What a COA is (and is not)
A COA is a lot-specific analytical summary: it reports what a defined laboratory measured on a defined batch of material, on defined instruments, using defined methods. Because it is lot-specific, a COA from one lot does not certify a different lot of the same product — each production run must carry its own document tied to its own lot number.
A COA is not a safety document, an efficacy claim, or a substitute for a laboratory's own incoming-material verification. All materials described here are reference compounds intended for in-vitro and research use only, and are not for human or animal consumption. The COA answers a narrower, more important question: is this the correct molecule, at the stated purity, in the stated amount?
- Lot-specific: valid only for the batch number printed on it.
- Multi-method: identity and purity are established by independent techniques that should agree.
- Traceable: each result should map to a raw instrument file the vendor can produce on request.
The identification block
The top of a COA identifies the article unambiguously. Expect the product name, the sequence or molecular formula, the CAS number where one exists, the lot or batch number, a manufacture or test date, and often a recommended retest date. The lot number is the anchor for everything else on the page and is what makes the phrase verified to the lot meaningful.
Cross-check these fields against the compound you intended to source. A mismatch between the stated molecular formula and the named peptide, or a CAS number that does not correspond to the sequence, is an immediate red flag that the template was reused without care.
- Product name and synonym(s)
- Sequence (one-letter or three-letter) and/or molecular formula
- CAS number, where an assigned one exists
- Lot/batch number, manufacture date, retest or reanalysis date
Appearance and physical description
Appearance is a simple but genuine test. Most lyophilized research peptides are described as a white to off-white powder or cake; some hygroscopic or highly concentrated materials present as a fluffy or filmy solid. The COA states the observed appearance so it can be compared against the vial received.
A discrepancy — for example, a COA stating white powder against a vial containing a discolored or oily residue — is a signal that the material may have been mishandled, degraded, or does not match its paperwork.
HPLC purity — and why the chromatogram is the evidence
High-performance liquid chromatography (typically reversed-phase, RP-HPLC) separates the main peptide from synthesis-related impurities such as deletion sequences, truncation products, and oxidized or deamidated variants. Purity is reported as the area percentage of the main peak relative to total integrated peak area, commonly with UV detection at 214 nm (peptide bond absorbance) and/or 220 nm.
The purity number alone is not the evidence — the chromatogram is. A trace that shows a single dominant, symmetric peak with a flat baseline supports a high stated purity. A stated 99% next to a chromatogram with unlabeled shoulders, co-eluting humps, integration that excludes visible peaks, or a truncated time axis should be treated with suspicion. Always ask which detection wavelength, column, gradient, and run time produced the number, and whether the full chromatogram (not a cropped thumbnail) is available.
- Look for the method: column chemistry, gradient, flow rate, detection wavelength, run time.
- Read the baseline: drift, noise, and unintegrated peaks all matter.
- Peak symmetry and resolution from neighbors indicate a clean separation.
- Beware a purity figure presented without the underlying trace.
Mass-spec identity: observed vs theoretical mass
HPLC tells you how much of one thing is present; it does not by itself prove that thing is the intended molecule. Mass spectrometry (commonly ESI-MS or MALDI-TOF) establishes identity by measuring the molecular mass and comparing the observed value against the theoretical monoisotopic or average mass calculated from the sequence.
A credible COA lists both the theoretical mass and the observed mass, and they should agree within the instrument's expected tolerance. ESI spectra often show a series of multiply charged ions ([M+2H]2+, [M+3H]3+, and so on) that deconvolute to the neutral mass. A mass that is off by the weight of one or more residues suggests a deletion or addition; a mass shifted by ~16 Da can indicate oxidation. Identity confirmation is the step most often missing from thin COAs.
- Theoretical mass stated alongside observed mass.
- Agreement within stated tolerance confirms molecular identity.
- Characteristic mass shifts flag specific impurities (e.g. +16 Da oxidation).
Net peptide content vs HPLC purity
These two numbers are frequently conflated and they measure different things. HPLC purity is the fraction of peptide-related material that is the target sequence — a relative measure among the peptides present. Net peptide content (peptide content assay, often by nitrogen analysis or amino-acid analysis) is the fraction of the total dry mass in the vial that is actually peptide, as opposed to water, counterions, and residual salts.
A material can be 99% pure by HPLC yet only ~80% peptide by mass, because the balance is bound water and trifluoroacetate or acetate counterion. For quantitative work, net peptide content is the number that tells you how much peptide a given mass of powder contains; HPLC purity tells you how clean that peptide is. A rigorous COA reports both and does not present one as if it were the other.
- HPLC purity: how clean the peptide fraction is (relative).
- Net peptide content: how much of the vial mass is peptide (absolute).
- The gap between them is largely water and counterion.
Water, counterion, and residual solvents
Water content is typically determined by Karl Fischer titration and reported as a percentage; lyophilized peptides commonly carry several percent residual moisture. The counterion (frequently acetate or trifluoroacetate, TFA) is introduced during synthesis and purification and contributes to the mass that is not peptide. Some laboratories now prefer acetate salts and report a TFA removal or exchange step.
Residual solvents from synthesis (for example acetonitrile) may be quantified where relevant. Endotoxin or bioburden may appear on COAs for materials intended for sensitive in-vitro assays, though these are not required for all research articles. Each of these values further explains the difference between purity and net content.
- Water content by Karl Fischer titration.
- Counterion identity and content (acetate vs TFA).
- Residual solvents and, where relevant, endotoxin/bioburden.
Third-party testing and traceability
The strongest COAs are either produced by, or independently confirmed by, a laboratory that did not manufacture the material. Independent testing removes the conflict of interest inherent in a supplier grading its own product. Look for the testing laboratory's name, the analyst or approver, and instrument or method references.
Traceability means each printed result can be tied back to a raw data file — the actual chromatogram and mass spectrum for that lot — that the vendor can produce on request. When those files exist and match the summary, the COA is verifiable rather than merely asserted. This is the core of what verified to the lot means: every batch is tested and every batch's data is retrievable.
- Named testing laboratory and approver.
- Raw chromatogram and mass spectrum available for the specific lot.
- Results reproducible on the vendor's request.
A practical verification workflow
Working through a COA in a fixed order prevents you from anchoring on a single headline number. The sequence below moves from identity to purity to quantity, which mirrors how the measurements build on one another.
- 1. Confirm the lot number on the vial matches the COA.
- 2. Verify the product name, sequence/formula, and CAS in the identification block.
- 3. Confirm identity: observed mass agrees with theoretical mass.
- 4. Assess purity from the actual HPLC chromatogram, not just the percentage.
- 5. Read net peptide content to know how much peptide the mass contains.
- 6. Check water and counterion to explain the purity/content gap.
- 7. Confirm the testing lab and that raw data is retrievable for the lot.
COA red flags
The following patterns recur on low-quality or fabricated documents. Any one warrants a request for the underlying data; several together are grounds to reject the material.
- A purity percentage with no chromatogram, or only a cropped thumbnail.
- No mass-spec identity confirmation (purity reported but identity never established).
- Net peptide content and HPLC purity conflated or one silently omitted.
- No lot number, or a generic COA reused across every batch.
- Molecular formula, CAS, or mass inconsistent with the named peptide.
- No named testing laboratory and no way to obtain raw files.
- Chromatogram with unintegrated peaks or a suspiciously truncated axis.