Atom Percent (Atom %): the absolute number of atoms of a given isotope in 100 atoms of total element. For example, the nitrogen-15 content of air nitrogen is 0.3663 Atom %. For calculation, At% = [R_s / (R_s + 1) * 100] where R_s is the ratio of the light isotope to the heavy isotope of the sample.

Continuous-Flow (CF): refers to an automated preparation device and mass spectrometer in which sample analysis is conducted in a continuous stream of helium carrier gas.

Delta Units (d): are expressed in molecules per thousand, or “per mil”. For example, d¹⁵N_Air = 12 per mil means that the sample was analysed against a reference material and found to be 12 molecules per thousand more than Air - the accepted zero point for expression of nitrogen-15 in per mil notation. For calculation, d = [ (R_s / R_r) - 1 ] * 1000 where R_s is the ratio of the heavy isotope to the light isotope of the sample and R_r is the ratio of the heavy isotope to the light isotope of the reference.

Dissolved Inorganic Carbon (DIC): in both sea and fresh water DIC occurs as four species: carbon dioxide (CO₂); carbonic acid (H₂CO₃); bicarbonate (HCO₃) and carbonate (CO₃).

Doubly Labelled Water: water which has been enriched in both deuterium and oxygen-18 which is used to determine energy expenditure in free living humans or animals.

Dual Inlet (DI): an inlet method in which a pure gas sample is admitted into an isotope ratio mass spectrometer (IRMS) by a variable volume bellows. A reference gas is admitted into the IRMS via a second variable volume bellows. The bellows are balanced to provide sample and reference signal responses of equal intensity.

Elemental Analyser (EA): an automated sample preparation instrument in which samples are automatically converted into pure gases for isotope ratio analysis. An elemental analyzer contains the following elements: (i) furnace for combustion, reduction or pyrolysis of sample material; (ii) chemical traps for analyte gas purification; (iii) gas chromatography for time separation of these analyte gases.

Equilibration: a sample preparation technique whereby a aqueous sample in a sealed container is allowed to exchange and equilibrate molecules and isotopes with a headspace gas.

Fractionation: the enrichment or depletion of a stable isotope caused by natural or artificial processes.

ISCIRA (Internal Standard Carbon Isotope Ratio Analysis): method used in isotope analysis of honey for adulteration with cane sugar or corn syrup.

Isotopes: can be defined as atoms whose nuclei contain the same number of protons but a differing number of neutrons.

Isotope Ratio Mass Spectrometry (IRMS): a mass spectrometer is an instrument for separation of molecules based upon their mass-to-charge ratio. In IRMS the mass spectrometer used separates isotopes of different mass within a magnetic field and precisely measures the ratio of two, or more, isotopes.

Isotope Ratio: the ratio of the minor isotope over the major isotope. For example, nitrogen in air contains 0.3663 Atom % nitrogen-15 and 99.6337 Atom % nitrogen-14, giving an isotope ratio of 0.3663 / 99.6337 = 0.003676466).

Natural Abundance: the concentration of isotopes as found in nature.

Per Mil (‰): see Delta Units

Pyrolysis: a high temperature means of converting sample material to pure gas in the absence of oxygen.

SCIRA (Standard Carbon Isotope Ratio Analysis): method used in isotope analysis of honey for adulteration with cane sugar or corn syrup.

Stable Isotope: a non-radioactive isotope in which the number of protons and neutrons in the atomic nucleus is constant through time. Stable isotopes pose no known physiological risk.

Vienna-Canyon Diablo Troilite (V-CDT): a FeS meteorite used as the accepted zero point for expression of sulfur-34 in delta units. V-CDT is thought to have a sulfur-34 value close to the universal mean.

Vienna-Pee Dee Belemnite (V-PDB): a belemnite from the Cretaceous Pee Dee formation of South Carolina, US which is used as the accepted zero point standard for expression of carbon and oxygen isotope abundance in delta units.

Vienna-Standard Mean Ocean Water (V-SMOW): the accepted zero point standard for expression of hydrogen and oxygen isotopes of water samples in delta units.