What holds the sample: laboratory glass, closures and how a seal is proved

Every result a laboratory produces depends on something nobody writes in the method: the container the material sat in before anyone measured it. A sample spends minutes on a balance and seconds in an instrument. It can spend months in a container. During those months the container is in contact with it the whole time, on every surface, at every temperature the material passes through. If the container gives something to its contents, takes something from them, lets air or water in, or lets light through, that change is in the result before the analysis begins.

This guide is about that container. It covers the kinds of glass a laboratory uses and how they are classified, what water and light do across a glass wall, how stoppers, septa and caps are made and what they release, how the tightness of a sealed container is tested, and what a complete container specification should tell you. It is written as a reference, not a catalogue. No brand, supplier or product is named or judged.

A container is not packaging. It is the first piece of equipment the material meets, and the one it meets for longest.

Glass is not one material

Ordinary window and bottle glass is soda-lime glass: mostly silica, with sodium oxide added to lower the melting temperature and calcium oxide added to make the result durable. It is cheap, clear and easy to form. It is also comparatively soft chemically, because the sodium that makes it easy to melt is held loosely in the network and leaves the surface when water sits against it.

Laboratory glass is mostly borosilicate, in which boron oxide takes over much of the work the sodium did. Two families matter. Borosilicate 3.3 is the glass of beakers, flasks and reagent bottles. Its name is its coefficient of linear expansion, 3.3 millionths of its length per kelvin, which is low enough that it survives being moved from a hot plate to a cold bench. Borosilicate 5.1, sometimes called neutral glass, carries a little more alumina and alkali. It expands a little more, but it can be drawn into thin, precise tubing, and that is why small sealed containers are usually made from it.

The difference that matters most day to day is not strength but thermal shock. When one face of a glass wall is heated or cooled faster than the other, one face tries to grow or shrink while the other holds still. The stress is proportional to how much the glass expands per degree. A soda-lime container expands nearly three times as much as borosilicate 3.3 for the same step, which is why it is the one that cracks when it goes from a freezer into warm water.

Values are typical published figures for orientation. A drawing or certificate for the specific item is always the authority.

What water does to a glass wall

Glass looks inert. It is not. When water sits against a glass surface, hydrogen ions from the water trade places with sodium and potassium ions in the glass. The water slowly becomes alkaline, and the surface layer of the glass slowly becomes depleted in alkali. Over long contact, or at high temperature, the silica network itself begins to dissolve. For most purposes this is far too slow to notice. For a small volume of a sensitive solution kept for a long time, a shift in pH of a few tenths and a few parts per million of dissolved glass can be enough to change what is measured later.

This is why glass containers are classed by hydrolytic resistance, which simply means how much the glass gives up to water under a standard test. The tests are blunt and reproducible. A sample of the glass, either crushed to a fixed grain size or as a filled container, is held in water at about 121 °C in an autoclave for a fixed time, and the alkali that comes out is titrated with dilute acid. Less acid needed means less alkali released, which means a more resistant glass.

Containers are then grouped into classes. The ones you will see on a specification are usually called Type I, Type II and Type III:

• Type I is borosilicate glass, resistant through its whole thickness because of what it is made of.
• Type II is soda-lime glass whose inner surface has been treated, usually with sulphur compounds during forming, so that the alkali at the surface is removed. It is resistant only where it was treated, and only for as long as that thin layer lasts.
• Type III is untreated soda-lime glass, adequate for many dry or non-aqueous contents and not intended for sensitive water-based ones.

A subtler failure, and the one that worries people who make small sealed containers, is delamination. When tubing is heated to form the neck and the base, some of the borate and alkali in the glass evaporates from the hottest zones and condenses again on cooler parts of the inner wall. Those patches have a different composition from the glass around them. Under the wrong contents, especially solutions at high pH or containing certain buffers, the patches can loosen and shed thin, glittering flakes. The remedy is in the forming: lower temperatures at the base, careful control of the flame, and a check of the inner surface on samples from each production run.

Tubing glass and moulded glass

Small containers are made in one of two ways, and the method leaves a signature. Tubing glass begins as a long, precisely drawn tube. A machine heats a section, forms the neck and the shoulder, cuts it off and closes the base. The walls are thin and even and the dimensions are tight, which matters for anything that has to fit a closure exactly or run on an automated line. The weakness is the heating described above, which is where delamination starts.

Moulded glass is formed from a gob of molten glass blown into a mould. The walls are thicker and less even, the containers are heavier and stronger against knocks, and the inner surface has been through less reheating. Moulded containers are common for larger volumes and for contents where wall uniformity is less important than robustness.

Neither is better in general. The useful question is whether the specification says which one it is, because the answer predicts which failure to look for.

