When you reconstitute a lyophilized peptide, the liquid you choose affects how long the solution stays usable and how many times a vial can be accessed. The three most common laboratory diluents are bacteriostatic water, sterile water and sterile saline. They look identical, but they are not interchangeable.
This article compares them in plain terms. For the technique itself, see our reconstitution guide.
- Bacteriostatic water contains 0.9% benzyl alcohol, which slows microbial growth in a multi-use vial.
- Sterile water has no preservative and is intended for single use once opened.
- Saline contains salt and suits some peptides better than others.
- The best choice depends on the peptide, the protocol and how many times the vial will be accessed.
- Always follow the supplier’s solubility notes for the specific compound.
The three diluents at a glance
| Diluent | Contains | Typical use | Once opened |
|---|---|---|---|
| Bacteriostatic water | Water with 0.9% benzyl alcohol | Multi-access vials | Commonly kept up to 28 days under refrigeration |
| Sterile water | Pure sterile water, no preservative | Single-use preparations | Discard remainder promptly |
| Sterile saline | Water with 0.9% sodium chloride | Where isotonic conditions are needed | Discard remainder promptly |
Bacteriostatic water
Bacteriostatic water is sterile water with 0.9% benzyl alcohol added as a preservative. The alcohol does not sterilise anything; it inhibits growth of microbes that may be introduced each time a needle pierces the stopper. That is why it is preferred when a single vial will be accessed repeatedly over days or weeks.
Benzyl alcohol is not suitable for every experimental system, particularly some cell cultures and sensitive assays. Check that it is compatible with your protocol.
Sterile water
Sterile water for injection or laboratory use contains nothing but water. It is the cleanest choice when preservatives could interfere with an assay. The trade-off is that nothing prevents microbial growth after the vial is opened, so leftover liquid should be used promptly or discarded.
Sterile saline
Saline contains 0.9% sodium chloride, matching the salt concentration of many biological fluids. Some peptides dissolve more readily or stay more stable in a buffered or salted medium, while others can aggregate. Solubility data from the supplier is the best guide.
How to choose
- Read the solubility information for your specific peptide.
- Decide how many times the vial will be opened or pierced.
- Check for compatibility with your assay (preservatives can interfere).
- Record the diluent on the vial label, together with the date.
Whichever you choose, use sterile technique and sterile supplies. Our calculator can help work out the concentration.
Frequently asked questions
Is bacteriostatic water the same as sterile water?
No. Bacteriostatic water contains 0.9% benzyl alcohol as a preservative. Sterile water contains no additive, so it is only suited to single-use preparations.
How long does bacteriostatic water last once opened?
Multi-dose vials of bacteriostatic water are commonly discarded 28 days after first puncture. Check the manufacturer’s labelling for the product you have.
Can I use tap water or distilled water?
No. Tap and ordinary distilled water are not sterile and can contain microbes, endotoxins and minerals that compromise research results.
Does the diluent change the peptide?
It can. pH, salts and preservatives influence solubility and stability, which is why the supplier’s guidance for each compound matters.
Sources and further reading
Products sold by Lodge are intended for laboratory research use only and are not for human or animal consumption. This article is general educational information, not medical or legal advice.
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