These septa are made from fluoroelastomer FKM which is highly resistant to a broad range of solvents.
An elastomeric study done at Sigma-Aldrich compared the solvent compatibility (resistance) of FKM and EPDM rubber, Fig. 1. A Sealability Factor was determined for each elastomer. Septa materials with lower sealability factor values, such as the Fluoroelastomer FKM, correlate with better solvent compatibility.