How to read a peptide certificate of analysis

Everything else on a vendor's website is a claim about a product. A certificate of analysis is a claim about one specific batch of that product, attributed to whoever ran the tests, and it either holds up under reading or it does not. That makes it the one piece of evidence worth learning to read properly.

This guide covers what the two core tests prove, what the headline purity figure does not tell you, which tests are routinely left out, and how to tell a laboratory report from a marketing document with a chromatogram pasted into it.

What the document actually is

A COA records the results of tests performed on a single production lot. It is not a certificate of quality, it is not a regulatory approval, and it is not a statement that the material is safe to put in a human being. It is a set of measurements, and it carries exactly as much weight as the laboratory that produced it and the lot number printed on it.

Two things follow from that. A COA that cannot be tied to the vial in your hand tells you nothing about the vial in your hand. And a COA produced by the seller, about the seller's own product, with no independent laboratory named anywhere on it, is self certification.

The two tests that matter, and what each one proves

Nearly every peptide COA rests on two analyses. They answer different questions and neither substitutes for the other.

A certificate showing 99% by HPLC with no mass spectrum tells you the vial contains one very pure substance, which could be almost anything. A mass spectrum with no chromatogram tells you the correct peptide is in there somewhere, in an unknown amount, alongside an unknown quantity of everything else. You need both, and a COA offering only one of them is incomplete rather than merely brief.

Why 99% purity does not mean 99% peptide

This is the single most misread figure in the trade, and it is worth being precise about.

HPLC purity is an area percentage. The detector traces absorbance over time, the software integrates the peaks, and purity is the area of the main peak divided by the total area of all peaks. It is a statement about the relative proportions of what came off the column and absorbed light. It is not a statement about the mass of peptide in the vial.

Lyophilised peptide powder is not pure peptide by weight. It also contains:

• Counterions left over from purification, most often trifluoroacetate from the TFA used in the mobile phase, sometimes acetate. These can account for a substantial share of the dry mass.
• Bound and residual water, which lyophilised material readily reabsorbs.
• Residual salts and buffer components from the process.

The figure that captures this is net peptide content, measured by amino acid analysis or by elemental nitrogen determination. For a TFA salt it commonly lands somewhere in the region of 70 to 85 percent of gross mass, though it varies considerably with the sequence and the process.

The identifiers, and why the lot number carries the document

Before reading any result, check that the certificate is anchored to something. A complete header carries the product name, the amino acid sequence, the molecular formula and molecular weight, the batch or lot number, the manufacture date, the analysis date, the quantity, and the identity of the laboratory with a report reference and a signature.

The lot number is the load bearing element. It is what connects a set of measurements to the physical material. A certificate with no lot number, or with a lot number that does not match the label on the vial you received, is not evidence about your product. It is a document about some other material, or about no material at all.

The molecular weight is worth thirty seconds of your own verification. Look the compound up on a public chemical database such as PubChem and compare the molecular formula and mass against what the certificate states. A mismatch here ends the exercise immediately.

Reading the chromatogram

The chromatogram should be a legible plot with a labelled time axis and a labelled absorbance axis, accompanied by an integration table listing each detected peak with its retention time, area, and area percentage.

That table is the point. Without it you are looking at a picture of a graph and taking the purity figure on trust. Any certificate that shows a chromatogram image but no peak integration data has given you decoration rather than data.

What to look at once you have it:

• Whether the main peak is on scale. A peak that runs off the top of the plot has saturated the detector and the integration underneath it is unreliable.
• Shoulders on the main peak. Closely eluting impurities in synthetic peptides are typically deletion sequences, oxidised variants, or epimers. They sit near the main peak precisely because they resemble it, and a short or poorly resolved run can hide them inside it.
• Run length. If the gradient ends shortly after the main peak elutes, late eluting impurities were never given the chance to appear.
• The stated method. Column, gradient, flow rate, detection wavelength, injection volume. A result produced by an undisclosed method cannot be reproduced or challenged by anyone.

Reading the mass spectrum

The certificate should give both the theoretical molecular weight and the observed value. Electrospray instruments usually produce a series of multiply charged ions that are deconvoluted to a single mass; MALDI typically reports the singly protonated ion.

