abundance of some stable isotopes

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abundance of some stable isotopes

• Isotope relative abundance ()
• 1H 99.985
• 2H 0.015
• 12C 98.90
• 13C 1.10
• 14N 99.63
• 15N 0.37
• 16O 99.76
• 17O 0.04
• 18O 0.2
• 19F 100
• 32S 95.02
• 33S 0.75
• 34S 4.21
• 36S 0.02
• 35Cl 75.77
• 37Cl 24.23
• 79Br 50.69
• 81Br 49.31

Isotope relative abundance () 24Mg 78.70
25Mg 10.13 26Mg 11.17 28Si 92.21
29Si 4.70 30Si 3.09 39K
93.10 40K 0.0118 41K 6.88 40Ca
96.97 42Ca 0.64 43Ca 0.145
44Ca 2.06 46Ca 0.0033 48Ca
0.18 54Fe 5.82 55Fe 91.66 56Fe
2.19 56Fe 0.33
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Why isotope measurements?

• Small differences in nuclear masses do not change
  general behavior

Physical and chemical effects cause slight
differences in the isotope distribution in Nature
Small variations in isotope distribution are
fingerprints for those effects which are often
preserved
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Thermodynamic equilibrium

• Equilibrium reaction AX BX ? AX BX
• Equilibrium constant

Q partition function Qtot Qtr Qrot Qvib
Qel Main component vibrations
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Isotope fractionation zero point energy
Density of states (transition states)
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d-notation
Units
Isotope ratio
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international standards for isotope measurements

• 13C VPDB (Vienna PeeDeeBelemnite)
• 18O VPDB-CO2
• VSMOW (Vienna Standard Mean Ocean Water)
• 17O VSMOW
• 2H VSMOW
• 15N atmospheric nitrogen

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Fractionation factor (constant)

• Equilibrium reaction
• Kinetic reaction
• a fractionation factor
• R ratio heavy light isotope
• k1, k2 rate constants of light/heavy isotope
• Fractionation constant e (a -
  1) 1000

Note Today often inverse definition!!!
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Rayleigh-fractionation
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Isotope scale conversion

• Scale conversion
• c Sample
• b lab ref
• a international std

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CH4-lines, TDLAS
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Ion optics
Up to 8 collectors
Electron impact ionization (70 eV) Small source
slit, wide collectors, Ion currents 10-9-10-8 A
High stability Relative measurment (versus
reference compound)
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Peak center, CO2 isotopes
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Isotope distribution in CO2
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Calculate isotope rations from isotopomer ratios
(CO2)

• measure 45R, 46R, 2 signatures

The 3 unknowns 13R, 17R and 18R cannot be
determined from two equations, need additional
relation
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Mass independent fractionation
Mass dependent fractionation
d17O ? 0.52 d18O
Now define
D17 O d17O 0.52 d18O
Excess 17O-enrichment Mass independent
fractionation
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Calculate isotope rations from isotopomer
ratios(O2)

• measure 33R, 34R, 2 signatures

The 2 unknowns 17R and 18R can be determined
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Isotope effects in water
H2O, H218O
less
Difference is temperature dependent
difference

1
more
H2O, H218O
Verdunstung von Wasser
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The isotope thermometer
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climate of the past 400.000 years
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Isotope measurements on atmospheric trace gases

• Trace gases from different sources often carry a
  characteristic isotope signature

Two examples Carbon monoxide (CO) from
industrial processes in enriched in 18O Nitrous
oxide (N2O) produced in soils is depleted in 15N
und 18O

• Fractionations are associated with sink processes

• Information about global (isotope) budgets

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Mixing of two reservoirs

• b background mixing ratio
• t total mixing ratio
• (?13C)s ? value of local source
• (?13C)b ? value of background
• ?13C measured ? value
• Isotope balance
• or
• As t? ?, i.e. at infinite contamination from a
  local source we obtain the isotopic composition
  of that source
• ? plot ? vs. 1/t

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CH4 and isotope observations in Russia
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CH4 source signatures
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CO at high northern latitudes(Spitzbergen, Alert)
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Annual cycle in CO concentration
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Isotope information
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Summer-Minima
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Photosynthesis
12CO2 is assimilated faster than 13CO2
13CO2 is depleted in the plant by about 2
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?13C of CO2 in the atmosphere
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natural 14C
Units ?14C 1000 ((Ra Rstd)/Rstd 1)
pmC percent modern Carbon 100 pmC corresponds
to 95 of the activity of an oxalic acid standard
in 1950
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bomb 14C
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Sunspots