Peptide Reconstitution Math: Concentration and Dilutions
Reconstitution is part technique, part arithmetic. Getting the math right is what makes a stock solution accurate and your dilutions reproducible.
Once you have chosen a solvent and reconstituted a lyophilized peptide, the next question is purely quantitative: what concentration did you just make, and how do you dilute it to the working concentration your assay needs? The math is straightforward, but small slips here propagate through every downstream result.
The core formula
Stock concentration is mass divided by volume. If you dissolve a known mass of peptide in a known volume of solvent, the concentration is simply mass ÷ volume. For example, 10 mg of peptide dissolved in 2 mL of solvent gives a 5 mg/mL stock. The single most important habit is recording the exact solvent volume you added — a guessed volume makes every later calculation wrong.
Converting to convenient units
Lab work often calls for smaller units. A 5 mg/mL stock is the same as 5,000 µg/mL, or 5 µg/µL. Keeping your units consistent before you calculate a dilution prevents the most common order-of-magnitude errors.
Write the units next to every number as you work. Most reconstitution mistakes are not arithmetic errors — they are unit errors, like mixing mg with µg or mL with µL.
Planning a dilution
To dilute a concentrated stock to a lower working concentration, the standard relationship is C1 × V1 = C2 × V2, where C1/V1 are the concentration and volume of the stock you take, and C2/V2 are the concentration and volume of the diluted solution you want. Solve for V1 — the volume of stock to draw — then add solvent to reach V2.
Worked example: you have a 5 mg/mL stock (C1) and want 1 mL (V2) of a 1 mg/mL working solution (C2). Rearranging gives V1 = (C2 × V2) ÷ C1 = (1 × 1) ÷ 5 = 0.2 mL of stock, topped up with 0.8 mL of solvent to reach 1 mL total.
Accounting for net peptide content
For most purposes the labeled mass is used directly. When higher precision is required, remember that a lyophilized vial is not 100% peptide by weight — it also contains water, salts, and counter-ions. Net peptide content (often on the COA) lets you correct the effective mass if your work demands that level of accuracy.
A quick checklist
- Record the exact solvent volume added to the vial
- Keep all values in consistent units before calculating
- Use C1 × V1 = C2 × V2 to plan dilutions
- Label every prepared solution with concentration and date
This article is provided for laboratory and in-vitro research context only. Pulse Peptide Labs products are not for human consumption, diagnostic, therapeutic, or medical use, and nothing here is medical advice.
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