How to read a Certificate of Analysis

A Certificate of Analysis, or COA, is the single most useful document a research-compound supplier can hand you, and also the one most often faked, omitted, or quietly misread. At its core it is a laboratory record of what one specific batch of material actually contains, measured rather than asserted. Learning to read one is what turns 'trust me' into 'here is the measurement,' and that shift is the entire point of this guide.

This is a research-quality question, not a dosing question. Nothing below is a recommendation to use any compound. It is about telling a tested substance from an untested one.

The five checks, at a glance

If you read nothing else, these are the five things to confirm before you treat a Certificate of Analysis as evidence. The rest of this guide is each one in detail.

1. Match the lot number. The lot on the COA has to match the lot printed on your vial. A COA from a different batch, even the same product and seller, does not describe your material.
2. Confirm identity by mass spectrometry. The observed mass should match the theoretical mass for the sequence to within a fraction of a dalton. Purity means nothing if the molecule is the wrong one.
3. Check purity by HPLC, and know what it excludes. The percentage is UV-detected peak area, not mass, and it should come with a chromatogram and stated method conditions.
4. Check water and counterion. Karl Fischer water content and the named counterion are what turn a purity percentage into an actual milligram figure. Their absence is why a 10 mg vial is rarely 10 mg of peptide.
5. Verify the testing laboratory. Named, contactable, with a report number, and ideally independent of the seller rather than the seller's own unnamed bench.

First, is it even a COA?

Before reading any numbers, check that the document is tied to a real, traceable test. A legitimate COA names four things: the compound and batch/lot number, the date of testing, the methods used, and the laboratory that ran them, ideally an independent, third-party lab rather than the seller's own unnamed bench. A pretty PDF with a purity percentage but no batch number and no method is a graphic, not a result. It cannot be linked back to the vial in front of you.

Identity: is it the right molecule?

Purity is meaningless if the substance is the wrong thing to begin with. Mass spectrometry (MS) answers the identity question by measuring the molecular weight of the compound and comparing it to the known, expected mass for that peptide. The COA should show an observed mass that matches the theoretical mass for the sequence. If a COA reports a high purity number but no identity confirmation, it is telling you the sample is pure something, without confirming what.

Purity: what the percentage actually measures

Purity is most often measured by high-performance liquid chromatography (HPLC) with ultraviolet detection, which separates the sample into its components and reports the target compound as a percentage of the total. You will typically see a chromatogram, a plot with peaks, and a headline figure like 98%. This is the most-quoted number on any COA and the most widely misread, so it is worth being precise about what it is.

The gap between them comes from what the detector can see. UV detection for peptides is normally run at 214 nm, where the peptide bond itself absorbs, or 280 nm, where only aromatic residues (tryptophan, tyrosine, phenylalanine) absorb. Anything in the vial without a chromophore at the detection wavelength contributes nothing to the chromatogram and is therefore excluded from the arithmetic. Water is invisible. Counterion salts are effectively invisible. Inorganic residues are invisible. A sample can be legitimately 99% pure by peak area while a substantial fraction of what you weigh out is not peptide at all.

Two further things a single number hides:

• What the other percent is. A 98% purity means 2% is something else, related peptide fragments, deletion sequences, or synthesis byproducts. The chromatogram shows whether that remainder is one small peak or a cluster of them, and a cluster is the more concerning shape.
• The method conditions. A credible HPLC result states the column, the detection wavelength, and the gradient used. Their absence is a sign the number was typed in rather than measured, and it also makes the result impossible for anyone else to reproduce.

The three numbers that decide how much peptide you have

If you want to know the mass of peptide in a vial, and you do, because every concentration calculation depends on it, purity is only the first of three figures. The other two are reported less often and matter just as much.

Worked through, the difference is not academic. Take a vial labeled 10 mg, with a COA reporting 98% purity by HPLC, 6% water by Karl Fischer, and 8% trifluoroacetate. The gross powder is 10 mg. Subtract water and counterion and about 86% of that mass is peptide-plus-impurity; apply the 98% purity figure and roughly 8.4 mg is the target peptide. You did not lose anything and nobody misled you. The vial simply never contained 10 mg of peptide, and the COA said so if you read all three lines instead of one.

Counterion: why trifluoroacetate is not just dead weight

Solid-phase peptide synthesis uses trifluoroacetic acid for cleavage and deprotection, so peptides, especially basic ones, come off the process as trifluoroacetate (TFA) salts unless a deliberate exchange step is performed. That has an arithmetic consequence, covered above. It also has a biological one that is easy to miss.

Residual trifluoroacetate has been reported to inhibit the proliferation of osteoblasts and chondrocytes in cell culture, and counterion identity has been shown to alter both the antimicrobial activity and the measured cytotoxicity of antimicrobial peptides. In other words the salt can be biologically active in exactly the assays a researcher is likely to be running. If a result in a cell-based experiment is unexpectedly cytotoxic, the counterion is a genuine candidate explanation before the peptide is. Acetate and hydrochloride salts are the usual alternatives, and a COA that names the counterion is telling you something a COA that omits it is not.

