"On August 20, 1942, a trace quantity of this element was isolated and measured for the first time. About 50 micrograms of plutonium-239 combined with uranium and fission products was produced and only about 1 microgram was isolated."

So small were the amounts of early produced plutonium, than new methods to study it had to be used.

https://www.scientificamerican.com/article/ultramicrochemist...

Of course we have now military plutonium production reactors.