Colour is a filter, not a decoration

Light carries energy, and the shorter the wavelength, the more energy each photon carries. Ultraviolet and blue light are energetic enough to break or rearrange bonds in many organic molecules. A clear container on a sunny windowsill, or under fluorescent tubes on an open shelf, is exposed to a steady stream of exactly that light, hour after hour.

Amber glass gets its colour from iron and sulphur compounds added to the melt. It absorbs strongly below roughly 450 nanometres and lets longer wavelengths through. In practice that means it blocks most ultraviolet and much of the blue, while still letting you see the contents. The standards that govern light-resistant containers set a maximum transmission at each wavelength across the ultraviolet and visible range, scaled to the wall thickness, and an amber container either meets that limit or it does not.

Two cautions. First, amber glass is not a substitute for darkness. It reduces exposure; it does not remove it. Material that is sensitive to light is still stored in a closed box or cabinet. Second, the iron in amber glass can itself leach in small amounts into some solutions. For contents where trace metals matter, clear glass kept in the dark is sometimes the better choice. Neither colour is correct in general. The right one depends on what the contents are sensitive to.

The closure is half the container

A glass container is only as good as what closes it, and closures are far more complicated materials than glass. There are three common families.

Elastomer stoppers are pressed into the neck and held down by a crimped aluminium seal. Most are made from bromobutyl or chlorobutyl rubber, chosen because butyl rubbers are unusually poor at letting gases and water vapour through. A rubber stopper is not a single substance. It is a compounded mixture: the base polymer, a filler such as clay, vulcanising agents that cross-link the rubber, pigments, and processing aids. Some of those ingredients, or the products of the vulcanising reaction, can migrate out of the rubber into whatever touches it.

For that reason many stoppers are given a barrier film on the face that meets the contents, often a fluoropolymer. The film reduces what the rubber can give up and stops the stopper sticking to the glass. Stoppers are also commonly washed, siliconised so they slide into the neck cleanly on a machine, and sterilised. Each of those steps is a choice the specification should state.

Septa are thin discs, usually silicone faced with PTFE, sitting under a screw cap or crimp cap with a hole in the top. They exist so an autosampler probe can pass through without the cap being removed. The PTFE face is chemically inert; the silicone behind it gives the septum its spring and its ability to reseal. Once the PTFE face has been pierced, the silicone is exposed, and repeated piercing slowly cores the septum and weakens the seal.

Screw caps with liners are the closures of reagent bottles. The cap is usually polypropylene or phenolic resin. The seal is made not by the cap but by the liner inside it, which might be PTFE, polyethylene foam, or a laminate. A cap can be screwed down hard on a poor liner and still leak, and a cap left loosely closed is not closed at all.

Extractables and leachables

Two words come up whenever closures are discussed, and they are easy to confuse. Extractables are everything that can be pulled out of a material under aggressive conditions: strong solvents, high temperature, long contact. An extractables study is a deliberately harsh experiment that answers the question "what could possibly come out of this?"

Leachables are what actually does come out under real conditions, into the real contents, over the real storage time. Leachables are a subset of extractables. A leachables study is slower and narrower, because it has to be run on the real combination of container, closure and contents, often for the whole intended storage period.

The relationship is the useful part. A good extractables study tells the person running the leachables study what to look for. A substance that never appears among the extractables cannot reasonably turn up as a leachable. A substance that appears in quantity among the extractables needs a specific check in the leachables work.

What a seal has to keep in, and keep out

A sealed container is expected to do several things at once. It must keep the contents in. It must keep microorganisms out. It must keep air, and in particular oxygen, out. It must keep water vapour out, or in, depending on whether the contents are dry or liquid. And it must go on doing all of this through every temperature the container will see, because rubber stiffens in the cold and glass and metal shrink at different rates.

The property is called container closure integrity, and the important idea behind it is that a seal is not simply tight or leaking. Every seal has a leak rate, even if it is extremely small. The questions are how small, and whether that is small enough for the contents and the storage time. A path one micrometre across will let gas through long before it lets a liquid or a bacterium through. A dry material sealed under nitrogen can be spoiled by moisture through a leak far too small to see.

Because of that, the method used to test a seal matters as much as the result. Some methods only find large leaks. Others can find leaks thousands of times smaller. A container that "passed the integrity test" has told you very little until you know which test.

Sensitivities are ordinal, for orientation. Each method has its own validated limit of detection, and that limit belongs in any report that uses it.

The headspace tells the story

The gas above the contents of a sealed container is called the headspace, and it is one of the most useful things a laboratory can measure without opening anything. If a container is filled and sealed under nitrogen, the oxygen in its headspace should start near zero and stay there. If the oxygen rises over weeks, air is getting in. If the container was sealed under partial vacuum and the pressure rises, the same is true.

