|Stable Isotope Principles|
Isotope: an isotope is an atom whose nuclei contain the same number of protons but a different number of neutrons. Isotopes are broken into two specific types: stable and radioactive. There are over 300 known naturally occurring stable isotopes. The Stable Isotope Geosciences Facility focuses on the measurement of naturally occurring light element stable isotopes of nitrogen, carbon, hydrogen and oxygen.
Isotopic Abundances: light elements contain different proportions of at least two isotopes. Usually one isotope is the predominantly abundant isotope. For example, the average natural abundance of 14N is 99.64%, while the average abundance for 15N is 0.36%. Knowing these abundances helps the researcher determine if a sample is enriched or depleted in a specific isotope once the data is in hand. The table below outlines the average isotopic abundances of elements that are most commonly measured for stable isotope measurements in this facility.
Natural Isotopic Abundances of some light stable isotopes
Heavy isotopes undergo all of the same chemical reactions as light isotopes, but, simply because they are heavier, they do it ever so slightly slower. These tiny differences in reaction rates cause the products of reactions to have different isotope ratios than the source materials. Knowing the precise isotope ratios in plant and animal tissues allows us to know about the processes by which the materials were formed. This can tell if a plant's roots are tapping recent rain or deep groundwater, the water-use efficiency of whole forests, what an animal has eaten throughout its life and where it sits on the food chain, and the global sources and sinks for carbon dioxide in the atmosphere. Historical materials, including those that may be many thousands of years old, can be analyzed in the same manner, allowing us to compare modern and ancient environments.
Isotopic Fractionation: Isotopic fractionation causes stable isotopic abundance variations. Fractionation is caused by the differences in the chemical and physical properties of a certain atomic mass and concerns the concepts of isotope exchange and kinetic processes in reaction rates. Changes in temperature are just an example of an isotope exchange process that can cause fractionation in an isotopic ratio. This is why temperature stability is a priority in many instrumentation facilities. Gas pressure can also have a significant role in determining the magnitude of fractionation effects. Some examples of a kinetic isotope processes are evaporation and condensation, diffusion, and dissociation reactions.
Delta value: Understanding the processes that may affect the isotopic relationship in a specific sample type is an important step toward understanding how isotopic delta values (d) are calculated. An average difference in isotopic composition between the sample and the reference gas is determined using this equation:
[(Rsample-Rstandard)/(Rstandard)] x 1000 = δsample-standard
Rsample is the ratio of the heavy isotope to the light isotope in the sample
Rstandard is the ratio of the heavy isotope to light isotope in the working reference gas which is calibrated against an internaionally known IAEA or NBS standard
δsample-standard is the difference in isotopic composition of the sample relative to that of the reference, expressed in per mil (‰)
Rstandard absolute ratio values for international standards
Primary Reference Scales
V-SMOW (Standard Mean Ocean Water) - used for δ2H and δ18O isotope measurement. The standard is an average of different ocean samples from around the world.
SLAP (Standard Light Antarctic Precipitation) - used for δ2H and δ18O isotope measurement. The standard is an average of different ocean samples from around the world.
V-PDB (Pee Dee Belemnite) - used for δ13C measurement. The standard is a CaCO3 from a belemnite from the Pee Dee formation in South Carolina.
Atmospheric Air - used for δ15N measurement. The air has a very homogeneous isotopic composition making this an ideal reference.
V-CDT (Canyon Diablo Troilite) - used for δ34S measurement.
Levels of Stable Isotope Ratio Specificity
Now that you understand what stable isotopes are, how fractionation can affect isotope ratios, and how stable isotopes are measured, it is important to realize that there are several levels of specificity at which different materials can be measured. There are currently 3 levels of specificity for isotope analysis (bulk, compound, and position specific). The example shown below illustrates how a plant sample can be analyzed down to each of these levels for different purposes depending on the research being done.
International Reference Material Calibration
Typically, samples are measured along with several internal standards as a check on internal instrument precision. These internal standards are usually a matrix standard (which means that we analyze the sample along with a similar matrix standard: plant samples are analyzed with plant standards, animal tissues are measured with animal tissue standards, water samples are measured with water standards, carbonate samples are measured with carbonate standards, etc). Before analyzing samples along with internal standards they need to be calibrated for accuracy. For this calibration we use international reference materials. These reference materials are provided by the International Atomic Energy Agency (IAEA), National Institute of Standards and Technology (NIST) and the United States Geological Survey (USGS) and are typically in limited quantities. These reference materials are, in turn, calibrated against the above discussed primary reference scales. In some cases the samples are measured along with the international reference materials, rather than an internal standard. Samples measured in a dual inlet system are measured against an isotopically known reference gas. The MAT 253, for instance, has a known CO2 reference gas for carbonate analysis. Samples are measured against this gas and steps are taken to watch for any slight changes in calibration.
IAEA Stable Isotope Reference Materials
2H and 18O in water samples
Materials with known 2H, 13C, 15N and 18O isotopic composition
Materials with known 13C, 18O, and 7Li isotopic composition