Stable Isotopes

Question 1

Delta Notation

Answer 1

• Means of expressing the relative abundance of the heavier stable isotope in a mixture of atoms
• units are per mil

d = (Rsample/Rstandard-1)*1000
R = molar ratio of heavy-to-light isotope (ex: 13C/12C)

Question 2

Founding Fathers of Isotope Ecology

Answer 2

Tom Hoering (Carnegie), Harmon Craig (Scripps), Harold Urey (U of Chicago), Sam Epstein (Caltech)

Question 3

Hutchinson’s ecological niche

Answer 3

n-dimensional set of points in a space whose axes represent environmental variables

Question 4

Electron

Answer 4

tiny subatomic particle with negative charge

Question 5

proton

Answer 5

subatomic particle with a positive charge

Question 6

Neutron

Answer 6

Subatomic particle with no charge. Similar mass to protons

Question 7

Atomic Number (Z)

Answer 7

Integer that expresses the number of protons in the nucleus

Question 8

Mass Number (A)

Answer 8

The sum of protons and neutrons in a nucleus

Question 9

Atomic Mass

Answer 9

The mass of a neutral, noncharged, atom of a nuclide

Question 10

Atomic Mass Unit (AMU)

Answer 10

A measure of atomic mass

1 AMU = 1/12 mass of 12C

Question 11

Radioactive Decay

Answer 11

Decay of an element to a different form with a more stable energy configuration

Question 12

Half-Life

Answer 12

Time required for one half of the atoms in a sample of a radioactive isotope to decay

Question 13

Isotopes

Answer 13

Atoms which have the same number of protons in their nuclei, but different numbers of neutrons

Question 14

Isotopologues

Answer 14

Isotopic species of the same molecule

Carbon dioxide has 12 isotopologues: 12C16O16O, 12C16O17O, etc

Question 15

Isotopomers

Answer 15

Isotopologues that differ from one another only in the position of the isotopic elements

Question 16

International Reference Standard for Hydrogen

Answer 16

V-SMOW, Vienna Standard Mean Ocean Water

Question 17

International Reference Standard for Oxygen

Answer 17

V-SMOW, Vienna Standard Mean Ocean Water

V-PDB, Vienna Pee Dee Belemnite

Question 18

International Reference Standard for Carbon

Answer 18

V-PDB, Vienna Pee Dee Belemnite

Question 19

International Reference Standard for Nitrogen

Answer 19

Air, Atmospheric Air

Question 20

Fractionation

Answer 20

Isotopic separation/sorting

Isotopes of the same element undergo the same chemical reactions because they have the same protons and electrons. Isotopes have different thermodynamic properties due to their different masses (melting point, vapor pressure, diffusion coefficient, and equilibrium and kinetic rate constants). Thus, different isotopes react at different rates in chemical reactions, which leads to fractionation.

Question 21

Results of fractionation

Answer 21

abundance of heavy isotopes in the reactant being different from the abundance of heavy isotopes in the product

lighter goes faster

Question 22

Isotopic fractionation factor (alpha)

Answer 22

alpha = (1000 +delta of reactant)/(1000+delta of product)

Question 23

Exchange - Equilibrium Fractionation

Answer 23

• equilibrium
• complete back-reactions occur
• product/reactant offset by constant fractionation factor
• physical state changes (water to vapor)
• chemical transformations (carbonic acid to carbon dioxide and water)

ex: liquid droplet formation from vapor in clouds, evaporation at the boundary layer (100% RH) of body of water

Question 24

Transport/Diffusive Fractionation

Answer 24

• non-equilibrium
• kinetic fractionation dealing with flux rather than chemical reaction
• incomplete back-flux is the rule

ex: diffusion through boundary layer over water, out of leaves, through soils

Question 25

Kinetic Fractionation

Answer 25

- non equilibrium - incomplete back reactions or none occur - product and reactant delta values can evolve in concert (closed system) or product composition determines reactant composition (open system) ex: biosynthesis (enzyme mediated), evaporation of water (<100% RH)

Question 26

Temperature effect on equilibrium fractionation

Answer 26

alpha approaches zero (less fractionation) at higher temperatures

Question 27

Rayleigh Distillation/Fractionation

Answer 27

Evaporation - vapor is formed from the liquid and the fraction remaining of original liquid declines Can also describe the process as vapor mass condenses (rainfall)

Question 28

Kinetic Reactions

Answer 28

Forward reaction rate is accelerated relative to backward reaction and opportunity for backward mixing diminishes. Ratio of reaction rate constants (k) determine fractionation Most biological (enzyme) reactions

Question 29

5 Water World Rules to Remember

Answer 29

1. expect fractionation during phase changes (liquid to vapor) 2. temperature will impact isotopic composition (lower temps = larger isotopic affects) 3. Isotope composition of any water is a result of fractionation, mixing, or both 4. Always anticipate that both equilibrium and kinetic effects may be at play 5. Biology and physiology can influence water isotope values too.

Question 30

Mixing

Answer 30

A process that combines different substrates into the product (the making of a homogenous whole - e.g. baking)

Question 31

Global Meteoric Water Line

Answer 31

Plots d2H against d18O slope =~8 Intercept = +10 per mil