Stable isotopes time considerations

Considerations in Stable-Isotope Experiments. Isotopic tracer additions should label the specific pool of interest quickly, before chemical or biological transformations of no interest distribute the tracer throughout the ecosystem. For calculation of transfers of material from one pool to another the isotope must be well mixed within the ecosystem in the compound it is supposed to trace, and the isotopic content between both pools should be given time to reach equilibrium. 

For a long time" , it has been known that there is rearrangement of 1-alkynes to 2-alkynes under basic conditions (for instance by alcoholic potassium hydroxide or powdered potassium hydroxide) at 175 C. It has been shown that in fact an equilibrium mixture is obtained . It contains isomeric compounds, mainly the starting 1-alkyne and isomeric 1,2-alkadiene and 2-aIkyne, but the latter, more stable from thermodynamic considerations, is predominant. Therefore the method could be used to prepare 2-alkynes from corresponding 1-alkynes. The reverse reaction is possible disubstituted acetylenes can be converted to sodium derivatives of 1-alkynes by sodium or sodamide . 1-Alkynes are recovered by hydrolysis. We have tried to apply both reactions to prepare acetylenes deuterated in defined positions with high isotopic purity. 

Isotope ratio mass spectrometers (IRMS) are specialized mass spectrometers that produce precise and accurate measurements of variations in the natural isotopic abundance of hght stable isotopes. The breakthrough in classical IRMS was the introduction of the dual inlet mass spectrometer by Urey in 1948 [26]. Since this time, the instrument has been considerably developed and automated, leading to the systems commercially available today. 

In the case of radioactive materials contained in living organisms, an additional consideration is made for the reduction in observed activity due to regular processes of elimination of the respective chemical or biochemical substance from the organism. This introduces a rate constant called the biological half-life (Tbioi) which is the time required for biological processes to eliminate one-half of the activity. This time is virtually the same for both stable and radioactive isotopes of any given element. 

The abundances of radioactive isotopes over time in the galaxy can be modeled based on the above considerations. With an approximately constant production rate, the abundance of a stable nuclide will grow and will be proportional to the time over which it has been produced. In contrast, the abundance of a radionuclide will reach a steady state between production and decay in about eight mean lifetimes. (We will use mean life (t) instead of... 

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