Laboratory reagents: grade, lot control and the shelf

Specifying and controlling molecular biology reagents

1. Match the grade to what the method is sensitive to. Analytical grade for quantitative chemistry, chromatography grade where trace impurities raise a baseline, molecular biology grade where nuclease or nucleic acid contamination matters, and technical grade for cleaning and for bulk uses where purity is irrelevant.
2. Read the certificate of analysis rather than the label. The certificate names the impurities tested and their limits for that lot. Two suppliers' analytical grades can certify different impurity sets, and the one that matters to your method may be on only one of them.
3. Record the lot number with the result, not with the invoice. When an assay drifts, reagent lot is the first variable to check and it is usually unrecorded. Writing the lot into the method record costs nothing and turns a week of investigation into one query.
4. Date every container on receipt and on opening. Shelf life runs from manufacture and many reagents degrade faster once opened. A bottle with no opening date is a bottle nobody can judge, and it will be used long after it should have been discarded.
5. Keep the safety data sheet and label every decant. Every hazardous chemical needs its sheet available and every container other than one in immediate use needs its identity and hazards on it. An unlabelled bottle is the single most common laboratory inspection finding.

Inventory that people actually use

A chemical inventory is a legal expectation as well as a convenience, and the one that works is the one at the point of use: a barcode scanned when a bottle is taken and when it is finished. An inventory maintained by an annual walk round is out of date within a fortnight.

Review for expiry and for the bottles nobody has touched in three years. Disposal of old chemicals is expensive and gets more so as labels fade and contents become uncertain.

Storage and segregation

Segregate by hazard rather than alphabetically, keep flammables in the appropriate cabinet, and use secondary containment for acids and solvents. Alphabetical shelving is how an oxidiser ends up beside an organic solvent.

Temperature sensitive reagents need the right refrigerator, not the nearest one, and anything flammable in cold storage needs a spark free interior. This is a recognised cause of laboratory fires.

Bridging a lot change without losing the assay

Where a reagent materially affects a result, overlap lots: run the new lot alongside the old on the same samples before switching. It costs one run and it separates a lot effect from a real change.

Reserve a lot for the duration of a study where the supplier allows it. Suppliers will usually hold material if asked and will rarely offer, which makes it the cheapest insurance available in a long study.

Sodium phosphate buffer or tris: the species matters as much as the pH

Two buffers at the same pH are not interchangeable. Phosphate precipitates with divalent cations and inhibits several enzymes; tris shifts pH substantially with temperature, so a buffer titrated warm is not at that pH in the cold; and both contribute ions that matter to a separation.

Choose the species by the chemistry and the temperature of the experiment, titrate at the temperature of use, and record the recipe rather than the name. A protocol that says only the pH has under-specified the buffer.

Balanced salt solutions and what is deliberately left out

A balanced salt solution without calcium and magnesium exists because those ions support the adhesion molecules that hold cells to each other and to a surface. Omitting them is what lets a wash loosen cells before dissociation, and including them is what keeps a tissue intact.

Using the wrong one is a common and invisible protocol error: a wash that removes cells, or a dissociation that will not start. Formulations with a bicarbonate buffer also need the right atmosphere, while an organically buffered one holds pH on the bench.

Reducing agents and reagents that do not keep

A thiol reducing agent oxidises in solution and loses activity, which is why stocks are frozen in aliquots and added fresh. A protocol that specifies it and a stock kept in the fridge for a month are not the same protocol, and the failure appears as a protein that will not stay reduced.

The same applies to nucleotide solutions, enzymes in dilute solution and anything light-sensitive. Where a reagent's activity decays, the storage instruction is part of the method, and a dated aliquot regime costs nothing.

Nuclease-free is a tested claim

Labelling on a tube or a tip box states that material was tested for nuclease activity at a stated sensitivity, and certification varies in what was tested and how. It is not a synonym for sterile or for pure, and autoclaving does not reliably destroy the enzyme that degrades RNA.

For RNA work the practical regime is certified consumables, dedicated solutions in single-use aliquots, and handling that does not reintroduce the enzyme from skin or from a shared pipette. Buying certified material is usually cheaper than proving your own.

Lysogeny broth, Sf9 media and other media: buy or make

Standard microbial broths are cheap to make from powder and are the clearest case for making your own, provided the recipe and the powder lot are recorded. Insect and specialised primary-cell media are complex, sensitive to preparation and normally bought.

The dividing line is how many components there are and how much the result depends on them. A two-component broth is a preparation; a serum-free medium with defined growth factors is a product, and reproducing it in house rarely saves what it costs in variability.

Where idte and other resuspension buffers differ from water

A dilute Tris and chelator buffer is the default for resuspending nucleic acids for a reason: the chelator sequesters the divalent metals that nucleases need, and the buffer holds a pH at which DNA is stable rather than slowly depurinating. Water is fine for an oligonucleotide going straight into a reaction and poor for anything stored, because the pH of unbuffered water drifts acidic as it takes up carbon dioxide.

The reason to know which you used is downstream: the chelator will inhibit a reaction that needs magnesium if the volume carried over is large enough, which is the usual explanation for an amplification that fails only on a concentrated template. Resuspend in the buffer for storage, dilute into water or into the reaction buffer for use, and record which, because a stock in one behaves differently from a stock in the other.

Where a dnase 1 or a dna polymerase 1 belongs on the list

The enzymes a molecular laboratory keeps divide into the ones that cut, the ones that copy, the ones that join and the ones that clean up, and the last group is the one people order late. A nuclease for removing DNA from an RNA preparation and the classic bacterial polymerase used for nick translation and fill in are both in it, and both are supplied in units that only mean anything against the assay they were defined on.

Buy these with their buffers and record the lot: an enzyme stock that has been thawed a dozen times is the usual explanation for a protocol that worked last year. Keep single use aliquots of anything used rarely, and keep the datasheet, because the unit definition is on it and nowhere else.

Common questions

What grade of laboratory reagents do I need?

The grade the method is sensitive to. Analytical for quantitative chemistry, chromatography grade for low wavelength ultraviolet and mass spectrometric detection, molecular biology grade for enzymatic work, technical for cleaning and bulk use.

Does reagent lot really matter?

Often more than the supplier does. Lot to lot variation in serum, enzymes, antibodies and even some salts is real, and an assay that drifts after a reorder is a lot change until proven otherwise. Record the lot with every result.

How long do reagents last once opened?

It depends entirely on the reagent and on how it is stored, which is why the opening date on the bottle matters more than a generic rule. Hygroscopic solids, reducing agents and anything light sensitive degrade fastest.

Should we standardise on one supplier?

For convenience and price, largely yes, with a second source for anything whose absence stops work. What should not vary within a study is the supplier and the lot of the reagents the result depends on.

Are two buffers at the same pH interchangeable?

No. Phosphate precipitates with divalent cations and inhibits some enzymes; tris shifts pH with temperature. Choose the species for the chemistry, titrate at the temperature of use and record the recipe rather than the name.

Why is the calcium and magnesium free formulation used for washing?

Because those ions support the adhesion molecules holding cells together and to the surface. Omitting them loosens cells before dissociation; including them keeps tissue intact. Using the wrong one is an invisible protocol error.

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