
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Spatial distribution of deuterium excess in the US
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