Guides / Sourcing

Why Chromatographers Reach for PEEK Over Stainless

September 26, 2026

Sources: manufacturer datasheets and published standards. See editorial standards.

Walk into any HPLC lab built in the last fifteen years and the fittings, unions and injector parts are more likely to be PEEK than stainless. That shift did not happen because PEEK is cheaper. It happened because a chromatography fluid path has a job that stainless does not always do well: staying out of the way of the sample.

The problem an inert flow path solves

A fitting, union or valve body sits between the sample and the detector on every run. If that surface reacts with the sample, adsorbs part of it, or sheds metal ions into the mobile phase, the chromatogram reflects the hardware as much as the sample. Three failure modes show up in real labs:

• Metal ion leaching. Stainless steel is an alloy, and acidic or halide-containing mobile phases can pull trace iron, nickel or chromium off exposed surfaces. For ICP-MS, ion chromatography or any trace metal method, that leaching is the interference, not a side effect.
• Adsorption and carryover. Some biomolecules and metal-sensitive analytes stick to steel surfaces, particularly at low concentration. A peak that tails or a recovery that drifts downward across a run can be the flow path talking, not the column.
• Corrosion under aggressive buffers. Chloride-containing buffers, common in ion chromatography, pit stainless over time. A fitting that develops microscopic corrosion becomes a new source of adsorption and a leak risk.

PEEK does not react with the sample and does not corrode in these conditions, which is the entire case for it in fittings, unions, frits and injector components that stay wetted for the life of the system.

Pressure: the number PEEK cannot argue its way around

Inertness is not free. PEEK’s mechanical strength is lower than stainless, and pressure rating is where that shows up directly.

	Machined PEEK fitting	316 stainless fitting
Typical pressure rating, standard bore	6,000 to 10,000 psi	15,000+ psi, higher with reinforced designs
Behavior at elevated temperature	Rating drops as it approaches the 143°C glass transition	Rating holds well above HPLC operating temperatures
Metal ion leaching	None	Present under acidic or halide-rich mobile phases
Ferrule reseal after repeated use	Good, holds a seal across many connect cycles	Good, but a re-cut ferrule is standard practice
Relative cost per fitting	Higher	Lower

Standard analytical HPLC runs comfortably inside PEEK’s pressure range. UHPLC systems that push past roughly 15,000 psi are a different story, and most of them keep stainless or titanium in the highest pressure sections of the flow path even when PEEK handles everything downstream of the pump. Check a system’s actual operating pressure against the fitting’s rating rather than assuming PEEK covers every case, since what tolerances are realistic on a machined PEEK part also affects how consistently a fitting seals at the top of its rated range.

The “biocompatible” flow path, and what that word actually means here

Instrument vendors increasingly market “biocompatible” HPLC systems, and in this context the word has nothing to do with implants or tissue contact. It means a metal-free wetted flow path: PEEK or PEEK-lined tubing, PEEK fittings, and a titanium or ceramic pump head instead of stainless, all chosen to keep metal ions and adsorptive metal surfaces away from the sample.

This matters most for:

• Biopharma methods analyzing proteins and peptides sensitive to metal-catalyzed degradation
• Trace metal and speciation analysis where the instrument itself cannot be the contamination source
• Ion chromatography running corrosive halide buffers day after day

A metal-free system is not automatically the right choice for every method. It costs more, and for routine small-molecule work with well-behaved analytes, an all-stainless system performs fine for a fraction of the price. The decision belongs to the method, not to a general preference for one material.

Where PEEK falls short in a fluid path

• Solvent limits. Concentrated chlorinated solvents and high percentages of THF can swell PEEK over time. Check compatibility for the specific mobile phase before committing a long method to PEEK tubing. The full chemical resistance picture covers what does and does not attack it.
• Lower pressure ceiling. Covered above. It is a real limit, not a marketing footnote.
• Ferrule and fitting geometry matters more than with steel. A slightly out-of-tolerance bore or sealing face leaks or introduces a dead volume that a steel equivalent would tolerate better. See PEEK vs PTFE for lab fittings and ferrules for how the seal itself behaves under repeated tightening.

Specifying it on a drawing

A chromatography fitting drawing should state the bore and OD to the tolerance the seal actually needs, the thread or port standard the instrument uses, and the maximum operating pressure and temperature the part sees in service. None of that is exotic, but leaving it off a drawing is how a fitting that looks right on paper leaks at the bench.

Have a fitting, union or manifold drawing for an HPLC or chromatography system? Send the drawing with the pressure and port standard, and we match you with shops that already cut analytical instrument parts.

Frequently asked questions

Keep reading

• Clearance, Grade and Fit for PEEK Pump Wear Rings and Bushings
• What a Shop Needs to Machine Spinal Cage Prototypes in PEEK