Reading the chromatogram, not just the number

The chromatogram is the evidence behind the purity figure, which is precisely why its absence matters. A single number can be typed; a plot is harder to fake convincingly and much easier to sanity-check. What you are looking for is one dominant peak that is sharp and symmetrical, sitting on a flat, quiet baseline.

• Shoulders or tailing on the main peak suggest a closely related impurity co-eluting with the target, which integration software may quietly fold into the main peak area and report as purity.
• A cluster of late-eluting peaks points to hydrophobic impurities, often truncated or modified sequences from an incomplete synthesis.
• A noisy baseline paired with a very clean summary figure is internally inconsistent. If the trace is that untidy, the integration is doing a great deal of interpretive work.
• No chromatogram at all, only a summary table, means you are being asked to accept the conclusion without the measurement.

Contaminants: what shouldn't be there

Identity and purity describe the intended molecule. Contaminant testing describes what else came along. Depending on the compound and how it was made, a thorough COA may include:

• Heavy metals (such as lead, arsenic, cadmium, mercury), residues from manufacturing, reported against a limit.
• Endotoxin / bacterial endotoxins (LAL test), relevant for anything intended to be dissolved, because endotoxin is a potent inflammatory contaminant that purity testing alone will not catch.
• Residual solvents and water content, leftovers from synthesis and lyophilization that affect both stability and how much actual compound is in a labeled milligram.

Tying the document to the vial in front of you

A COA can be entirely genuine and still tell you nothing about your material, because it was issued for a different lot. This is the most common failure in practice, and it is not usually deception, it is a supplier linking one representative document from a product page. Five checks close the gap:

1. The lot number on the COA matches the lot number printed on your vial. Not the product name, the lot. If they differ, the document describes someone else's material.
2. The test date falls after the manufacture date, and both are recent enough to be meaningful. A test dated before the batch existed is a clerical error at best.
3. The testing laboratory is named, with a report or reference number that could be quoted back to it.
4. The compound name and the observed mass match what you ordered. A COA for a closely related sequence is easy to mistake for the right one at a glance.
5. The document can be located independently, in the supplier's published COA library by compound and lot, rather than only arriving as an attachment when you ask.

Red flags

• No lot number, or a document that claims to cover "all batches." Batch-specific testing that is not batch-specific is a contradiction.
• No named laboratory, or a logo with no report number, address, or contact details.
• A purity figure with no identity confirmation. Pure something is not the same as pure this.
• A summary table with no chromatogram, no spectrum, and no stated method conditions.
• Identical documents across different lots, down to the trace shape or the decimal places. Real batches vary.
• Purity claimed above 99.9% for a synthetic peptide. It is not a plausible routine result for solid-phase synthesis, and the claim is doing marketing work rather than analytical work.
• Water and counterion omitted entirely while a headline purity figure is given prominence.
• Dates that do not reconcile, or a test date years old on material sold as current stock.

Where we stand on this

It would be reasonable to ask why a site that earns affiliate revenue is publishing a guide to catching suppliers out. Fair question, so the answer in plain terms: this site is funded by referrals to a supplier, some outbound links here are affiliate links, and every standard in this article applies to that supplier exactly as it applies to any other. A checklist that a partner is exempt from is not a checklist, it is an advertisement with steps.

The same principle runs through the rest of the database. Female-evidence fields stay marked unstudied when no qualifying study exists, including for compounds we link to, because the alternative, filling gaps with plausible-sounding text, is the exact failure this article teaches you to catch on a COA. A number without a measurement behind it is a number without a measurement behind it, whoever printed it.

A quick checklist

1. Lot number present on the COA and matching the vial in your hand.
2. Named issuing laboratory, ideally independent and third-party, with a test date after the manufacture date.
3. Identity confirmed by mass spectrometry, observed mass matching theoretical to within a fraction of a dalton.
4. Purity by HPLC with the chromatogram shown and the column, wavelength and gradient stated.
5. Water content by Karl Fischer and the counterion named, so net peptide can actually be computed.
6. Contaminant panel appropriate to the compound: heavy metals, bacterial endotoxin, residual solvents.
7. Every number tied to a method, rather than a bare percentage on a branded page.

The habit worth building is simple: treat the COA as the claim's evidence, and treat the headline purity figure as the beginning of the reading rather than the end of it. If a supplier can hand you a lot-linked, method-backed document that survives those seven questions, you are looking at tested material. If the paperwork evaporates under them, you are looking at marketing, and no purity figure printed on marketing is worth more than the page it is on.

Once the document checks out, the next thing that decides whether your material behaves as labeled is how you handle it. That is covered in peptide reconstitution and storage, including the arithmetic for turning a net peptide mass into a working concentration.