You can either use the stainless-steel fittings that are like the ones that come with your UHPLC system, or you can use EXP fittings. The EXP fittings can be used up to 20,000 psi when tightened with a wrench. In any case, always make sure you are using a fitting with zero dead volume so that the high efficiency provided by UHPLC is not compromised by extra dead volume.

