Radiometric dating rock samples
So accurate determinations require very pure samples, very accurate and selective detectors, or both.
The true age of a sample is self-explanatory, but unless the material dates from historic times, the true age is rarely known.
Argon, an inert gas, is not chemically bonded to neighboring atoms at all, and even minor thermal disturbance allows them to escape.Crystallization of a mineral is a good way to close a system. Any disturbance of the system effectively resets the clock to zero by allowing decay products to escape or reshuffling the abundances of elements.