Couldn't exactly get your question. Food grade silicon has been widely used for WFI application (found no problems during audits). You can very well go for EPDM and PTFE also.
FDA doesn't approve materials of construction unless you are making a medical device and the material is implanted. Silicon happens to be highly permeable to oxygen. You may wish to consider a sanitary EPDM/PTFE envelope gasket.
FYI, the "moral equivalent" of (incorrectly) so-called "FDA-approved" materials would be something akin to USP Class V or Class VI implantation results. Personally, I prefer to use certificates of analysis for traceability purposes in case an FDA inspector asks.
