PEEK stands for Poly Ether Ether Ketone. They can withstand upto 100°C. In many cases where stainless steel is too reactive it is good to use PEEK. It is absorptive for solvents & analytes such as methylene chloride, sulphuric acid & nitric acid. PEEK can't be used at high pressure for long time. PEEK ferrules are used for harder tubing (Tefzel, Polyurethane). Perfluoroalkoxy is a fluorcarbon with chemical and mechanical properties similar to those of FEP, but with greater rigidity.