What ‘5 mg’ on a Peptide Vial Actually Means
The label states gross lyophilisate mass. Counter-ions and residual water are part of it, and the arithmetic is worth doing once.
A vial labelled 5 mg does not contain 5 mg of peptide. This is not a trick and it is not specific to any supplier — it is a consequence of how peptides are made and how vials are filled, and the arithmetic is worth doing once.
What the number on the label refers to
The stated quantity is gross lyophilisate mass — the total mass of solid in the vial. That solid is not pure peptide. It contains, in descending order:
- The peptide itself
- Counter-ions, usually trifluoroacetate from purification. Typically 5–20 per cent by mass, rising with the number of basic residues in the sequence
- Residual water, a few per cent even after thorough drying
- Trace residual solvent
The fraction that is actually peptide is called peptide content, and for a typical research peptide it runs somewhere between 75 and 90 per cent.
The arithmetic
Take a 5 mg vial at 99 per cent HPLC purity and 82 per cent peptide content:
- Gross lyophilisate: 5.0 mg
- Peptide present: 5.0 × 0.82 = 4.1 mg
- Of which the correct peptide: 4.1 × 0.99 = 4.06 mg
The 99 per cent purity figure is accurate and the label is accurate. They are simply answering different questions — purity asks what fraction of the peptide present is the right one, content asks what fraction of the vial mass is peptide.
Why this matters, and when it does not
For qualitative work — does the compound do the thing at all — a ten to twenty per cent difference in actual mass rarely changes the conclusion.
For quantitative work — dose–response curves, IC50 determinations, anything where concentration is the measured variable — it is a systematic error of exactly that size, applied to every point. It will not average out with replication and it will not be visible in the data.
If concentration accuracy matters, ask for the peptide content figure. It comes from a separate assay — amino acid analysis or nitrogen determination — and not every supplier commissions it. Whether they can answer is itself informative.
Fill tolerance
A second, smaller source of variation. Vials are filled by mass or by volume of a solution before lyophilisation, and neither is exact. Tolerances of a few per cent are normal and are not a defect.
The practical consequence is that a vial labelled 5 mg may contain 4.9 or 5.1 mg of lyophilisate before any of the content arithmetic is applied. For most work this is negligible; for the most precise work, weighing is the answer, and weighing requires an analytical balance and dry handling rather than a label.
Comparing suppliers properly
Price per vial is close to meaningless as a comparison. Two 10 mg vials at the same price are different products if one is 90 per cent peptide and the other 75 per cent.
The comparable figure is cost per milligram of peptide:
cost per mg peptide = price ÷ (vial mass × peptide content × purity)Most suppliers do not publish peptide content, which means for most comparisons you are working with two of the three numbers. That is worth knowing rather than working around — and it is one more reason the willingness to answer a specific analytical question separates suppliers more reliably than their websites do.
Our content versus purity article covers the distinction in detail, the counter-ion article covers where the extra mass comes from, and the pricing article covers what drives cost.
All products and information referenced are for in-vitro research and laboratory use only. Nothing here is medical advice, and no therapeutic claim is made or implied.