Why a plastic that is safe to touch may not be safe to test with
Leachables, and why material selection is a chemistry question.
A plastic can be entirely safe to touch, wear, or drink from, and still be unsafe to build a sensitive manifold from, because the trace chemicals it sheds into a liquid can sit at concentrations far too low to concern human health while still being large enough to throw off a measurement built to detect changes at a similarly small scale.
The taste of a new water bottle
A brand new plastic water bottle often gives its first few fillings a faint plastic taste that fades with use. That taste comes from trace manufacturing residue and additives leaching into the water. Nobody drinking it is at any real risk, and the taste disappears once the small reservoir of readily available material near the surface is used up. A sensitive chemical or biological assay is a far more discerning tongue, able to notice the equivalent of that taste at concentrations far below anything a mouth could detect, which is why a material judged perfectly safe by food-contact standards can still interfere with a measurement.
Almost no manufacturing plastic is a single pure compound. It is a base polymer blended with additives that give it useful working properties: a plasticiser for flexibility, a stabiliser against light and heat, a mould-release agent, sometimes a residue of unreacted starting material. None of these are locked permanently in place, and given time in contact with a liquid, especially one with any solvent-like character, some of each migrates out, a process called leaching.
A fault with no visible symptom
Leaching gives no visible sign, rarely any smell, and no change in the plastic's appearance. It usually surfaces as a puzzling assay result: a reaction running slower than expected, a baseline drifting upward over a run, or two manifolds made to the same material specification behaving differently because their batches or suppliers used different additive packages. Tracing that back is hard, since the culprit already passed every safety and mechanical check the project ran.
A food-contact rating assumes a container used occasionally and rinsed between uses. A manifold may touch the same class of liquid continuously for months, and leaching keeps drawing on the same reservoir of additive for as long as contact continues, so a material that shed an undetectable trace during a brief regulatory test can shed a steadily accumulating amount over an instrument's working life. Analytical and pharmaceutical work therefore uses a separate qualification step, called extractables and leachables testing, which characterises what a candidate material sheds into the actual liquids and conditions it will see in service.
Choosing a manifold material is a chemistry decision as much as a mechanical one. Polypropylene is favoured in this kind of work for its comparatively low additive content and general chemical inertness, and a substitution that looks harmless on a mechanical drawing, swapping one grade of plastic for a cheaper or more easily machined one, can be the single change that makes a sensitive instrument lose its accuracy with nothing on the drawing to explain why.