On a low resolution instrument, agreement within roughly one mass unit on the average mass is ordinary. Systematic differences are informative rather than random: a result heavier by about 16 units points to oxidation, and a difference matching the mass of a single amino acid residue points to a deletion or substitution in the sequence. A stated observed mass that simply does not correspond to the stated formula is the clearest possible signal to stop.

Specification is not result

Well constructed certificates use two columns. The specification column states the acceptance criterion, for example "not less than 98.0%". The result column states what was actually measured, for example "98.7%".

Certificates that print only the specification have told you what the material was supposed to be, not what it was found to be. Similarly, the word "conforms" against a quantitative test is a non answer. Appearance and solubility can reasonably be reported as conforming. Purity cannot.

The tests most certificates leave out

Research grade peptide certificates typically stop at HPLC and mass spectrometry. The tests below are standard for pharmaceutical material and are usually absent from research material, which is one of the practical differences between the two categories.

Their absence is normal for material sold for laboratory research, and that is exactly the point. It is a large part of why such material is not equivalent to a prescribed medicine, whatever the purity figure says. Chapter references above are to the United States Pharmacopeia, published at usp.org; other pharmacopoeias carry equivalent chapters under different numbering.

Who ran the test

An in house certificate is produced by the manufacturer's own quality control. That is standard practice across the chemical industry and is not inherently suspect, but it is the seller marking their own work.

A genuine third party certificate is issued on the testing laboratory's letterhead, carries the laboratory's own report number and contact details, and names the analyst or authorising signatory. A PDF bearing only the vendor's logo, with no laboratory identity anywhere on it, is not third party testing regardless of how it is described on the product page.

There is one further question worth asking, and few vendors are asked it: was the sample drawn independently, or supplied by the vendor to the laboratory? A vendor supplied sample demonstrates the quality of that sample. Whether it represents the batch is a separate matter, and one the certificate cannot settle.

Eleven red flags

1. No batch or lot number, or one that does not match the vial you received.
2. The same certificate served for multiple products, or reused unchanged across successive batches.
3. Purity given as a specification only, with no measured result.
4. A chromatogram image with no integration table, no axis labels, or no scale.
5. No analytical method stated, so the result cannot be reproduced.
6. Purity quoted at implausible precision, such as 99.99%, which exceeds what routine RP-HPLC meaningfully resolves.
7. Mass spectrometry missing entirely, or an observed mass inconsistent with the stated molecular formula.
8. No laboratory name, no report number, no signature or authorising analyst.
9. Missing dates, or an analysis date that precedes the manufacture date.
10. A purity figure with no net peptide content stated anywhere, on a product priced per milligram.
11. A flattened image or cropped screenshot rather than a complete document, particularly where the header or footer has been removed.

The checklist

Working through any certificate you are handed:

• The lot number on the certificate matches the lot number on the vial.
• The product name, sequence and molecular formula are all present.
• The stated molecular weight matches an independent chemical database.
• Both HPLC and mass spectrometry are present.
• Purity is a measured result, not a specification.
• A peak integration table accompanies the chromatogram.
• The analytical method is described in enough detail to be repeated.
• The observed mass agrees with the theoretical mass.
• Manufacture date and analysis date are both present and in the right order.
• The testing laboratory is named, with a report reference and a signature.
• Net peptide content is stated, or its absence is acknowledged by the vendor.

What a certificate cannot tell you

A clean COA is a floor, not a ceiling. Even a fully compliant certificate is silent on several things that matter.

It cannot confirm that the vial you were sent came from the lot that was tested. It says nothing about how the material was stored or shipped after testing, and peptides are sensitive to heat and to repeated temperature cycling. It does not make an unapproved compound approved, and it has no bearing whatsoever on whether the compound does anything useful in a human being. Purity and efficacy are unrelated questions, and a very pure sample of something with no supporting evidence is simply a pure version of an unknown.

That last question is a separate assessment, and we handle it separately. Every compound covered on this site is assigned an evidence tier that caps what we are willing to say about it, described in full on the method page.

Back to PeptoPurity