Fractionation dating method
There are two situations that limit an age; the first is that the measured Fm is smaller than that of the corresponding process blank measured in the same suite of samples on the AMS.If this is the case, then the reported age will be quoted as an age greater than the age of the process blank. The typical background age for organic combustions is 48,000 years and for inorganic carbon samples, 52,000 years.One other situation that limits the age (if not already limited by the background age) is the error of the AMS result.If twice the reported error of the Fraction Modern (let's call this 2sigma) is larger than the sample Fraction Modern, then a limiting age is reported.After acceleration and removal of electrons, the emerging positive ions are magnetically separated by mass and the C counts per second are collected.It is expected then, for a 5,570 year (1 half-life) or 11,140 year old (2 half-lives) sample that 125 or 63 counts per second would be obtained.
In order to remove the effects of isotopic fractionation, the Fraction Modern is then corrected to the value it would have if its original δC value to which all radiocarbon measurements are normalized.) The fractionation correction is done using the 13/12 ratio measured by the AMS system.
Fractionation is the term used to describe the differential uptake of one isotope with respect to another.
While the three carbon isotopes are chemically indistinguishable, lighter C, reflecting the difference in mass.
This correction is performed as follows: $$Fm_ = Fm_ ( Fm_ - Fm_b)\frac$$ Where \(M\) is sample mass, and \(M_b\) and \(Fm_b\) are the mass and Fm of the blank.
Fraction Modern is a measurement of the deviation of the C is also affected by natural isotopic fractionation.