A stock solution is a concentrated, well-characterised solution that working solutions are diluted from. Preparing one carefully, once, is more accurate than weighing small amounts for every experiment, and it keeps a series of experiments consistent.
1. Choose the stock concentration
Pick a concentration 100 to 1000 times higher than your highest working concentration. That keeps transfers in a comfortable pipetting range and keeps any co-solvent low in the final solution. The stock must also stay below the compound’s solubility in the chosen solvent, with some margin.
2. Account for purity and net content
Lyophilised peptides are rarely 100% peptide by mass. The powder also contains counter-ions (often trifluoroacetate or acetate) and residual water. Certificates of analysis may report:
- Purity (often by HPLC): the fraction of peptide-related material that is the target sequence.
- Net peptide content: the fraction of the powder’s mass that is peptide. Values well below 100% are common for salt forms.
To make a solution of known peptide concentration, divide the required mass by the net content (as a fraction):
mweigh = (c × V × M) / p
Worked example
Target: 10 mL of a 10 mM GHK stock (M = 340.38 g/mol), from powder with 80% net peptide content.
Peptide needed: 0.010 mol/L × 0.010 L × 340.38 g/mol = 34.04 mg. Mass to weigh: 34.04 ÷ 0.80 = 42.55 mg.
In 500 µL aliquots, the stock fills 20 tubes.
3. Weigh accurately
- Let a cold vial reach room temperature before opening, so moisture does not condense on a hygroscopic powder.
- Weigh well above the balance’s minimum weight. If the required mass is too small, make a larger volume or a higher concentration and dilute later.
- Record the actual mass weighed, then recalculate the actual concentration from it. Do not chase the exact target mass.
4. Dissolve, then make up to volume
Choose the solvent using the supplier’s guidance and the peptide’s properties (see amino acid properties). As general laboratory practice:
- Peptides with a net positive charge often dissolve in water or dilute acetic acid. Those with a net negative charge often dissolve in water or a dilute basic buffer.
- Hydrophobic peptides may need a small volume of an organic solvent such as DMSO first, followed by slow dilution with buffer while mixing.
- Test solubility on a small portion first when you are unsure.
For accurate molarity, dissolve in part of the final volume and then make up to the mark. When the whole contents of a vial are dissolved directly, use the volume of solvent mode of the calculator. For dilute solutions the volume change on dissolving is negligible.
5. Aliquot, label and record
- Divide into single-use aliquots to avoid repeated freeze–thaw cycles.
- Label each tube with the compound, concentration (with units), solvent, date and your initials.
- In your notebook, record the lot number, the mass actually weighed, the purity or net content used, the final volume and the calculated concentration.
Common mistakes
- Ignoring net peptide content, which can leave the real concentration 10–30% below the label.
- Calculating with the salt-form molar mass and correcting for net content. That counts the counter-ion twice.
- Making a stock too concentrated for the peptide’s solubility, then diluting a suspension.