|Isotope mass||13.003355 u|
Detection by mass spectrometry
A mass spectrum of an organic compound will usually contain a small peak of one mass unit greater than the apparent molecular ion peak (M) of the whole molecule. This is known as the M+1 peak and comes from the handful of molecules that contain a 13C atom in place of a 12C. A molecule containing one carbon atom will be expected to have an M+1 peak of approximately 1.1% of the size of the M peak, as 1.1% of the molecules will have a 13C rather than a 12C. Similarly, a molecule containing two carbon atoms will be expected to have an M+1 peak of approximately 2.2% of the size of the M peak, as there is double the previous likelihood that any molecule will contain a 13C atom.
In the above, the mathematics and chemistry have been simplified, however it can be used effectively to give the number of carbon atoms for small- to medium-sized organic molecules. In the following formula the result should be rounded to the nearest integer:
C = number of C atoms X = amplitude of the M ion peak Y = amplitude of the M+1 ion peak
13C-enriched compounds are used in the research of metabolic processes by means of mass spectrometry. Such compounds are safe because they are non-radioactive. In addition, 13C is used to quantify proteins (quantitative proteomics). One important application is in "Stable isotope labeling with amino acids in cell culture" (SILAC). 13C-enriched compounds are used in medical diagnostic tests such as the urea breath test. Analysis in these tests is usually of the ratio of 13C to 12C by Isotope ratio mass spectrometry.
The ratio of 13C to 12C is slightly higher in plants employing C4 carbon fixation than in plants employing C3 carbon fixation. Because the different isotope ratios for the two kinds of plants propagate through the food chain, it is possible to determine if the principal diet of a human or other animal consists primarily of C3 plants or C4 plants by measuring the isotopic signature of their collagen and other tissues. Deliberate increase of proportion of 13C in diet is the concept of i-food, a proposed way to increase longevity.
Uses in earth science
Due to differential uptake in plants as well as marine carbonates of 13C, it is possible to use these isotopic signature in earth science. In aqueous geochemistry, by analyzing the δ13C value of surface and ground waters the source of the water can be identified. This is due to the fact that atmospheric, carbonate, and plant derived δ13C values all differ with respect to Pee Dee Belemnite (PDB) standard.
To calculate δ13C:
The same calculation can be used to distinguish between biomass produced by C3 and C4 metabolic pathways (e.g. trees versus tropical grasses), which allows for spatiotemporal reconstruction of species and ecosystems range shifts based on the isotopic characterization of soil organic matter.
Within C3 plants processes regulating changes in δ13C are well understood, particularly at the leaf level, but also during wood formation. Many recent studies combine leaf level isotopic fractionation with annual patterns of wood formation (i.e. tree ring δ13C) to quantify the impacts of climatic variations and atmospheric composition on physiological processes of individual trees and forest stands. The next phase of understanding, in terrestrial ecosystems at least, seems to be the combination of multiple isotopic proxies to decipher interactions between plants, soils and the atmosphere, and predict how changes in land use will affect climate change.
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|Carbon-13 is an
isotope of carbon
|Decay product of: