The reasoning is as follows: the atmosphere does not only contain Ar as being atmospheric argon.However, this only works if all the excess argon did indeed come from the atmosphere.The potassium-argon (K-Ar) isotopic dating method is especially useful for determining the age of lavas.
The selected size fraction is cleaned in ultrasound and acid baths, then gently oven-dried.
Because Ar has a very short half-life, it is guaranteed to be absent in the sample beforehand, so it's a clean indicator of the potassium content.
The advantage is that all the information needed for dating the sample comes from the same argon measurement. This method is commonly called "argon-argon dating." The Ar-Ar method is considered superior, but some of its problems are avoided in the older K-Ar method.
The severity of this problem decreases as the accuracy of our instruments increases.
Still, as a general rule, the proportional error in K-Ar dating will be greatest in the youngest rocks.