Peptide purity versus potency

Purity and potency are used interchangeably in research peptide marketing and they measure entirely different things. A 99+% pure peptide can be inactive; an 85% pure one can perform well in an assay. Understanding the distinction prevents both over-reliance on a purity figure and the assumption that a disappointing assay means a bad batch. All Alphex products are for laboratory and in-vitro research only.

Purity

An analytical measurement: what fraction of the peptide content is the target sequence, calculated from HPLC peak areas at 220 nm. It is a property of the material as supplied, measurable by any competent laboratory, and reported on the Certificate of Analysis. It says nothing about biological activity. See how purity is calculated.

Potency

A biological measurement: how much effect the material produces in a defined assay, at a defined concentration, relative to a reference. Potency is a property of the molecule in a system, not of the vial, and it can only be measured by running the assay. In pharmaceutical development it is determined with validated bioassays; in research it is usually implicit in whatever readout the laboratory uses.

Why they come apart

• Concentration error. The commonest cause of "low potency" in a pure peptide. Gross vial mass is not net peptide mass, so a nominal 5 mg/mL stock may be 4 mg/mL of peptide. See net versus gross.
• Adsorption and aggregation. Material lost to tube walls or to aggregates never reaches the assay. See adsorption.
• Degradation after receipt. A pure peptide stored badly becomes a less pure one. See recognising degradation.
• Stereochemistry. A peptide of correct mass and high HPLC purity can be the wrong isomer and inactive. This is why the reference-standard comparison matters. See D-amino acids.
• Active impurities. In the other direction, an impurity that is itself active can flatter a batch's apparent potency, which is one argument for a tight purity specification. See 98% versus 99+%.
• Assay conditions. Buffer, temperature, cell passage, serum content and incubation time move potency readings far more than a percentage point of purity.

What a supplier can and cannot promise

Alphex certifies purity and identity: 99+% by HPLC at 220 nm against a reference standard, batch-matched, independently tested. That is what the purity guarantee covers. We do not certify potency, because potency depends on your assay, your handling and your concentration determination, none of which we control. A supplier claiming a "potency guarantee" for a research reagent is either promising something unverifiable or describing purity in different words.

Diagnosing a disappointing result

1. Check the concentration arithmetic and the net-content assumption. See calculating concentration.
2. Prepare fresh from a sealed vial and repeat; if the fresh preparation works, the problem was the stock, not the batch.
3. Inspect for haze; check plastic and dilution steps for adsorption losses.
4. Run the vehicle and scrambled controls. See controls and design.
5. If all of that is clean and the material still underperforms, re-analyse it by HPLC against the certificate and contact us with the batch number and trace.

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 23 July 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.