What aliquoting is for
A reconstituted vial that is frozen and thawed twenty times has been through twenty ice-crystal formation cycles, twenty warmings, and twenty exposures to the air-liquid interface at the stopper. Splitting the same solution into twenty portions and freezing them separately means each portion is thawed once.
The cost is one extra handling session and a set of tubes. The benefit is that degradation stops accumulating across the life of the vial, which for aggregation-prone or otherwise fragile peptides is the difference between a usable last portion and a questionable one.
Sizing the portions
The natural portion size is one use, because that is what makes each thaw terminal. Where a single use is a very small volume, portioning into two or three uses is a reasonable compromise: two thaws is still an enormous improvement on twenty.
The constraint from the other direction is measurement. A portion so small that it cannot be pipetted accurately, or that loses a meaningful fraction of itself to the tube wall, defeats the purpose. Below about 50 microlitres, tube geometry and surface adsorption start to matter.
Why the calculator reserves a waste allowance
Transferring a solution into twenty tubes leaves a little behind at every step: in the pipette tip, on the vial wall, in the transfer. The tool reserves a percentage of the total volume against those losses before dividing, so the count it reports is a count you can actually fill.
Setting the allowance to zero gives the pure arithmetic. Five percent is a reasonable default for careful pipetting into a modest number of tubes, and more is sensible for many small portions.
Storing the portions
Portions should be labelled with the peptide, the concentration, the volume and the date, because a rack of identical unlabelled tubes is unusable within a week. Low-binding tubes are worth using for dilute solutions, since the surface-to-volume ratio in a small tube is much higher than in a vial.
Freezing at minus 20 degrees Celsius is adequate for most peptides in the short term; minus 80 is better for long storage. Whichever you use, the point of aliquoting is defeated if the tubes are stored in a self-defrosting freezer that cycles above freezing.
How the aliquot count is calculated
A concentration, a reserved allowance, and a floor division. The count is floored rather than rounded because a partial aliquot is not one.
concentration = peptide mass (mg) / diluent volume (mL) usable volume = total volume x (1 - waste allowance) aliquot volume (mL) = requested volume, or requested mass / concentration aliquot count = floor( usable volume / aliquot volume ) mass per aliquot = concentration x aliquot volume
- Establish the concentration. From the vial mass and the diluent volume, or entered directly if you already know it. Everything else depends on this figure.
- Reserve the waste allowance. A percentage of the total volume is set aside before dividing, so the reported count survives real transfer losses instead of assuming perfect recovery.
- Resolve the aliquot size to a volume. A size given in millilitres is used directly. A size given in micrograms is divided by the concentration first, so both routes end at the same physical quantity.
- Divide and floor. Usable volume over aliquot volume, rounded down. The remainder is reported separately as leftover rather than being silently absorbed.
- Report mass per aliquot. Concentration times aliquot volume, in both milligrams and micrograms, so the tube label can carry the amount as well as the volume.
What this method cannot tell you
- •It assumes uniform concentration throughout the vial, which requires the peptide to have fully dissolved before splitting.
- •The waste allowance is a flat percentage, not a model of your pipetting. Very small aliquots lose proportionally more than the default assumes.
- •It does not model adsorption to tube walls, which is significant for dilute solutions of hydrophobic peptides in standard polypropylene.
- •It says nothing about how long the frozen aliquots remain usable, which depends on the peptide, the temperature and the container.
Peptide aliquot calculator: frequently asked questions
A measured portion taken from a larger solution. In peptide handling it usually means splitting one reconstituted vial into several small single-use tubes so that each one is frozen and thawed only once.
To stop freeze-thaw damage accumulating. A vial thawed twenty times has been through twenty cycles of ice formation and rewarming; twenty aliquots are each thawed once.
For peptides prone to aggregation or oxidation the difference is substantial, and it is concentrated at the end of the vial's life, where the last withdrawal has taken the most punishment.
Divide the usable volume by the volume of one aliquot and round down. A 2 mL vial split into 0.25 mL portions gives eight, before allowing for transfer losses.
The calculator reserves a waste allowance from the total first, so the number it reports is one you can actually fill.
One use per tube is the ideal, because it makes each thaw terminal. Where a single use is an awkwardly small volume, two or three uses per tube is a sensible compromise.
- •One use per tube: maximum protection, most tubes, most handling.
- •Two to three uses per tube: still a large improvement over a single shared vial.
- •Below roughly 50 microlitres: measurement error and wall losses start to dominate.
Transfer losses. Liquid stays in the pipette tip, clings to the vial wall and is left behind at each step, and those losses are what make the last tube come up short.
Five percent is a reasonable default for careful work with a modest number of tubes. Increase it if you are making many small aliquots, and set it to zero if you want the pure arithmetic.
Sterile polypropylene microcentrifuge tubes are the usual choice. Low-binding tubes are worth the extra cost for dilute solutions, because the surface-to-volume ratio in a small tube is much higher than in a vial.
Screw-cap tubes hold a seal through freeze-thaw cycling better than snap-caps, which matters if the aliquots will sit for months.
Minus 20 degrees Celsius is adequate for most peptides over weeks to a few months. Minus 80 is better for long-term storage.
Avoid self-defrosting freezers. Their defrost cycles warm the contents above freezing repeatedly, which is exactly the cycling that aliquoting was meant to prevent.
Yes. A rack of identical unlabelled tubes becomes unusable within days, and a mislabelled concentration is worse than no label at all.
- •Peptide name
- •Concentration
- •Volume in the tube
- •Date the aliquots were made
The storage label generator produces printable labels carrying exactly these fields.
You can, but it undoes the reason for aliquoting. A tube that is thawed, partly used and refrozen has entered the same cycling pattern as a shared vial, just on a smaller scale.
If this happens routinely, the aliquots are too large. Recalculate with a smaller portion size.
Slowly, at refrigerator temperature or in the hand, then swirl gently rather than shaking. Rapid warming and vigorous agitation both promote aggregation.
Never use a microwave or hot water. The local temperature spike denatures peptide even if the bulk solution never feels warm.
The handling carries a small cost: additional pipetting, more surface contact, and a period at room temperature while you work. It is far smaller than the cost of repeated freeze-thaw cycling.
Work quickly, keep the source vial cold while you split it, and get the tubes into the freezer promptly.
Weighing out milligram quantities of a light, static-prone powder accurately requires an analytical balance and care, and it exposes the powder to atmospheric moisture each time.
For most people, reconstituting once and aliquoting the solution is both more accurate and less damaging than splitting the powder.
The concentration multiplied by the aliquot volume. At 2.5 mg/mL, a 0.2 mL aliquot holds 0.5 mg, which is 500 micrograms.
The calculator reports this in both milligrams and micrograms so it can go straight onto the tube label.
It is reported separately rather than being folded into the count. Usually it is the reserved waste allowance plus whatever does not divide evenly into a whole aliquot.
If the leftover is close to a full aliquot, adjusting the portion size slightly often yields one more usable tube.
It is most worthwhile for peptides that will be used over many sessions, and for those known to be fragile in solution. A vial that will be finished within a few days does not benefit much.
The freeze-thaw risk calculator ranks how much cycling a given peptide is likely to tolerate, which is a reasonable way to decide where the effort is best spent.
Pre-filling syringes introduces its own problems: plunger seals interact with the solution over time, and the barrel is not designed as a storage container.
Tubes are the better container for storage. Draw into a syringe at the point of use.
Only if you enter the corrected mass. By default it uses the figure you give it, which is usually the label amount rather than the actual peptide content.
For work where the absolute amount matters, run the label figure through the net peptide content calculator first.
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