USP carbon dioxide facility

I know that there are a lot of carbon dioxide facilities throughout the united states. However, there are only a handful of facilities that are considered certified to sell USP grade carbon dioxide. I know of a c02 facility that tests their product very stringently, and their product is more pure then what is considered USP. This company manufactures their CO2 as a byproduct off of their ammonia process. What would this particular company have to do to become authorized to sell USP grade Carbon Dioxide?