Specific samples may vary significantly from the expected isotopic composition due to real deviations from earth's average isotopic abundances.

However, helium is unusual in that its isotopic abundance varies greatly depending on its origin.

It also led to explanations for the isotopic abundances of present-day planetary atmospheres.

Moreover, even more stringent constraints, placed by study of certain isotopic abundances in the Oklo natural nuclear fission reactor, seem to indicate no variation is present.

There is a known variation in the isotopic abundance of chlorine-37.

Another application is in radiochemistry, where this may refer to isotopic ratios or isotopic abundances.

The year-to-year variation in tree-ring widths and isotopic abundances gives clues to the prevailing climate at that time.

Unlike most elements, helium's isotopic abundance varies greatly by origin, due to the different formation processes.

Such effects could account for shifts in the isotopic abundances of hydrogen and nitrogen in carbonaceous chondrites.

These differences in isotopic abundance are used as "labels" to identify the different sources of CO2 found in an atmospheric CO2 sample.