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Techniques for Stable Isotope Analysis

Isotopes are atoms of the same element that have different mass numbers. This means that while the number of protons and electrons remain constant, the number of neutrons in the nuclei varies. This doesn't change the element, but does give it more or less mass. Isotopes can be stable or radioactive, and most people are familiar with radioactive decay, which uses the ratios of decayed radioactive elements to determine age. Stable isotopes don't decay. Rather, they are captured in tissues and ice cores during formation.

Common Stable Isotopes
- The isotopes of foraminifera species can be used to determine geographic region.
  
  There are a number of different elements that display stable isotope ratios. Depending on the element, different ratios indicate different environmental factors affecting a region or organism. Oxygen isotopes in shells, bones and teeth can indicate geographic region by comparison to water sources. The stable isotopes of carbon and nitrogen are indicative of paleodiets. The main food sources of animals and people can be determined by the relative abundances of 15N and 13C. Sulfur isotopes can identify whether organisms were benthic (sea-floor dwelling) or pelagic (floating in the water column).
Mass Spectrometry
- Molesules can be separated into their respective isotopes through the use of a mass spectrometer.
  
  A mass spectrometer, or MS, measures the mass-to-charge ratio of a sample or molecule. A sample is vaporized and then ionized (typically by bombardment of an electron beam or by other methods). Ionization forms charged particles which are then separated by their mass-to-charge ratio through the use of an electromagnetic field. The ions are then detected and processed into mass spectra, providing a numerical ratio. This ratio is compared to a standard, such as the Vienna Standard Mean Ocean Water, or VSMOW, which is the standard for oxygen isotope comparison.
Uses of Mass Spectrometry Data
- A compound's composition and structure can be determined using a mass spectrometer.
  
  The results provided by an MS analysis can provide several results for use in a scientific study. This type of analysis can determine the isotopic composition of a molecule or element. It also helps to identify unknown compounds, when there is doubt regarding what elements make up the composition. This process can also identify the structure of molecules. The structure of a molecule is important when determining geochemistry of a mineral or gemstone or any geological sample.
Data Analysis
- Plants are producers and will display a different isotopic ratio than a consumer that eats plants.
  
  Once a ratio of any particular stable isotope is obtained, the value can be substituted into a formula that will then give the ratio a qualitative value, using parts per million.
  
  δX=[(R sample/R standard) - 1] * 1000 permil
  
  The X represents the final numerical value of how many parts per million of a particular isotope are contained within the sample. The R is the isotopic ratio of the sample compared to the isotopic ration for the standard. For instance, this could be a formula to determine the amount of 18O in a sample, using the ratio of 18O to 16O. The final outcome of the equation can be applied to understand what types of food were being digested, what trophic level an organism lived on, whether is was a producer, such as a plant, or a consumer, such as a cow, and so on.

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