The amount of carbon 14 in the atmosphere today (about .0000765%), is assumed there would be the same amount found in living plants or animals since the plants breath CO2 and animals eat plants. Since sunlight causes the formation of C-14 in the atmosphere, and normal radioactive decay takes it out, there must be a point where the formation rate and the decay rate equalizes. Let me illustrate: If you were trying to fill a barrel with water but there were holes drilled up the side of the barrel, as you filled the barrel it would begin leaking out the holes.
At some point you would be putting it in and it would be leaking out at the same rate.
Water samples should be stored in inert plastic bottles with tightly sealed screw tops and a negligible air gap.
Samples are normally taken within the laboratory where we have all the necessary facilities.
Extensive laboratory testing has shown that about half of the C-14 molecules will decay in 5730 years. After another 5730 years half of the remaining C-14 will decay leaving only ¼ of the original C-14. In theory it would never totally disappear, but after about 5 half lives the difference is not measurable with any degree of accuracy.
This is why most people say carbon dating is only good for objects less than 40,000 years old.
This energy converts about 21 pounds of nitrogen into radioactive carbon 14.
The current estimated turnaround time for commercial samples submitted now is: Samples should be well protected in packaging that is not likely to contaminate the samples.
Plastic bags are normally fine but it is best to use those designed for use with food as these are likely to have lower plasticiser levels, and to wrap the samples in them in aluminium foil.
For small samples, glass tubes with screw on tops are preferable - use aluminium foil under the lids to prevent the samples sticking to the lids, which are usually plastic.
For inorganic materials, such as rocks containing the radioactive isotope rubidium, the amount of the isotope in the object is compared to the amount of the isotope's decay products (in this case strontium).
The object's approximate age can then be figured out using the known rate of decay of the isotope.