Step 1: Read the Vial Before You Draw
Every calculation starts with two numbers from the label and COA: the fill mass (total lyophilized weight) and the net peptide content (the fraction that is actual peptide, typically 70–90%). Dose math that ignores content overstates the delivered peptide by 10–30%, which is the single most common source of irreproducible results at the bench.
Check the batch number on the vial against the Certificate of Analysis before anything else. If they do not match, stop and contact the supplier — a mismatch means you cannot trust any other number in the document.
Step 2: Choose the Right Solvent
Bacteriostatic water (sterile water with 0.9% benzyl alcohol) is the default for multi-use storage: the preservative keeps the solution stable across repeated draws for 1–2 weeks refrigerated. Sterile water or saline suits single-use experiments.
Sequence-specific exceptions: cationic peptides dissolve better with a splash of dilute acetic acid; hydrophobic or aggregation-prone sequences may need DMSO first, then dilution (keep DMSO under ~10% for cell work). Gentle inversion or brief sonication beats vortexing, which foams and denatures.
Step 3: The Arithmetic, Worked
Concentration = net peptide mass ÷ solvent volume. Example: a 5 mg vial at 80% content contains 4 mg peptide; adding 2 mL gives 2.0 mg/mL. On a U-100 insulin syringe (100 units = 1 mL), each unit then delivers 0.02 mg.
Work backwards from your target dose: units = (target mg ÷ concentration) × 100. Write the whole chain down before the first draw — arithmetic errors made while holding a syringe are not recoverable.
Step 4: Storage and Shelf Life
Lyophilized peptide: 18–24 months at −20 °C, protected from light and moisture. Reconstituted: 1–2 weeks at 2–8 °C in bacteriostatic water, or 2–3 months frozen in single-use aliquots. Never refreeze a thawed vial — aliquot before freezing instead.
Light-sensitive families (melanocortins) and rapidly degrading sequences (some incretin analogues) warrant the conservative end of every range. Label each aliquot with sequence, batch, concentration, solvent and date.