Purity is a function of relativity, the very relativity we must instill in all manmade systems.
First, what is pure? Pure sugar is what? I think you are asking for absolutisms that do not exist in life, for our world and everything in/on it is an ever-expanding fractal of relativity.
Second, capitalism is not a system but a symptom omnipresent in many manmade systems. Countries that promote socialism or even communism use capitalism, the exchange of money for goods and services rendered, as the basis for assigning trust in merit. Conversely, societies we describe as capitalistic depend on a fair amount of socialism to protect their oligarchies.
Third, the purest form of capitalism is the one subjugated to a dynamic meritocracy derived from a relativity theory of freedom. A type of capitalism we do not have today for we have failed to comply with fundamental principles that respect the freedom to each his own and establishes imperative paradoxical rules to freedom. And since our freedom is currently oligarchically controlled, so is our current distribution of capital, unable to trace and exploit the full capacity of human ingenuity.
Our implementation of capitalism must change (see the above chart) following the rules of nature to explore the full capacity of a product of nature: people.
A change I would consider to be a better, and more importantly, a constantly improving proxy of pure capitalism.