The results are reported of the study of materials used in the CERN Closed Loop recirculation
gas system currently under test with the RPC muon detectors in the CMS experiment
at the LHC. Studies include a sampling campaign in a low-radiation environment (cosmic
rays at the CERN ISR test site). We describe the dedicated RPC chamber tests, the
chemical analysis of the filters and gas used, and discuss the results of the Closed Loop
system
