Peptide Purity: What a Certificate of Analysis Actually Tells You

Ask a peptide seller for proof and you will usually be sent a PDF headed "Certificate of Analysis", with a chromatogram, a number like 99.2%, and a laboratory logo. Most people look at the big percentage, decide it is fine, and stop reading.

The big percentage is the least informative number on the page.

This post is about how to read the rest of it. It contains no doses, no vendors and no product recommendations, because none of those would help you and two of them would compromise us.

Why any of this matters

Start with what happens when someone actually tests what is sold online.

In the closest published analogue we have, researchers bought forty-four products marketed and sold over the internet as selective androgen receptor modulators, then analysed them using anti-doping laboratory procedures. Only twenty-three of the forty-four, 52%, contained any of the compounds they were sold as. Another seventeen, 39%, contained a different unapproved drug entirely, including a growth hormone secretagogue and two other experimental agents [1].

That study is about SARMs rather than peptides, and we are not going to pretend otherwise. It is the same distribution channel, the same regulatory vacuum and the same customer, and it is the best-quality data of its kind. Roughly half the products were not what they claimed to be.

For peptides specifically, the regulator has said something narrower and arguably worse. Reviewing BPC-157, FDA stated that the name is a common name rather than an official adopted one, and that the agency has encountered multiple salts and derivatives, including different active moieties, sold commercially under that same common name [2]. Two products, same label, potentially different molecules, with nobody obliged to tell you which you have.

That is the environment a certificate of analysis is supposed to protect you from. Here is what it can and cannot do.

Purity and content are two different numbers

This is the distinction that does the most work, and the one almost nobody makes.

Chromatographic purity is what the 99% figure nearly always refers to. The sample is run through high performance liquid chromatography, the detector records peaks, and purity is expressed as the area of the main peak as a share of the total detected area. It answers one question: of the material this detector could see, what fraction was the biggest single component?

Peptide content is a different measurement entirely. It asks what share of the powder's mass is peptide at all. Synthetic peptides come out of manufacture carrying counter-ions from the purification process, along with bound water and residual salts. All of that is mass in the vial that is not peptide, and none of it shows up as an impurity peak in the purity assay, because the assay is not looking for it.

So a product can be accurately described as 99% pure and still be materially less than 99% peptide by weight. Both statements are true at once. The purity figure is not a lie; it is an answer to a question most buyers did not realise they were asking.

If a certificate reports only purity and never mentions peptide content, you do not know how much peptide is present. You know what the largest peak was.

Purity is not identity

The second thing a chromatogram does not tell you is what the peak is.

Purity says the sample was mostly one substance. Identity says which substance. Those are established by different methods, and identity is normally confirmed by mass spectrometry, which measures molecular weight and lets the analyst check it against the expected value for the declared sequence.

A certificate carrying a purity chromatogram and no mass spectrometry data is telling you the sample was clean and declining to say what it was clean of. Given FDA's finding that different active moieties circulate under one common name [2], that is not a pedantic objection.

The questions a certificate must answer

Read any certificate against this list. The gaps are the finding.

Does the batch number match the vial? A certificate is a statement about one specific production lot. If the number on the PDF does not appear on the vial in your hand, the document is about somebody else's product. This is the most common failure and the easiest to check.

Who issued it? An independent accredited laboratory, named, with its own contact details, is evidence. A document produced in-house by the seller is a self-assessment. Both are PDFs; only one is testimony from someone with nothing to gain.

Is there an identity test as well as a purity test? Mass spectrometry alongside the chromatogram, as described above.

Is peptide content stated separately from purity? If not, one of the two numbers you need is missing.

When was it tested? Peptides degrade, and a certificate dated long before your batch was filled describes a different moment in that material's life.

Does it cover sterility and endotoxin? For anything intended for injection this is the safety question, and purity and identity do not touch it. FDA's specific concern with compounded peptides is immunogenicity arising from uncharacterised impurities and peptide aggregation, particularly in injectable formulations [2]. A purity percentage does not address that at all.

Is the document machine-generated and complete? Cropped images, missing axes, absent retention times and a chromatogram with no scale are all reasons to treat the number on the front as decorative.

What a clean certificate still does not buy you

Suppose every box above is ticked: independent accredited laboratory, matching batch, identity confirmed, content and purity both stated, recent date, sterility and endotoxin covered.

You now know what was in one sample of one lot on one day. That is genuinely worth having, and it is the ceiling of what the document can do.

It does not make the compound approved. Most of these substances are not authorised medicines in the US or the UK and are not lawful dietary supplements either, which is why they are sold labelled for research use only, a phrase describing the seller's legal position rather than the contents. It does not tell you whether the compound works, which is a question for clinical trials that in most cases have not been run. It does not tell you the risk of using it, which no study has established for most of this market. And if you compete in any tested sport, it has no bearing on the fact that many of these compounds are prohibited at all times [4].

A certificate of analysis is a quality control document. It was never designed to answer the questions people are actually asking when they wave it around, and treating it as a safety guarantee is the most common mistake in the entire category.

The honest summary

Read the batch number first, the issuing laboratory second, and the big percentage last. Ask whether identity was tested, not just purity. Ask what share of the mass is peptide, not what share of the chromatogram is the main peak. And keep the document in proportion: it describes a sample, not a decision.

If you want this reasoning applied compound by compound, The TFC Peptide Guide covers thirty-three of them, including a chapter on judging any provider yourself: what a licensed prescriber looks like, what a real certificate of analysis contains, and the questions that end a conversation with a seller. No doses, no vendor names, no affiliate links.