The journal
Reconstitution arithmetic, done properly
Reconstitution is one equation. Almost every error made with it is a units error, and almost every units error is made in the same three places. Here is the arithmetic in full, and where it goes wrong.
The equation
Concentration is mass divided by volume:
C (mg/mL) = mass (mg) ÷ volume (mL)
A 10 mg vial reconstituted with 2 mL of diluent gives 10 ÷ 2 = 5 mg/mL. Multiply by a thousand for micrograms per millilitre: 5,000 mcg/mL. Divide by a hundred for the mass sitting in one 0.01 mL graduation: 50 mcg.
That is the whole of it. The calculator does this and nothing else, including a reverse solver for working back from a target concentration to the volume of diluent needed.
Error one: confusing mg and mcg
A thousandfold error, and the easiest one to make because both units appear on the same certificate and in the same conversation. 1 mg = 1,000 mcg. A figure that looks a thousand times larger or smaller than expected is almost never a surprising result; it is this.
Error two: the volume you added is not the volume you have
Dissolving a solid displaces liquid. For the milligram quantities in these vials the displacement is negligible — ten milligrams of peptide occupies a volume too small to matter against two millilitres — but the habit of assuming that is worth examining rather than inheriting. Where the solid is a substantial fraction of the final volume, added volume and final volume are different numbers and the distinction is real.
The related and more common version: reading the barrel of the measuring device rather than the meniscus, or measuring to a graduation while air sits in the line.
Error three: nominal mass is not measured mass
A “10 mg” vial is a fill target. The certificate records what was actually there. And if you are working from net peptide content rather than gross powder mass, the two figures diverge further — a powder that is 80% peptide by mass gives a real concentration a fifth below the one the label arithmetic predicts.
Which of those numbers is the right one to divide by depends on the work. That is a decision for the researcher; our job is to make sure the numbers on the certificate are true.
Why the calculator refuses to round prettily
A 10 mg vial in 3 mL is 3.333 mg/mL. Not 3.33, and not 3.3333333.
Our calculator rounds to four significant figures and drops trailing zeros, because a decimal place you did not measure is a claim about precision you do not have. The vial mass is a nominal fill. The equipment you measured the diluent with has a tolerance of its own. Printing 3.33 implies three-figure confidence; printing 3.3333333 implies eight. Four is already generous.
What the calculator will never do
It takes no target quantity to administer. It returns no volume to withdraw for one. It speaks in no unit that belongs to administration equipment.
Those outputs are administration guidance, and everything sold here is supplied for laboratory research use only. Concentration is the honest half of the arithmetic and the half that belongs on this site. The other half is not a feature we have not got round to building — it is one we have decided against.
Handling, while you are here
- Direct the diluent down the wall of the vial, not into the powder cake. Peptides are surface-active and a stream driven into the cake will foam it.
- Swirl gently or simply wait. Never shake. Agitation costs you material to foam, and what the foam takes does not come back.
- Label the vial the moment it is reconstituted: compound, lot, concentration, date. An unlabelled vial in a fridge is an unknown within a week.
All products supplied by Alpha Aminos are for laboratory research use only and are not for human or animal consumption. Nothing on this page describes or is intended to describe an effect of any compound. No statement here has been evaluated by the Food and Drug Administration, and nothing here is intended to diagnose, treat, cure, or prevent any disease.