Polypropylene polyethylene and polystyrene

A plasmid prep, a wash bottle and a clear assay plate are three ordinary jobs, and they do not want the same polymer. This note applies polypropylene, polyethylene and polystyrene to those jobs and states the limit of a resin-class claim. Tubes, tips and plates as a buying decision are discussed in when to use plastic tubes, tips and plates. When the solvent or the volume pushes you back to glass, use choosing laboratory glassware.

A concrete bench, three resins

Consider a small molecular workflow. You prepare a buffer to a stated concentration. That step belongs in a volumetric flask of a known class, at the flask's reference temperature. NIST weights and measures practice is why the moulded line on a bottle does not take this role. You then aliquot the buffer into polypropylene microtubes because they centrifuge cleanly, cap reliably, and tolerate aqueous salt solutions for the time the method states. You rinse a bench bottle of ethanol or water from a low-density polyethylene wash bottle, remembering that a solvent the chart dislikes should never have been in that squeeze bottle. You read an aqueous colourimetric assay in a polystyrene plate because the well is clear and the instrument was built for that footprint.

None of those three successes promotes the resin to a universal container. The limit of the claim is the next liquid.

Polypropylene in the rotor and the autoclave

Polypropylene's useful range on a research bench is aqueous chemistry at moderate temperature, plus the autoclave and centrifuge duties a given product marks on its label. The mark is the product's, not the polymer's in the abstract. A tube rated to a stated relative centrifugal force in a stated rotor is not rated to a higher speed because the wall looks thick. A cap marked for steam is not a promise that every polypropylene item in the room, including a rack you forgot to check, will keep its shape.

Nucleic-acid work lives in this plastic for good practical reasons. It is also where adsorption of dilute samples becomes visible, which is a surface question rather than a solvent question. When the analyte is precious and dilute, the tube specification has to say whether a low-binding claim is required and for which molecule. An ordinary polypropylene tube remains the right default for a concentrated buffer.

Polyethylene as a bottle, not as a measurement

High-density polyethylene stores many aqueous reagents that would be wasted in borosilicate. The bottle is light, it survives a knock, and the liquid is visible only as a level, not as a colour you can compare carefully. Low-density polyethylene wash bottles deliver a stream. Both pick up solvent exposure badly when someone refills them with a convenient organic. The wall swells, the print lifts, and the next user inherits a contaminated squeeze.

Do not autoclave a polyethylene bottle because a polypropylene cap once survived. Distortion is the usual result, and a warped mouth no longer matches the cap. Glass media bottles in borosilicate, the low-expansion type distinguished in ASTM E438, remain the reusable steam vessel. Polyethylene's honest application is room-temperature aqueous storage and rinsing with liquids the chart allows.

Polystyrene where optics and cells need clarity

Polystyrene plates and dishes are optical and biological surfaces. Tissue-culture treatment is a surface claim on top of the resin. An untreated dish and a treated dish are not the same culture vessel. The assay insert names the plate colour and the polymer when both matter. Clear polystyrene suits absorbance of an aqueous assay. It does not suit luminescence or fluorescence by default, and it does not suit acetone.

The failure mode is unmistakable and still gets repeated: a pipette wetted with an organic solvent is emptied into a polystyrene well "just to dilute it". The well crazes, the optical path changes, and neighbouring wells may be contaminated by a creeping solvent. Retire the plate. Do not read around the damaged well and call the row usable.

Polystyrene also warps in a drying oven or an autoclave. If you need a sterile empty plate, buy it sterile or follow a method the plate maker allows. Do not steam a research dish and expect the lid to fit.

Marks that travel with the piece

Write the resin on the tray, not only in the purchasing note. A clear polystyrene plate and a clear PETG bottle look similar on a crowded bench, and only one of them is a plausible optical aqueous plate. A milky polyethylene bottle and a polypropylene jar look similar, and only one of them is a usual autoclave candidate, and only when that item is marked. The application succeeds when the person at the bench can see the class without opening a catalogue.

When the application changes mid-protocol

The branch is the first solvent, the first heat, or the first optical claim. If an extraction switches from aqueous buffer to chloroform or phenol, the tube decision is new. Glass or a resin the chart allows takes the organic phase. If a plate assay moves from absorbance to luminescence, the colour decision is new even though the resin may still be polystyrene. If a bottle must be sterile and chemically clean, steam on borosilicate may be required, and a fresh polystyrene dish does not answer the bottle problem.

A failed control is informative. If an enzyme blank fails only in washed polystyrene and passes in a new plate, look at residue and at solvent carry-over before you replace the enzyme. If a centrifuge tube cracks, retire the lot from that chemical and that speed until you know which limit you crossed.

Safety and institutional limits

Cultures in polystyrene dishes and tubes follow your biosafety rules. The WHO laboratory biosafety manual and the CDC BMBL are the background references, not a containment assignment. Solvent waste in the wrong bottle is a chemical-hygiene incident. This page does not authorise a diagnostic assay because a plate is clear. Research use, with the assay insert and the resin chart in front of you.

What to put in the request

Name the resin, the format, the duty (centrifuge, optical, storage, autoclave), the liquids, and the quantity. Keep volumetric glass as its own line when a concentration will be declared. The laboratory plasticware catalogue is where these three resins are browsed as products, and the quote request is where the duty description belongs. Glass that the plastic must not silently replace is in the laboratory glassware catalogue.