Buy peptides: what to check before you do

A certificate of analysis, an HPLC purity figure, a peptide content figure and a batch number are the four things worth checking before you buy any research peptide. Three of them are numbers. The fourth is what ties those numbers to the vial in front of you.

What a certificate of analysis proves, and what it does not

A certificate belongs to a batch, not to a product name. It records what a laboratory measured in one particular fill: on what day, by what method, with what result. A certificate with no batch number on it is not a certificate, it is an advertisement. It becomes checkable only when the number on the label matches the number on the document, and both can be traced to a laboratory that says the same thing.

What a certificate cannot do matters as much. It says nothing about a fill that came from a different batch. It says nothing about storage or transport after the measurement was taken. And it says nothing about what the substance does: an analysis describes a material, not an outcome. The long version is in our guide to reading a certificate of analysis.

Purity and peptide content are two different numbers

This is where most listings go soft. HPLC purity is a ratio: of the peptide present, what share is the sequence you asked for. Peptide content is a quantity: of the dried material in the vial, what share is peptide at all. The remainder is residual water, counter-ions and salts left behind by the synthesis.

Solid-phase synthesis and the purification that follows it normally leave a peptide as a trifluoroacetate salt, and how much counter-ion it carries tracks the number of positively charged residues in the sequence, which is why removing it takes a deliberate step rather than happening on its own (Pharmaceuticals, 2025). So a lyophilisate can report a very high purity and still hold appreciably less peptide than the label implies. Anyone quoting one of the two numbers is, as a rule, quoting the higher one. Ask for both, and ask by what method the peptide content was determined. There is a separate guide on purity and peptide content.

Identity is a mass, not a name

Mass spectrometry answers a different question from HPLC. It does not say how pure something is; it says which molecule is there. A measured mass that agrees with the mass calculated from the sequence is the actual proof of identity. Without one, the name on the label is only a name on a label.

That is not a theoretical objection. Cyclised and modified peptides trade under the same short name as their linear or unmodified versions, and fragments trade under the name of the full-length compound, although those are different substances with different masses. A listing that gives a name and no mass has not said which one it means.

A chromatogram says more than a percentage

A percentage can be asserted. A chromatogram shows what it rests on: the main peak, the side peaks, the baseline, the resolution between neighbouring signals. Two materials reported at the same purity can have entirely different traces. One large impurity and four small ones are not the same material, and a headline figure hides which of them you have. Whoever shows the trace makes themselves checkable. Whoever gives only the number does not.

Water content

Lyophilised peptides are hygroscopic. Residual water, usually determined by Karl Fischer titration, belongs in the description of a batch and explains part of the gap between purity and peptide content. It is almost never mentioned in a listing, and it is one of the reasons a vial is kept cold and closed until it is used, which the guide to reconstitution and storage covers.

Batch numbers, and what they should let you do

A batch number is not decoration. It should let you do three things: find the certificate covering that exact fill, compare the number on the document with the number printed on your own label, and ask somebody other than the seller whether the result exists. The third is the one that fails most often. A laboratory that publishes a finding under its own report number makes it checkable somewhere the seller does not control, and testing laboratories can hold accreditation to ISO/IEC 17025 from national accreditation bodies (ILAC).

A certificate that exists only as an image on a seller's own website proves little. It is a document, and documents are editable.

What a supplier should be able to answer

• Which batch number does this certificate cover, and is that same number printed on the label?
• When was it measured, and by which method?
• Purity and peptide content, stated separately.
• Is there a mass determination, and does it agree with the calculated mass?
• Is there a chromatogram, or only a figure?
• Does the certificate cover the fill size on offer, or a different one?
• What happens to batches that miss the supplier's own threshold?

The last question tells you more than the others. A supplier whose published batches all pass is publishing a selection, not a test.

Warning signs

A certificate as an image file with no batch number. The same document reused across several fill sizes. A laboratory that is named but cannot be checked. A purity figure with no date. One certificate that covers a product rather than a lot. And statements about what a substance does in the body on a page that also says the material is for laboratory use only: those two cannot both be true.

What research use only means as a legal category

The phrase is neither a formality nor a licence. Whether something counts as a medicine does not turn on its composition alone. Under the UK Human Medicines Regulations 2012 a medicinal product includes any substance "presented as having properties of preventing or treating disease in human beings", and the definition the EMA publishes turns on purpose and physiological action in the same way. The MHRA says as much in its own borderline guidance: the claims made for a product, explicit and implicit, are among the things that decide the question.

The consequence is that presentation moves a product between categories, and selling or offering an unauthorised medicinal product is prohibited outright. A page that calls a material research use only and then describes what it does in people has presented it as a medicine, whatever the footer says. Peptorium makes no statement about what any material does in a body; the research declaration sets out the terms on which anything here is described. None of this is legal advice.

Frequently asked questions

Does a high purity figure mean the vial is mostly peptide?

No. Purity is a share of what the column separated. Peptide content is a share of the material that was weighed out. The second is usually the lower number, and it is the one more often left out.

Is a certificate for the product good enough?

No. A certificate covers one batch. If the document names no batch, or names one that is not on your label, it describes something other than what you are holding.

What if the laboratory report number cannot be found?

Then the result has not been published independently and rests entirely on the seller's word. That may be acceptable, but it is a different thing from a verifiable result.

Does research use only make a compound legal to buy?

It is not a status a seller can confer by writing it on a page. What the material is, how it is presented and what is claimed for it all bear on how the law treats it, and where the answer matters it is a question for a lawyer rather than for a shop.

Why does purity differ between two sellers of the same compound?

Because they are quoting different batches, and possibly different methods. Purity belongs to a lot, not to a compound, so a figure with no batch number attached cannot be compared with anything.

What this page gives you

This page names the compounds that are explained here and sets out how an offer can be examined, whoever is making it: what a certificate of analysis should contain, and which entries in it decide the question.