I only have information from a question that was answered in the NBA forum.
The information which I pasted below states not to use the NI after the hyphen in chemical isotopes.
This question was asked March 25, 2009.
Subject: ELI and NI in chemical isotopes
Question: When brailling isotopes in chemistry such as U-235, Ba-142, Kr-91, etc., are the ELI and NI used?
Does it make a difference if the text is narrative or showing a reaction equation?
Answer: (Dorothy asked a Chemical Code Expert and then answered)
Here is the answer I got: the Chem Code says that the symbol for an element is not an abbreviation by rule, so ... no ELI with a single element symbol and no NI with the number following the hyphen. I know it looks really weird, but that's the way it is.