Every research peptide supplier in the UK advertises the same figure: 99%+ HPLC verified. We do too. It has become so universal that it has stopped carrying information — if every vendor claims it, it cannot be a differentiator.
It is also widely misread. A 99% HPLC purity figure does not mean 99% of the powder in the vial is peptide. Those are two different measurements, and the gap between them is large enough to matter when you are calculating concentrations.
What HPLC purity actually measures
High-performance liquid chromatography separates the components of a sample and produces a chromatogram — a trace with peaks, each corresponding to a compound eluting at a particular time. Purity is calculated as the area of the target peak as a percentage of the total peak area.
So “99% purity” means: of everything the detector saw, 99% of the signal was your target peptide. It is a statement about the peptide-related content of the sample — the target versus its synthesis by-products, truncated sequences, and deletion sequences.
What the detector does not see is just as important.
The three things HPLC purity ignores
1. Counter-ions
Peptides are almost always supplied as salts. Solid-phase synthesis and the subsequent purification step typically leave the peptide as a trifluoroacetate (TFA) salt, and less commonly as an acetate or hydrochloride. The counter-ion is chemically bound to the peptide but has no chromophore — it is invisible to the UV detector doing the purity measurement.
Depending on the peptide's sequence and how many basic residues it carries, TFA can account for a meaningful share of the total mass of the solid. A peptide rich in arginine or lysine binds more counter-ion than one that is not.
2. Residual water
Lyophilised peptides are hygroscopic. Even a well-sealed vial retains some bound water, and more is absorbed each time a cold vial is opened in a warm room before it has equilibrated. That water contributes to the mass on the balance and nothing to the peptide content.
3. Residual solvent
Traces of acetonitrile and other solvents from the purification step can remain in the lyophilised cake. Usually small, but again invisible to a UV purity measurement.
Net peptide content: the number that answers the mass question
The measurement that tells you how much of the powder is actually peptide is net peptide content, sometimes called peptide content or peptide loading. It is usually determined by amino acid analysis or elemental (nitrogen) analysis, and it is reported separately from purity.
The two figures answer different questions:
| Measurement | Question it answers | Typical method |
|---|---|---|
| HPLC purity | Of the peptide present, how much is the right sequence? | RP-HPLC, UV detection |
| Net peptide content | Of the powder in the vial, how much is peptide at all? | Amino acid or nitrogen analysis |
| Identity | Is it the sequence it claims to be? | Mass spectrometry (LC-MS) |
A vial can legitimately be 99% pure by HPLC and still contain noticeably less peptide by mass than the label suggests, because purity and content are simply not the same axis.
Why this matters for your calculations
If you reconstitute a 10 mg vial with 2 mL of bacteriostatic water, our reconstitution calculator will tell you the concentration is 5,000 mcg/mL. That calculation — like every reconstitution calculator on the internet — assumes the vial contains 10 mg of peptide.
If the vial actually contains 10 mg of lyophilised solid at, say, 85% net peptide content, the true concentration is lower than the arithmetic suggests. For qualitative work that may not matter. For anything quantitative, it does, and the only way to know is to have the figure in front of you.
This is also why suppliers who label vials by peptide mass rather than by total solid mass are doing something meaningfully different, and why it is worth asking which convention a supplier uses.
What to ask a supplier
If you want to know what is actually in a vial, these are the questions that get you there:
- Can I see the batch-specific COA, not a generic one? A specimen COA with no lot number tells you nothing about the vial you received.
- Which lab produced it, and is it independent of the manufacturer? A manufacturer testing its own output is a weaker claim than third-party analysis.
- Does it report net peptide content as well as HPLC purity? Many COAs report only purity.
- Is there mass spectrometry confirming identity? Purity without identity tells you the sample is clean, not that it is the right compound.
- What is the counter-ion? TFA, acetate and hydrochloride salts have different mass contributions.
- Is the vial labelled by peptide mass or total solid mass?
A supplier who cannot answer these is not necessarily selling you something bad. But they are asking you to take the number on the label on trust, and you should know that is what is happening.
Where we stand
We state plainly on every product page that our compounds are HPLC verified by our supplier to a 99%+ purity standard. That wording is deliberate: it is a manufacturer's figure, not an independent one, and we would rather say so than imply a level of verification we have not personally commissioned.
Every product page carries the CAS number, molecular formula, appearance and storage conditions for the compound — see Retatrutide 20mg or Tesamorelin 10mg for the format — and the full range is at All Peptides.
All compounds supplied by ASPeptides are for in vitro laboratory research use only. They are not medicinal products and are not approved for human or veterinary use.