Modern headspace analysers shine a tuned laser through the glass and read the absorption of a single gas line. The measurement takes seconds, does not touch the contents, and can be repeated on the same container month after month. That makes it a good way to watch a seal age, which is something a destructive test can never do, because a destructive test only ever sees each container once.

Moisture follows the same logic. A freeze-dried cake holds very little water and wants to take more from anywhere it can. Any leak, or any rubber that gives up its own absorbed water, shows up first as a rise in residual moisture and later as a change in the appearance of the cake. A closure that was not dried properly after washing can be the source rather than the air outside.

Where seals actually fail

The rubber and the glass are rarely the problem on their own. Most failures happen at the junction between them. The neck of the container has a finish, the shaped lip the closure seals against, and its dimensions are toleranced to a fraction of a millimetre. A stopper that is slightly too small for the neck, a neck finish chipped in handling, or a crimp applied with too little or too much force all leave a path along the interface.

Crimping is a good example of a process that has to be set, not guessed. The aluminium seal is folded under the neck flange by a crimping head. Too light a crimp and the stopper is not compressed enough to seal. Too heavy and the rubber is overcompressed, the glass can crack, or the stopper takes a permanent set and relaxes away from the glass later. Production lines measure the result with a residual seal force tester, which pushes on the cap and records how hard the compressed rubber pushes back. That single number is a good proxy for whether the crimp was right.

Temperature is the other common cause. Butyl rubber loses its elasticity as it approaches its glass transition, which for typical formulations is well below freezing but not far enough below for storage at very low temperatures. A seal that is tight at room temperature can open briefly when the rubber stiffens and the glass and aluminium contract at their own rates. Containers intended for deep-freeze storage are tested at that temperature, not just at room temperature, for this reason.

Labels on glass

A label is part of the container too, because a container whose identity cannot be read is a container of unknown contents. Paper labels with ordinary adhesive fail in exactly the places a laboratory needs them most. In a freezer the adhesive turns brittle and the label lifts at the edge. In a cold room, condensation forms when the container comes out, and a wet paper label smears and peels. Some solvents dissolve the adhesive outright.

Labels for cold storage are usually polyester or polypropylene film with an adhesive formulated to stay tacky at low temperature, and they must be applied to the container at room temperature, dry, before it goes into the cold. Printing is done by thermal transfer rather than ink jet, because thermal transfer print resists solvents and abrasion. A label printed and applied correctly will still be readable after years at very low temperature. A handwritten label on frosted glass often will not survive a week.

The content of the label matters as much as its material. At a minimum it should carry an identity, a lot or batch reference that ties the container to its records, a date, and any storage condition that matters. A barcode or a two-dimensional code makes the reference machine-readable, which removes transcription errors when the container is logged in or out.

Handling: the part nobody writes down

Most damage to a container happens between the shelf and the bench, and most of it is avoidable. A few habits account for the majority:

• Let cold containers warm before opening. A container taken from cold storage and opened at once draws in room air, and the water in that air condenses on the cold contents. Leaving it closed until it reaches room temperature avoids that.
• Avoid temperature shocks. Moving glass straight from very cold storage into warm water is the classic way to crack it, and even borosilicate has a limit.
• Hold glass by the body, not the closure. Lifting by the cap stresses the neck finish, which is where the seal lives.
• Store upright unless the specification says otherwise. Contents resting against a stopper for months give the rubber more time to exchange material with them.
• Keep the outer box. It is often the real light barrier and the place the lot information is printed in full.

Reading a container specification

A complete specification for a sealed container answers a predictable list of questions. When one is missing, it is worth asking for, because the missing answer is usually the one that matters.

• Glass: borosilicate or soda-lime, and the hydrolytic class (Type I, II or III).
• Forming: tubing or moulded, and for tubing, whether the inner surface is inspected for delamination risk.
• Colour: clear or amber, and whether light transmission meets a named limit.
• Closure: the elastomer type, whether it has a barrier film, and whether it was washed, siliconised, dried and sterilised.
• Extractables: whether a study exists for the closure, and what solvents and conditions it used.
• Seal: the integrity method used, its validated detection limit, and the temperature it was tested at.
• Headspace: the gas the container was sealed under, and whether headspace oxygen or moisture was measured.
• Label: the material and adhesive, and whether it is rated for the storage temperature.
• Lot reference: a reference on the container that leads to the records for both the container and its contents.

Why this matters more than it seems

It is easy to treat containers as a solved problem, because they usually work. That is exactly the reason they are rarely examined. When something does go wrong, a drifting result, a change in appearance, a value that slowly moves over months, the analysis is checked first, the method second and the material third. The container, which was in contact with the material the entire time, is often the last thing anybody thinks of.

The habit worth building is a simple one. Treat the container as part of the material's history. Know what it is made of, what closes it, how the seal was tested and how it has been handled. When those answers are written down, a container stops being an assumption and becomes a fact on the record, which is what it has been all along.