Peptide calculator: solution concentration

Research peptides are supplied as a lyophilizate, a freeze-dried powder sealed in a glass vial. Before any analytical work, the powder is dissolved in a suitable solvent, for example sterile water for laboratory use or an appropriate buffer, to produce a stock solution. The concentration of that solution follows from just two values: the mass of peptide in the vial and the volume of solvent added.

The calculation itself is simple division: concentration (mg/ml) = mass of lyophilizate (mg) divided by volume of solvent (ml). If a vial contains 10 mg of peptide and you add 2 ml of solvent, the resulting stock solution has a concentration of 5 mg/ml. Because 1 mg equals 1000 µg and 1 ml equals 1000 µl, the same concentration can be written as 5 µg/µl, which is often more convenient when working at microliter scale.

The second calculation works in reverse. When an experiment calls for a defined amount of peptide, the required volume of stock solution is: volume (µl) = amount (µg) divided by concentration (µg/µl). To obtain 250 µg from a 5 µg/µl stock, you would pipette 50 µl. The calculator above performs both calculations and flags results that fall outside a practical pipetting range.

For accurate results, work with a calibrated pipette, let the vial reach room temperature before dissolving, and label each vial with the batch, the solvent and the preparation date. At the masses typical for research vials, the dissolved powder adds a negligible volume, so the volume of solvent can be treated as the total volume of the solution.