Sterile water, bacteriostatic water or saline?

Three clear liquids in near-identical vials, and the choice between them decides how long a peptide stock lasts, whether it interferes with an assay, and in some cases whether the peptide dissolves at all. This note compares sterile water, bacteriostatic water and saline (with and without preservative) as diluents for research peptides, and gives a decision rule for each situation. All Alphex products are for laboratory and in-vitro research only.

The three, defined

• Sterile water for reconstitution. Purified water, sterilised, no additives. Single-use: once the vial is opened there is nothing to inhibit microbial growth.
• Bacteriostatic water. Sterile water with 0.9% w/v benzyl alcohol as a preservative, in a multi-dose vial. Usable for 28 days after first puncture. See the bacteriostatic water guide.
• Saline. 0.9% w/v sodium chloride in sterile water; isotonic. Available unpreserved (single-use) or as bacteriostatic saline with 0.9% benzyl alcohol (multi-use).

How they compare

• Shelf life of the stock: bacteriostatic water and bacteriostatic saline support refrigerated multi-sample stocks for two to four weeks; sterile water and plain saline are single-session or a few days.
• Assay interference: sterile water has none; benzyl alcohol interferes with cell culture at high concentration and with UV detection; sodium chloride adds ionic strength, which some assays want and some do not.
• Solubility: water and saline behave similarly for most short peptides. Ionic strength can slightly reduce the solubility of very hydrophobic or lipidated peptides (salting out) and slightly improve it for others; in practice, water first, buffer second, saline where isotonicity is the point.
• pH: none of the three is buffered. The reconstituted stock takes the pH the peptide's counter-ions give it. If pH matters, use a buffer; see how to make a buffer for reconstitution.
• Copper peptides: GHK-Cu is happy in any of the three; avoid phosphate and chelators rather than these.

Decision rule

1. Will the stock be sampled more than once over more than a couple of days? Yes: bacteriostatic water (or bacteriostatic saline if isotonicity is needed). No: sterile water.
2. Is the assay cell-based, preservative-sensitive, or analytical at 220 nm? Yes: sterile water or buffer for the working dilution, and calculate the final benzyl alcohol concentration if the stock was bacteriostatic.
3. Does the assay need defined pH or ionic strength? Yes: buffer, or saline for isotonicity, prepared as described in the buffer note.
4. Is the peptide lipidated or otherwise poorly soluble? Water alone may be hazy; a pH 7.4-8 buffer usually behaves better. See the solubility guide.

Common mistakes

• Using sterile water for a stock, then sampling it for three weeks. It will grow.
• Using bacteriostatic water for a cell-culture stock that will be added neat or at 1:10; that is 0.09% benzyl alcohol in the well, which many cells will not tolerate.
• Assuming "saline" means "buffered". Plain saline has no buffering capacity.
• Buying "bacteriostatic water" that turns out to be unpreserved sterile water relabelled. Check the label states benzyl alcohol 0.9% and a lot number.

Frequently asked questions

Which does Alphex sell?

Bacteriostatic water, 0.9% benzyl alcohol, 10 mL, because it is what most laboratories need for multi-sample peptide stocks. Sterile water and buffers are widely available from general laboratory suppliers.

Can I make bacteriostatic saline from bacteriostatic water?

Adding 0.9 g sodium chloride per 100 mL gives isotonic preserved saline; filter-sterilise afterwards. Only worth doing if isotonicity matters to the assay.

Which is best for GLP-3 (RT)?

A mildly alkaline buffer added slowly. If using water, bacteriostatic is fine for multi-use, but expect to need patience and no agitation. See the GLP-3 (RT) guide.

Alphex products are supplied for laboratory and in-vitro research by qualified researchers only. They are not authorised for human or veterinary use.

Research use only. Alphex products are supplied exclusively for laboratory and in-vitro research by qualified researchers. They are not authorised for human or veterinary use, and nothing in this note is advice on any such use.

Published 18 May 2026. Alphex reviews research notes when the testing method, the catalogue or UK guidance changes.

Alphex Research Team

Alphex Limited, UK research peptide supplier

The Alphex research team writes and maintains these notes for UK laboratories buying research peptides. Every batch Alphex lists is verified by reverse-phase HPLC with a batch-matched Certificate of Analysis. Content covers laboratory and in-vitro research only.