The concentration of a reconstituted peptide vial is fixed by two numbers and one division: concentration (mg/mL) = mass in the vial (mg) divided by the volume of solvent added (mL). Nothing about the peptide changes when you choose a different volume — only how concentrated the resulting solution is.
The two formulas
Concentration from volume:
concentration (mg/mL) = vial mass (mg) ÷ solvent volume (mL)
Volume for a target concentration:
solvent volume (mL) = vial mass (mg) ÷ target concentration (mg/mL)
Worked examples: a 100 mg vial
| Solvent added | Resulting concentration | Per 0.1 mL |
|---|---|---|
| 1 mL | 100 mg/mL | 10 mg |
| 2 mL | 50 mg/mL | 5 mg |
| 3 mL | 33.3 mg/mL | 3.33 mg |
| 5 mL | 20 mg/mL | 2 mg |
| 10 mL | 10 mg/mL | 1 mg |
The practical trade-off: a larger volume gives a more dilute solution that is easier to measure accurately in small aliquots, but uses more solvent and leaves a larger volume to store. A smaller volume concentrates the material but makes small-volume transfers less precise.
Units: mg, mcg and mL
- 1 mg = 1000 mcg (micrograms)
- 1 mL = 1000 microlitres
- A 50 mg/mL solution contains 50,000 mcg per mL, or 5000 mcg per 0.1 mL
IU is a different kind of unit entirely — it measures biological activity, not mass, and cannot be converted to milligrams without a substance-specific factor. This matters for material supplied in IU, such as HCG. Definitions are in the peptide glossary.
The correction most people skip
The formulas above use the label weight, which is gross powder mass. The powder also contains counter-ions, bound water and residual salts, so the actual peptide mass is lower:
peptide mass (mg) = vial mass (mg) × net peptide content (%)
A 100 mg vial at 80% net peptide content holds 80 mg of peptide. Reconstituted in 2 mL, the true peptide concentration is 40 mg/mL, not 50 mg/mL — a 25% overestimate if you use the label figure. For qualitative work this may not matter; for anything quantitative it does. Full explanation in net peptide content vs gross weight.
Serial dilution
To go from a stock solution to a lower working concentration, the relationship is C1 × V1 = C2 × V2.
Example: from a 50 mg/mL stock, to prepare 1 mL at 5 mg/mL — V1 = (5 × 1) ÷ 50 = 0.1 mL of stock, made up to 1 mL with solvent.
Practical notes
- Add solvent slowly down the vial wall, not directly onto the powder cake, and swirl rather than shake — foaming denatures peptide at the air-liquid interface.
- Let the vial reach room temperature before opening. Cold powder condenses moisture.
- Record the volume you added on the vial. The concentration is unrecoverable later if you do not.
- Bacteriostatic water is the standard solvent when the vial will be entered more than once; preservative-free water is for single-use preparations.
Full procedure: reconstituting lyophilized peptides. Solvent comparison: bacteriostatic water vs sterile water vs WFI.
All products are intended for laboratory and research use only. They are not medicines, are not for human or veterinary use, and are not for diagnostic or therapeutic application. Nothing here is guidance for use in humans.