The Girdler sulfide (GS) process, also known as the GeibSpevack (GS) process, is an industrial production method for filtering out of natural water the heavy water (deuterium oxide = DO) which is used in particle research, in Deuterium NMR spectroscopy, deuterated solvents for proton NMR spectroscopy, in heavy water nuclear reactors (as a coolant and moderator) and in deuterated drugs.
Deuterium Oxide cannot get past the GS process.
We are told that GS process is a method for filtering out Deuterium oxide, but aren't told at what percent it can remove it.  Is it 100% removal, or does some get through after filtering?  It's not mentioned. 