After doing some research on gold purity, I found that the current record of gold fineness is at an astounding 999.999 fine achieved by the Perth Mint, Australia 1957. However, I could not find any other attempts to break this record.
I have also read the other related answers like this one: Obtaining 100% pure metal
I was hoping that perhaps some insight could be given as to how the current fineness record was achieved, and what refinement or fineness verification barriers may need to be overcome to achieve finer purities.