Oxygen and Hydrogen Isotope Measurements in Plant Cellulose Analysis
Isotopes are atoms containing the same number of protons but a different number of neutrons in their nucleus. Most of us are familiar with unstable isotopes (e.g., Carbon-14) which decay to another element (e.g., Carbon-14 decays to Nitrogen-14). This chapter, however, is concerned with stable isotopes which do not decay and thus produce no radioactivity. The most abundant elements found in plant biomass have two or more stable isotopes. Carbon exists either as carbon-12 (6 protons and 6 neutrons) or carbon-13 (6 protons and 7 neutrons), hydrogen can be found either as deuterium (1 proton and 1 neutron) or protium (1 proton), and oxygen can be found either as oxygen-16 (8 protons and 8 neutrons), oxygen-17 (8 protons and 9 neutrons), or oxygen-18 (8 protons and 10 neutrons). Isotopes having the least number of neutrons are usually called the lighter isotopes, and those having more neutrons are called the heavier isotopes. The natural abundance of the isotope of those elements with the least mass (lower number of neutrons) is greater than those with the larger mass. Thus carbon in the biosphere is comprised of 98.89% carbon 12 and only 1.11% of carbon-13 (Nier and Gulbransen 1939). Hydrogen is comprised of 99.9844% protium and 0.0156% deuterium, and oxygen is comprised of 99.763% oxygen-16, 0.0375% oxygen-17 and 0.1995% oxygen-18 (Hoefs 1973). It is extremely difficult to measure absolute abundance of stable isotopes, thus they are usually measured and expressed relative to a standard.
KeywordsStable Isotope Isotope Ratio Hydrogen Isotope Carbon Isotope Ratio Cupric Oxide
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