Three argon isotopes are measured: Ar is determined by comparison to it.
Variations in this data may point to errors anywhere in the process, which is why all the steps of preparation are recorded in detail.
The rock sample to be dated must be chosen very carefully.
Any alteration or fracturing means that the potassium or the argon or both have been disturbed.
The learning curve has been long and is far from over today.
With each increment in quality, more subtle sources of error have been found and taken into account.
Then the gas sample is cleaned of all unwanted gasses such as H, nitrogen and so on until all that remains are the inert gasses, argon among them.