Convention tells us to match fitting material to tubing material: use PEEK fittings with PEEK tubing and use stainless-steel fittings with stainless-steel tubing. The main reason for that is simply system pressure. However, some stainless-steel fittings can be used with PEEK tubing. For example, EXP fittings utilize a titanium/PEEK hybrid construction and can be used with either PEEK or stainless-steel tubing. However, if inertness is a consideration, stainless-steel fittings should be avoided, and PEEK fittings are a better choice. Please note that PEEK is not recommended for use with some strong acid solutions, such as aqua regia, and it can be affected by halogenated solvents, as well as THF, under certain conditions. Stainless steel is safe for most mobile phase reagents and solvents, but it is less inert, particularly toward mineral acids, bases, and oxidizing agents.
EXP fittings utilize a titanium/PEEK hybrid construction and can be used with either PEEK or stainless-steel tubing.
Other types of tubing, such as PTFE and Tygon tubing, work well with PEEK or polyethylene (PE) fittings, unions, and adaptors. Many of these fittings are not screw-threaded but rather slip-on because of their use in low-pressure applications. A great example of this is mobile phase delivery in HPLC from bottle/reservoir to pump. This is a low-pressure delivery (HPLC and UHPLC systems hold high pressures from pumps to the end of the column), and PTFE tubing and PTFE or PE fittings are commonly used.
PEEK union connector (cat.# 27715)
Bluestem glass solvent filter (cat.# 26431)
