How often should research peptides be retested?
Retest intervals for supplier stock and laboratory stock, what a retest involves, and how ageing batches are handled.
A Certificate of Analysis describes a batch on the day it was tested. Material stored correctly changes very slowly, but it does change, and a laboratory that holds a vial for a year, or a supplier that holds a batch for eighteen months, should have a view on when the original result stops being a good description. This note sets out sensible retest intervals for lyophilised peptides in supplier stock and in the laboratory, what a retest involves, and how Alphex handles ageing stock. All Alphex products are for laboratory and in-vitro research only.
How fast lyophilised peptides change
Very slowly, if sealed and cold. Manufacturer stability data on freeze-dried peptides at -20 C typically show no measurable change over two to three years; at 2-8 C over one to two years; at ambient over months. The exceptions are peptides with methionine, tryptophan or cysteine (oxidation), free N-terminal glutamine (pyroglutamate formation), or a hygroscopic salt form in a compromised seal. See deamidation and oxidation in stored peptides.
Retest intervals for supplier stock
A reasonable policy for a research peptide supplier holding sealed vials at -20 C:
- Test at vialling: the batch COA.
- Retest at 12 months if the batch is still in stock, by HPLC at minimum; issue an updated certificate with the new date if it still meets specification, withdraw it if not.
- Retest sooner for known-sensitive compounds or if storage was interrupted.
Alphex's catalogue turns over faster than that for most compounds, so batches rarely reach a retest date; where one does, it is retested before further sale and the COA date reflects it. A supplier whose COA for a compound is two years old and whose stock has "never run out" is either holding an enormous batch or not testing. See why batch matching matters.
Retest intervals in the laboratory
- Sealed vials at -20 C: usable against the original COA for at least 12-24 months. Beyond that, treat as "probably fine" and verify by comparison with a fresh vial or by HPLC before critical work.
- Sealed vials at 2-8 C: 6-12 months, then verify.
- Reconstituted stocks: not retested; replaced. Weeks refrigerated, months as single-thaw frozen aliquots. See how long reconstituted peptides last.
- Any vial with a compromised seal or a temperature excursion: verify before use regardless of age.
What a retest involves
At minimum, reverse-phase HPLC at 220 nm under the original method, compared to the original chromatogram: same main-peak retention, no new peaks above the reporting threshold, purity still at or above specification. Mass spectrometry if there is any doubt about identity or if new peaks appear. Amino acid analysis is not needed for a stability retest. Any contract laboratory can do this on a milligram of material. See stability testing of peptides.
Frequently asked questions
Can I ask Alphex how old a batch is?
Yes; the COA carries the test date, and we can tell you the vialling date for any batch number.
If a batch is retested and passes, is the COA reissued?
Yes, with the retest date and results added, so the certificate describes the material as it is now.
Does the purity guarantee cover old stock?
The purity guarantee applies to material as shipped, tested against the certificate supplied with it, within a reasonable window; it is not a promise about how a laboratory stores it afterwards.
Sources and further reading
- ICH Quality Guidelines (Q1A stability, Q3A/Q3B impurities, Q6 specifications): the reference framework for purity, impurity and stability testing
Alphex research peptides 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 17 February 2026, last reviewed 16 August 2026. Alphex reviews research notes when the testing method, the catalogue or UK guidance changes.