When buyers compare peptide prices, the listed milligram count and the dollar figure are usually all they see. What those numbers represent, however, depends on three distinct measurements that vendors report inconsistently — and sometimes not at all. Understanding purity, potency, and net content as separate concepts is the prerequisite for any meaningful cost comparison.
Purity: what fraction of the solid is the target molecule
Purity describes the proportion of the material that is the intended peptide sequence, expressed as a percentage. The remainder consists of impurities: truncated sequences, deletion peptides, oxidation products, residual solvents, and synthesis by-products. Purity is most commonly measured by high-performance liquid chromatography (HPLC), which separates components by retention time and reports area-under-the-curve percentages. Third-party laboratories such as Janoshik and those catalogued by Finnrick routinely publish HPLC results on tested vials.
A product reported at 98% purity by HPLC contains, on paper, 98 parts target peptide per 100 parts total solid. The nature of the remaining 2% matters: some impurities are inert, others are structurally related fragments that may complicate interpretation of research results. Regulatory-grade peptide manufacturers such as Bachem and PolyPeptide Group publish detailed impurity profiles alongside purity figures, a standard that research-use suppliers rarely match.
Potency: biological or mass-based activity
Potency is a separate dimension that asks not "what fraction is my peptide?" but "how active is the peptide that is present?" In pharmaceutical manufacturing, potency is defined by the USP and similar pharmacopoeias as a measure of biological activity relative to a reference standard. For most research-grade peptides, however, "potency" is used informally to mean the confirmed mass of active material per stated unit — effectively a check on whether the label claim is accurate.
Some laboratories confirm potency through mass spectrometry (MS), which verifies molecular weight and sequence. Others use bioassay methods where a reference standard exists. The distinction matters because a sample can be high-purity (few impurities) but low-potency if the peptide itself is degraded, incorrectly stored, or partially oxidized in ways HPLC does not fully capture. It is well established in analytical chemistry that HPLC purity and biological activity can diverge meaningfully, underscoring that purity alone is not a complete characterization.
Net content: the number on the label versus what is actually present
Net content (sometimes called net peptide content or peptide content by weight) corrects for water and salt adsorbed into the lyophilized powder. Lyophilized peptides are hygroscopic — they absorb atmospheric moisture — and are frequently supplied as acetate or trifluoroacetate (TFA) salts from the synthesis process. A vial labeled "10 mg" may contain 10 mg of total solid, but the actual peptide mass can be substantially lower once moisture and counterion mass are subtracted.
Industry sources describe a common working estimate of roughly 70–85% net peptide content for typical lyophilized research peptides when water and salt are accounted for, though the figure varies by sequence, counterion, and storage history. Manufacturers like Bachem publish net peptide content as a certified specification. Most research-use vendors do not, meaning the stated milligram figure is total powder weight, not confirmed peptide mass.
Why price-per-milligram comparisons mislead
The interaction of these three measurements is what makes raw price comparisons unreliable.
| What you see | What it does not tell you |
|---|---|
| Price per mg (label weight) | Net peptide content; how much is actually peptide |
| HPLC purity % | Potency / biological activity; presence of TFA or moisture |
| "Third-party tested" | Which lab, which method, whether the lot matches |
| Lowest $/mg | Whether a competing product at higher price delivers more actual peptide |
A vial priced at half the cost but carrying 75% purity and no net-content correction can deliver less confirmed peptide mass than a more expensive vial at 99% purity with a stated net content of 85%. Aggregators including VialAudit and PeptideBenchmark have begun publishing side-by-side test results specifically to surface these gaps, and community-sourced databases are extending that coverage.
For research applications where reproducibility matters, practitioners and institutions increasingly request certificates of analysis that report all three figures — purity by HPLC, sequence confirmation by MS, and net peptide content — rather than relying on label weight alone.
Sources
- Bachem AG — quality and analytical standards overview: https://www.bachem.com
- PolyPeptide Group — peptide manufacturing and release testing: https://www.polypeptide.com
- Janoshik Analytical — third-party peptide testing: https://www.janoshik.com
- Finnrick — community test data aggregation: https://www.finnrick.com
- VialAudit — vendor and lot-level test tracking: https://www.vialaudit.com
- U.S. Pharmacopeia (USP) — potency and reference standards: https://www.usp.org