This nomenclature has recently been changed to VPDB (Coplen, 1994).
In summary, then, isotopic fractionation refers to the fluctuation in the carbon isotope ratios as a result of natural biochemical processes as a function of their atomic mass (Taylor, 1987).
The extent of isotopic fractionation on the 14C/12C ratio which radiocarbon daters are seeking to measure accurately, is approximately double that for the measured 13C/12C ratio.
If isotopic fractionation occurs in natural processes, a correction can be made by measuring the ratio of the isotope 13C to the isotope 12C in the sample being dated.
The ratio is measured using an ordinary mass spectrometer.
solid to gas, acetylene to benzene) then no fractionation will occur.
The delta C13 value for a sample can yield important information regarding the environment from which the sample comes, because the isotope value of the sample reflects the isotopic composition of the immediate environment.
In the case of shellfish for example, marine shells possess a d C13 value of between -1 and 4 per mille, whereas river shells possess a value of between -8 and -12 per mille, therefore, in a case where the precise environment of the shell is not known, it is possible to determine the most likely by analysis of the d C13 result.
One of the most commonly referenced reservoir effects concerns the ocean.
The average difference between a radiocarbon date of a terrestrial sample such as a tree, and a shell from the marine environment is about 400 radiocarbon years (see Stuiver and Braziunas, 1993).