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Laboratory
All Containers›Shop by use case›Laboratory›Closures and seals›Liners, and why PTFE keeps appearingShop all laboratory →
Closures and seals

Liners, and why PTFE keeps appearing

By Dwight Davis, Founder

Updated

On this page7
The liner is the part of the closure the sample actually touches, which makes it a material specification in its own right.

Why it is not a detail

A sample lying on its side in a cooler is in contact with the liner for the entire holding time. Everything the container pages say about leaching, adsorption and permeation applies to that surface as much as to the bottle wall.

PTFE facing is the inert answer

A liner faced with PTFE presents a fluoropolymer surface to the sample while something cheaper provides the compression behind it. That is how the FP material in Table II most often reaches a general laboratory: as a facing rather than as a whole bottle.

Pulp and plain foam are wrong here

They absorb, they break down against acid and base, and they can contribute to the sample. They are fine over a dry powder and they have no place against a preserved liquid, which is the mistake a general-purpose cap invites.

A polycone has no separate liner

The cap material itself forms a cone that seals into the neck, which removes the liner question by removing the liner. It seals well on thin liquids and the material in contact is then whatever the cap is made of, which still has to be appropriate.

What we publish

Liner is recorded on part of the range rather than all of it. Where it is published it is on the container page; where it is not, that is a question to ask us rather than an assumption to make, and in this property it is worth asking.

Match the liner to the preservative, not the sample

The acid or base is usually more aggressive than the analyte, and it is what the liner has to withstand for weeks. Specifying against the sample alone is how a liner that was fine in testing fails in the field.

References

40 CFR 136.3, Table II — Required Containers, Preservation Techniques, and Holding Times, which prescribes the container material, the preservation and the maximum holding time per parameter. Footnote 1 defines the codes: “P” is polyethylene, “FP” is fluoropolymer (PTFE), “G” is glass, “PA” is any plastic made of a sterilizable material. law.cornell.edu
Written from federal regulation, not from a bench. Where a method specifies a container, a preservative or a holding time, the method governs and nothing here overrides it. Read the method you are running rather than a summary of what methods generally say.