Isotope mass spectrometry
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Isotope ratio mass spectrometry is a specialist field of mass spectrometry, concerned with measuring the relative abundance of atomic species, rather than their mass.
A mass spectrometer is an instrument for analysing the mass of atomic or molecular particules. In the most general terms the instrument operates by ionising the sample of interest and subjecting the resultant ions to a physical process that separates them and their fragments into a spectrum by mass and charge.
Instruments have been developed based on several techniques for mass separation and tuned to a wide range of applications. This article describes one of these application areas, instruments adapted specifically to measuring the relative abundance of masses up to around mass number 44.
This field is of interest because the variation in mass between isotopes in this range is large enough to give rise to variation in chemical, physical and biological behaviour. This leads to measurable effects on the isotopic composition of samples due to their biological or physical history.
As a specific example, the hydrogen isotope deuterium (heavy hydrogen) is almost double the mass of the common hydrogen isotope. Water molecules containing the common hydrogen isotope (and the common oxygen isotope, mass 16) have a mass of 18. Water incorporating a deuterium atom has a mass of 19, over 5% heavier. The energy to vaporise the heavy water molecule is higher than that to vaporise the normal water so isotope fractionation occurs during the process of evaporation.
Thus a sample of sea water (standard mean ocean water VSMOW) will exhibit a quite detectable isotopic ratio difference when compared to arctic snowfall (standard light arctic precipitation SLAP).
[edit] 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 it's 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.
