Yes, you can very effectively use PEEK Fittings on Stainless Steel Tubing as long as you take into account the making Pressure Ratings of the Fitting you are using and the expected Pressures in your System.

Generally speaking, a PEEK Fitting is preferred for all HPLC Applications up to 5,000 psi and No, you should never use a Stainless Steel Fitting on PEEK tubing.

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