Applications
The variation outlined above has applications in hydrology since most samples will lie between these two extremes. Given a sample of water from an aquifer, and a sufficiently sensitive tool to measure the variation in the isotopic ratio of hydrogen in the sample, it is possible to infer the source, be it ocean water seeping into the aquifer or precipitation seeping into the aquifer, and even to estimate the proportions from each source.
Another application is in paleotemperature measurement for Paleoclimatology. For example one technique is based on the variation in isotopic fractionation of oxygen by biological systems with temperature.
Species of foraminifera incorporate oxygen as calcium carbonate in their shells. The ratio of the oxygen isotopes oxygen 16 and oxygen 18 incorporated into the calcium carbonate varies with temperature and the oxygen isotopic composition of the water. This oxygen remains "fixed" in the calcium carbonate when the forminifera dies, falls to the sea bed, and its shell becomes part of the sediment. It is possible to select standard species of forminifera from sections through the sediment column, and by mapping the variation in oxygen isotopic ratio, deduce the temperature that the forminifera encountered during life if changes in the oxygen isotopic composition of the water can be constrained.
In forensic science, research suggests that the variation in certain isotope ratios in drugs derived from plant sources (cannabis, cocaine) can be used to determine the drug's continent of origin.
Read more about this topic: Isotope-ratio Mass Spectrometry