What purity actually means

01 Two numbers, one label

Almost every research peptide sold anywhere carries a single headline figure — 98%, 99%, sometimes 99.5% — and almost every buyer reads it as the fraction of the vial that is peptide. It is not that, and the gap between what it says and what people assume it says is the most consistently misunderstood thing in this market.

The purity figure on a certificate of analysis is a chromatographic ratio. It describes how much of what was detected is the compound you ordered, as against related impurities. It says nothing at all about how much of the powder in the vial is peptide, because the things that make up the remaining mass are largely invisible to the instrument that produced the number.

There is a second figure that answers the mass question, called net peptide content, and it is reported far less often. A vial can honestly be 99% pure by HPLC and around 80% peptide by weight at the same time, with no contradiction and nobody lying.

This guide is about that difference: where the missing mass goes, when it matters, and what to ask.

02 What the purity figure measures

In reverse-phase HPLC the sample is separated into its components, and a UV detector records what comes off the column and when. The result is a chromatogram — a baseline with peaks on it. Purity is calculated by integrating the area under the main peak and expressing it as a percentage of the total area under all peaks.

That is a relative measurement, and the word doing the work is "relative". It compares the target peptide against the other things the detector saw. It is an excellent measure of synthesis quality: it catches deletion sequences, truncations, oxidised variants and incompletely deprotected material, which are exactly the impurities that matter for whether you received the right molecule.

What it cannot see is anything that does not absorb UV at the detection wavelength, or does not stay on the column at all. Peptide bonds absorb strongly at 214 nm, which is why that wavelength is used. Water does not. Inorganic salts do not. The counterion left over from purification does not, and it elutes in the void volume regardless.

So the number is accurate, and it is answering a narrower question than it appears to answer. Guide 03 covers how to read the chromatogram itself and how to spot a certificate that was never produced by an instrument.

A purity figure quoted without a wavelength, a method and a retention time is not a measurement, it is a claim. The chromatogram is the evidence; the percentage is a summary of it.

03 Net peptide content

Net peptide content — sometimes called peptide content assay — is the figure that answers the mass question directly. It states what proportion of the dry weight in the vial is actually peptide, with everything else counted as everything else.

It is determined by a different class of method from HPLC. Amino acid analysis hydrolyses the peptide into its constituent residues and quantifies them, which gives an absolute measurement rather than a relative one. Nitrogen determination and quantitative UV against a known extinction coefficient are also used.

For a typical lyophilised research peptide the figure usually lands somewhere between 70% and 90%. That is not a defect and it is not evidence of adulteration — it is the normal state of a solid-phase-synthesised, HPLC-purified, freeze-dried peptide, and the same is true of material from every reputable supplier in the world.

The reason it is reported less often than purity is straightforward: it requires an additional assay, it costs more, and it produces a number that looks worse to a buyer who has not read anything like this page. A supplier reporting 99% purity and a supplier reporting 99% purity with 82% net peptide content may be selling identical material, and the second one is telling you more.

04 The counterion

The largest single component of the non-peptide mass is usually the counterion, and it gets there as a direct consequence of how the peptide was purified.

Preparative reverse-phase HPLC uses trifluoroacetic acid as an ion-pairing agent. It improves peak shape and resolution, and it is very good at its job, which is why it is the near-universal default. The consequence is that the peptide leaves the column as a trifluoroacetate salt, with TFA associated with every basic site on the molecule.

How much mass that represents depends on how many basic residues the sequence carries. A peptide rich in arginine and lysine binds more counterion than a neutral one, and TFA content can range from a few percent to something north of twenty. For a highly basic peptide the counterion alone can account for a fifth of what you weighed out.

TFA is also not biologically inert. It is cytotoxic in cell culture at concentrations that are entirely achievable when a peptide stock is carried into the medium, and it can confound results in exactly the assays research peptides are typically used for. Where that matters, salt exchange to acetate or hydrochloride is available, and it is worth specifying rather than assuming.

05 Water, and where the rest of the mass goes

Lyophilisation removes most of the water, not all of it. A freeze-dried peptide typically retains a few percent by mass as residual moisture, and hygroscopic compounds retain more — and then continue absorbing whatever the atmosphere offers every time the vial is opened.

This is the practical reason for the advice, repeated across these guides, to let a vial reach room temperature before breaking the seal. Opening a cold vial condenses atmospheric moisture directly into the powder, which both adds mass and accelerates degradation. NAD+ is the clearest case in the catalogue: it is strongly hygroscopic, and moisture is its primary route to breaking down.

The remaining balance is residual solvent from synthesis and purification, and any inorganic salt carried through. Both are typically small, and both are invisible to the UV detector that produced the purity figure.

None of this is unusual and none of it is hidden. It is simply what a vial of lyophilised peptide is made of.

06 A 10 mg vial, worked through

Take a vial labelled 10 mg at 99% purity, with a moderately basic sequence. The figures below are illustrative rather than specific to any product, but they are representative.

Labelled mass

10 mg of lyophilised powder in the vial

HPLC purity

99% — of the UV-absorbing material detected, 99% is the target peptide

TFA counterion

roughly 12% of total mass, invisible to the purity assay

Residual water

roughly 5% of total mass

Residual solvent and salt

roughly 1% of total mass

Net peptide content

roughly 82% — approximately 8.2 mg of actual peptide

Both figures are true

99% pure and 8.2 mg of peptide describe the same vial from different angles

This is why concentration calculated from the label is an approximation. For qualitative and comparative work that is entirely adequate. For quantitative work where absolute concentration is the measurement, you need the net peptide content figure and should calculate from that instead.

07 When the difference matters

Most of the time it does not, and it is worth being clear about that rather than manufacturing anxiety about a normal property of a normal product.

It does not matter much for comparative work, where the same vial is the reference for every condition and a systematic offset cancels out. It does not matter for qualitative questions about whether a compound does something at all.

It matters when the absolute concentration is the result — dose-response curves, EC50 determination, anything where a number is being reported rather than a direction. An 18% overestimate of concentration propagates straight into every value derived from it.

It matters when TFA is a variable, particularly in cell culture, where the counterion carried in from the stock can produce effects that get attributed to the peptide.

And it matters when comparing suppliers. Two vials with the same label and the same purity figure can differ meaningfully in how much peptide they contain, and the only way to see that is a figure most suppliers do not publish.

08 What to ask

A supplier who can answer these without hesitation is telling you something about their analytical chain. One who cannot is also telling you something.

• Can I see the actual chromatogram for this batch, not a summary figure?
• At what wavelength was purity determined, and by what method?
• What is the net peptide content, and how was it determined?
• What is the counterion, and what is its approximate mass fraction?
• Is salt-exchanged material available if TFA is a problem for my application?
• What is the residual moisture content?
• Does the batch number on this vial match the batch number on the certificate?
• Was the analysis performed in-house or by an independent laboratory?