Organic materials, which require the most processing, are limited to younger ages by their corresponding process blank.
Due to counting and measurement errors for the blanks and samples, statistical errors are higher for very old samples.
However, in 1958, Hessel de Vries was able to demonstrate that the ratio had changed over time by testing wood samples of known ages and showing there was a significant deviation from the expected ratio.
This discrepancy, often called the de Vries effect, was resolved by the study of tree rings.
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.
Since it was created after the start of atomic testing, it incorporates bomb carbon, so measured activity is higher than the desired standard.However, limiting ages or "backgrounds" are also determined by process blanks which correspond to the method used to extract the carbon from the sample.» NOSAMS General Statement of C from contamination introduced during chemical preparation, collection or handling.Although one can simply measure older samples for longer times, there are practical limits to the minimum sample activity that can be measured.
At the present time, for a 1 milligram sample of graphite, this limiting age is about ten half-lives, or 60,000 years, if set only by the sample size.
Conventional radiocarbon ages have been corrected for isotope fractionation by normalizing to = -25 PDB or VPDB.