Why do most users privilege PEEK vs stainless steel fittings and tubing?
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Stainless steel (SS) fittings and ferrules are "swaged" onto the tubing. They must be wrench tightened and the ferrule is permanently attached to the tubing. Although this procedure gives a pressure resistant seal, the ferrule and tubing cannot be reused or removed without cutting and deburring the tubing and using a new ferrule and sometimes a new piece of SS tubing.
PEEK fittings offer a new and more practical approach to connecting a pump/autosampler to an injector, injector to guard/analytical column, analytical column to a detector etc…The nuts and tubings are not permanently swaged together which allows reusing both nuts and ferrules until the nut threads wear out and the ferrule totally deforms. The first generation of polymeric fittings and ferrules were not as resistant as the more modern PEEK versions that can withstand and resist usual HPLC column pressures and solvents without any problem and all that applying solely fingertight pressures to connect. The torque applied is sufficient to make proper connections and avoid any leaks. The ferrule can be reused many times when the fitting is finger-tightened, while it can be reused just a few times if the fitting is wrench- tightened for UHPLC use.
Therefore, stainless Steel (SS) fittings/tubing should be used as your best choice whenever extreme pressures are necessary and required for proper sealing and security while polymeric (PEEK) fittings are ideal whenever connections are changed frequently and pressure is less critical. Another great advantage of PEEK tubing is the flexibility compared to the SS tubing. Most PEEK tubing can be used up to the upper pressure limits of conventional HPLC equipment (6000 psi = 400 bar), but cannot be used at higher pressures with U-HPLC type applications. The smaller internal diameters should have higher burst strengths because of the greater wall thickness, but it is wise to check the tubing specifications if you expect to operate routinely at >4000 psi (275 bar) with PEEK tubing.
However, not all HPLC solvents are compatible with PEEK tubing. Aqueous buffers and salts are fine, as is acetonitrile (ACN), methanol (MeOH), and other alcohols. But tetrahydrofuran (THF) and the chlorinated solvents are not recommended for PEEK tubing. These solvents weaken the tubing, make it swell and cause it to become fragile.