At any given moment all living plants and animals have approximately the same percentage of carbon-14 in their bodies.
When a plant or animal dies it stops bringing in new carbon-14.
Archaeologists have long used carbon-14 dating (also known as radiocarbon dating) to estimate the age of certain objects.
Traditional radiocarbon dating is applied to organic remains between 500 and 50,000 years old and exploits the fact that trace amounts of radioactive carbon are found in the natural environment.
Nitrogen normally occurs in a seven proton, seven nuetron, nitrogen-14 state.
When it collides with an energetic neutron it becomes carbon-14, with six protons and eight neutrons and gives off a hydrogen atom with one proton and zero neutrons. Carbon-14 is an isotope of carbon, which exists only is small amounts in the environment (1 in one trillion carbon atoms is carbon-14).
Therefore, the amount of carbon-14 in an artifact decreases at a predictable rate while the amount of carbon-12 remains constant.
These collisions create secondary cosmic rays in the form of energentic neutrons.
When these neutrons collide with nitrogen-14 in the atmosphere carbon-14 can be created.
Imagine your sample contained 20% of the carbon-14 found in a living plant leaf.
Then, First, carbon-14 cannot be used to date biological artifacts of organisms that did not get thier carbon dioxide from the air.
The ratio of carbon-14 to carbon-12 in the atmosphere and on earth is nearly constant, although there has been some change in carbon-14 concentration over the last 10,000 years.