How to read a peptide Certificate of Analysis

A Certificate of Analysis (COA) is supposed to be the auditable record that ties a specific vial of peptide back to the lot it came from and the laboratory tests that validated it. In practice, most peptide COAs published by research-supply vendors leave out enough information that the document is closer to a marketing brochure than a quality record. This guide walks through the five numbers that actually matter on a per-lot COA, explains what each one tells you, and identifies the red flags that should send you to a different supplier.

1. Peptide purity (area %)

Peptide purity on a COA is almost always reported as an HPLC area percentage. This number is the integrated peak area of your target peptide divided by the total integrated area of all UV-absorbing peaks in the chromatogram, multiplied by 100. A peptide reported at 99.2% area means the target peak accounts for 99.2% of everything the detector saw at the analytical wavelength.

The catch: HPLC area percent is not the same thing as mass percent or true compound purity. UV-inactive impurities — counter-ion salts, residual solvents, water — do not show up in the chromatogram, so a 99% area peptide can still be only 80–85% actual peptide by mass after counting the salt form and water content. A meaningful purity claim requires both the HPLC area % and the peptide content by mass (sometimes labeled "net peptide content" or determined by amino acid analysis).

2. The chromatogram itself

A COA without an attached or embedded HPLC chromatogram is unsigned. The chromatogram is the actual evidence — the area-percent number on a stand-alone line in a PDF is just a number someone typed. Look for: (a) a visible chromatogram with the X-axis labeled in minutes (retention time) and Y-axis in absorbance units (AU or mAU), (b) the target peak clearly integrated with retention time and area printed next to it, (c) baseline noise low enough that you can see whether smaller impurity peaks were actually integrated or quietly ignored.

If the chromatogram shows a clean target peak with a flat baseline and no nearby shoulders, the area % claim is supportable. If the chromatogram shows multiple unresolved peaks near the target peak's retention time, the area % may be optimistic — those near-peaks are often counted into the target peak's integration if the integration method is set loosely.

3. Mass spectrometry confirmation

HPLC tells you how pure the compound is. Mass spectrometry (MS) tells you whether the compound is actually what the label says. A peptide can be 99% pure by HPLC and still be the wrong peptide — for instance, a peptide with one amino acid substituted will run at almost the same retention time as the intended sequence but have a different mass. MS confirms the molecular weight matches the theoretical mass calculated from the sequence.

For peptides under ~5 kDa, expect ESI-MS or MALDI-TOF data on the COA. The observed mass should match the theoretical mass to within 1 Da (often within 0.1 Da on modern instruments). If a COA omits MS confirmation, the supplier is asking you to trust that the HPLC peak labeled with a particular compound name actually is that peptide. That is not a trust you should extend without evidence.

4. Lot number and traceability

Every COA should be tied to a specific lot number, and the vial you receive should have that lot number on the label. A vial without a lot number, or a COA that says "representative lot" without specifying which lot the analysis was performed on, gives you no traceability. If you later identify a quality problem, you cannot trace which batch is affected, which means the supplier cannot perform a targeted recall.

The practical test: compare the lot number printed on the vial to the lot number printed on the COA published for that compound. They should match exactly. At Bench Grade Peptides this is enforced by the QR code on every vial — scanning the QR loads the COA for that specific lot, not a generic compound-level document.

5. Independent third-party signature

The single highest-leverage thing to look for on a peptide COA is whose laboratory ran the analysis. Three possibilities, in order of decreasing trust:

1. 1.Independent third-party lab. The peptide was manufactured by one entity and analyzed by another. The analyzing lab has no commercial relationship with the supplier and no incentive to inflate the numbers. This is the standard for any vendor claiming a premium QC posture.
2. 2.The synthesizer's own lab. The same facility that made the peptide also ran the analysis. The data is usually accurate, but a synthesizer with margin pressure has a known incentive to integrate area % generously or to round up. Acceptable, not premium.
3. 3.The supplier's own lab, downstream of the synthesizer. The vendor bought peptide from someone else and ran their own HPLC on it. This is the weakest signal — you have no insight into what was tested versus what was shipped, and most small vendors do not actually run their own analytics; they just receive the synthesizer's COA and put their logo on it.

If a COA does not name the laboratory that performed the analysis, treat it as category 3 by default. The decision to omit the lab name is usually deliberate.

Quick-reference red flag checklist

• ◆No HPLC chromatogram attached, only an area-percent number on a stand-alone line.
• ◆Theoretical mass listed without an observed mass.
• ◆Purity claim ≥99.9% with no chromatogram — that level of purity is unusual for solid-phase synthesis and the claim warrants visible evidence.
• ◆"Representative lot" or no lot number at all — no traceability.
• ◆No named analyzing laboratory on the document.
• ◆COA dated before the lot was synthesized (yes, this happens).
• ◆COA file name is generic (e.g. "semaglutide_coa.pdf") rather than lot-specific (e.g. "BG-SEM-2025-04-LOT-A12.pdf") — a strong signal the same document is being reused across lots.

A six-step verification checklist

1. 1.Match the lot number. The lot number printed on the vial label should appear on the certificate. A certificate for another lot, or one with no lot number, does not describe the vial in front of you.
2. 2.Identify the laboratory. The certificate should name the laboratory that ran the tests. Look the laboratory up, check its accreditation (for example ISO/IEC 17025) and what that accreditation covers, and confirm the report through the laboratory's own verification route where one exists.
3. 3.Compare the dates. The analysis date should fall after the lot was made and before it was shipped. A certificate dated long before the lot, or after it left the supplier, describes something else.
4. 4.Check identity by mass spectrometry. The observed mass should agree with the expected molecular weight within the tolerance the laboratory states. A purity figure alone does not show the material is what the label says.
5. 5.Check the purity method. An HPLC purity figure should state the method and the detection wavelength, and the chromatogram should be attached. A number with no method and no trace cannot be checked.
6. 6.Know how you would re-test. Decide which accredited laboratory you would send a sealed vial to, which tests you would ask for (identity by mass spectrometry, purity by HPLC), and what the supplier commits to in writing if the result disagrees.

What a COA cannot tell you

A Certificate of Analysis reports what one laboratory measured, on one sample, on one date, by the methods it names. It cannot tell you:

• ◆How the vial was stored and shipped after the sample was tested.
• ◆Whether every vial in the lot matches the sample the laboratory received.
• ◆Net peptide content, unless it was measured separately: HPLC area-percent purity is not the same measurement.
• ◆Contaminants the listed methods do not detect, such as residual solvents, endotoxin or heavy metals, unless those tests are reported.
• ◆Whether the material suits a particular experimental design.

How Bench Grade Peptides publishes its COAs

Every lot we ship has a per-lot COA published at benchgradepeptides.com/coa, reachable via the QR code printed on each vial label. The analyzing laboratory is named, the chromatogram is embedded, mass spec is included with both theoretical and observed masses, lot number ties to the vial label, and the analysis is performed by a US-based ISO 17025-accredited laboratory that is neither us nor the synthesizer. If our published COA disagrees with an independent third-party test you commission yourself, our guarantee refunds the order, the test, the shipping, and adds $100. Per-lot verification is the whole point — there is no version of our offering that does not include it.