How to Read a Peptide Certificate of Analysis

A certificate of analysis is the only objective evidence you have about what is actually inside a vial. It is also, in this industry, the document most frequently faked, recycled across lots, or reduced to a single unsupported number. Learning to read one properly takes about ten minutes and eliminates most of the risk in sourcing research peptides.

The header: lot number, date and method

Start at the top, not the purity figure. A legitimate COA identifies four things before it reports any result:

• The lot or batch number, which must match the number printed on the vial label you received.
• The date of analysis, not merely the date of manufacture. A two-year-old assay on material that has been sitting in a warehouse tells you about the past, not the present.
• The analytical method — column chemistry, gradient, flow rate, detection wavelength. "HPLC" alone is not a method.
• The laboratory performing the work, ideally with an accreditation reference.

If any of these four is missing, stop reading. The rest of the document cannot be verified against anything.

Chromatographic purity by HPLC

High-performance liquid chromatography separates the components of a sample by hydrophobicity. The output is a chromatogram: a baseline with peaks rising from it, each peak representing a compound eluting from the column at a characteristic retention time.

Purity is reported as the area of the main peak divided by the total area of all peaks, expressed as a percentage. A result of 99.2% means the principal component accounts for 99.2% of the detected UV-absorbing material.

Three things deserve attention:

• Is the actual chromatogram included? A number without the trace is an assertion. The trace is evidence. Any supplier can type 99.1% into a template.
• Is the baseline flat and the main peak symmetrical? A broad, tailing or shouldered main peak suggests co-eluting impurities that the integration software may have folded into the main peak area.
• What is the detection wavelength? Peptides are typically detected at 214 nm (peptide bond absorbance) or 220 nm. Detection at 280 nm only sees aromatic residues and will under-report impurities in sequences lacking tryptophan, tyrosine or phenylalanine.

Identity confirmation by mass spectrometry

HPLC tells you how pure the sample is. It does not tell you what the sample is. A 99.8% pure vial of the wrong peptide is still the wrong peptide, and chromatographic purity alone cannot distinguish them.

LC-MS resolves this. The instrument reports the observed molecular mass, which is compared against the theoretical mass calculated from the sequence. For a correct identification the observed and theoretical values should agree within a few mass units, allowing for the charge states and adducts that ESI produces.

On the COA you want to see both numbers side by side — theoretical and observed — and ideally the spectrum itself. A COA reporting HPLC purity with no mass confirmation has verified homogeneity without verifying identity.

Net peptide content

This is the figure most often omitted, and the one with the largest practical consequence for your dilution calculations.

A lyophilized vial labeled "10 mg" does not contain 10 mg of peptide. It contains 10 mg of lyophilized powder, which includes the peptide plus bound water and residual solvent. Net peptide content is typically 70–90% of gross mass depending on the sequence and its charge.

If your protocol requires molar accuracy, you need this number. Calculating molarity from gross vial mass will overstate your concentration by 10–30%, and that error propagates through every subsequent dilution.

Residual solvents and elemental impurities

Solid-phase synthesis and purification leave traces. Acetonitrile and TFA are the usual residuals. A thorough COA reports them with limits referenced to a recognized standard. TFA in particular is worth attention: it is cytotoxic at concentrations that are trivially achievable in cell culture if you assume it is absent, and it interferes with several common assays.

Elemental impurities — the category older documents call heavy metals — belong on the same certificate. They enter from reagents, resins and contact surfaces rather than from the sequence itself, so purity by HPLC says nothing about them. They are reported against recognized elemental limits, and a certificate that omits them is describing only part of what is in the vial.

Appearance, solubility and sterility

Appearance sounds trivial and is not. A lyophilized peptide cake should be white to off-white (GHK-Cu, being a copper complex, is an exception and is deep blue), intact rather than collapsed, and free of discoloration. A shrunken or yellowed cake indicates moisture ingress or thermal exposure during storage or transit.

Solubility notes and the recommended solvent save you from discovering at the bench that a hydrophobic sequence will not go into aqueous buffer without an organic co-solvent.

Red flags

Certain patterns recur often enough to be diagnostic:

• The same COA for every product. If the lot number, date or chromatogram is identical across two different compounds, the document is decorative.
• Purity above 99.9%. Achievable, occasionally. Claimed routinely across an entire catalog, it is not credible — real synthesis produces real variance.
• No chromatogram image. Numbers without traces are unverifiable by design.
• In-house testing only. A supplier grading its own work has an obvious incentive problem. Independent, accredited testing is the standard.
• Undated, unsigned, or lot-less. A COA that cannot be tied to a specific batch on a specific date is not a COA.
• A mismatch between COA lot and vial label. This is the single most important check, and it takes five seconds.

A practical checklist

Before you accept a batch into inventory, confirm: lot number matches the vial; analysis date is recent; the testing laboratory is named and independent; HPLC purity meets your specification and the chromatogram is attached; observed mass matches theoretical within tolerance; net peptide content is stated; residual solvents and elemental impurities are documented; and appearance matches the description.

Eight checks, under ten minutes, and it eliminates the overwhelming majority of the material-quality problems that surface later as unexplained variance in results.