Glossary of Terms & Symbols

a - the Greek symbol for alpha, used to symbolize the fractionation factor. See fractionation.

b - the Greek symbol for beta.

d - the Greek lower case symbol for delta, used for light isotopes to designate per mil (‰) deviation from a standard.

D - the Greek upper case symbol for delta, used as the measurement of the difference between a product and a reactant.

d - the symbol used for deuterium excess, defined as the intercept of the meteoric water line (MWL): dD = 8 d18O + d

D - used as an abbreviation for deuterium (2H). dD is the variation of 1H and 2H relative to SMOW.

e - the Greek symbol for epsilon. Used as the per 104 deviation of sample from a standard. Also used in fractionation studies to symbolize enrichment.

e - electron

l - the Greek symbol for lambda, the symbol used to express the decay constant (rate of radioactive decay) of a nuclide.

n - neutron

p - proton

abundance - the amount of an isotope of an element that exists in nature, usually expressed in relative terms as a percentage of the total amount of all isotopes of the element.

alpha decay - a type of radioactive decay in which an a particle (two protons and two neutrons) is emitted from the nucleus of an atom.

alpha particles - positively charged particles that consist of two protons and two neutrons (i.e., the nucleus of a helium atom), that may be emitted during radioactive decay.

amu - atomic mass units; see atomic mass.

anthropogenic - resulting from human activities. When applied to isotopes, the term usually refers to nuclides that are the product of the processing of nuclear fuels, reactor accidents, and nuclear weapons testing.

atom - a particle of matter consisting of a positively charged nucleus surrounded by a system of electrons, that cannot be reduced by chemical reaction

atomic mass - the mass of a nucleus in atomic mass units (amu), based on a standard of 12.00000 for . Syn. atomic weight.

atomic number - number of protons in the nucleus of an atom; the number determines the element's position on the periodic table. All isotopes of an element have the same atomic number

atomic weight - See atomic mass.

beta decay - type of radioactive decay in which an electron or positron is emitted from the nucleus of an atom. The emission of an electron results in an increase by 1 of the atomic number; emission of a positron results in the atomic number decreasing by 1.

beta particles - high-speed electrons or positrons, especially those emitted in radioactive decay.

cosmogenic - resulting from cosmic ray activity in the upper atmosphere. In reference to isotopes, this refers to those produced through spallation or from the shattering of a nucleus by cosmic rays. See also lithogenic.

daughter - the product isotope of radioactive decay; the parent and daughter atoms are of two different elements.

decay constant - the rate of radioactive decay of a nuclide, expressed by l (chance of decay/time).

delta - the lowercase delta symbol, d, is used for ‰ (per mil) deviation from a standard; whereas uppercase delta, D, is used as the measurement of the difference between a product and a reactant.

electron capture - a radioactive decay process in which an electron is spontaneously incorporated into a nucleus; the atomic number of the atom decreases by 1.

electron - an atomic particle that is negatively charged and has a mass that is .00055 the mass of a proton.

epsilon - the epsilon symbol, e, expresses per 104 deviation of sample from a standard; it is also used in fractionation studies to symbolize enrichment.

equilibrium fractionation - fractionation that occurs between two different phases that are in equilibrium, e.g., the transformation of water vapor to liquid precipitation. Although the process is in equilibrium, the rate of exchange for each isotope is different so that the result is an enrichment of one of the isotopes.

fractionation - During isotopic fractionation, heavy and light isotopes partition differently between two compounds or phases. Isotope fractionation occurs because the bond energy of each isotope is slightly different, with heavier isotopes having stronger bonds and slower reaction rates. The difference in bonding energy and reaction rates are proportional to the mass difference between isotopes. Thus, light elements are more likely to exhibit isotopic fractionation than heavy isotopes.

GMWL - global meteoric water line (GMWL), an equation defined by Harmon Craig that correlates the average relationship between 18O and 2H in meteoric waters throughout the world as follows: d2H = 8 d18O +10‰
See meteoric water line.

half-life - the time required for one-half of a given number of a radionuclide to decay (= t1/2).

 where l is the decay constant

 is the decay equation used to determine half life.

kinetic fractionation - fractionation that is unidirectional, where equilibrium is not attained. This type of fractionation applies to evaporation of surface waters and to most biogeochemical reactions, where the light isotope is faster reacting and becomes concentrated in the products.

lithogenic - refers to isotopes produced at the surface of the earth by direct cosmic ray irradiation of atoms in solid geologic materials.

mass number - total mass of an atom; the sum of neutrons and protons.

mass spectrometry - an analytical method used to identify chemical substances by ionizing the material, focusing the resulting ions into a beam, then separating them according to the ratios of their mass to their net electric charge

meteoric water line (MWL) - the relationship between 18O and 2H in meteoric water. Local meteoric water lines are calculated for given areas; variations from this model are due to isotope effects of kinetic fractionation.

neutron - atomic particles that have no charge and have a mass number of 1.

noble gases - the chemically inert elements of the helium group of the periodic table. The noble gas family consists of six different elements, five of which have stable isotopes (helium, neon, argon, krypton, and xenon), and one that has only radioactive isotopes (radon).

nuclide -a specific isotope of an atom

proton - an atomic particle that is positively charged and has a mass number of 1.

radioactive decay - the spontaneous emission of charged particles from the nucleus of an unstable atom in order to produce a stable nucleus.

radioactive isotope/nuclide - an isotope that undergoes radioactive decay.

radiogenic - formed by nuclear processes such as neutron capture or spallation.

radionuclide - a radioactive isotope; an isotope that undergoes radioactive decay.

redox - oxidation-reduction; a class of reversible chemical reactions in which one compound is oxidized and the other is reduced.

SMOW - the standard mean of ocean water; a standard used to compare oxygen and hydrogen isotopic compositions of water in parts per thousand deviation from the mean value of 18O/16O and 2H/H in seawater.

spallation reaction - the splitting of atoms in the upper atmosphere through the bombardment of cosmic rays. This results in the formation of new, often unstable, isotopes. Spallation reactions occur in the upper atmosphere when cosmic rays hit gas molecules and also at the surface of the earth by direct cosmic ray irradiation of atoms in solid geological materials.

stable isotopes - isotopes not subject to radioactive decay, whose mass does not change over time.