How to read a peptide Certificate of Analysis

1. What a COA actually is

A Certificate of Analysis is a batch-specific analytical report — not a marketing sheet. A legitimate COA is produced by an independent laboratory (ideally ISO 17025 accredited), references a specific batch or lot number, and reports the analytical methods, instrument conditions, and raw results used to characterize a single production run of a compound.

For research peptides, the two core assays on every COA are reverse-phase HPLC (for purity) and mass spectrometry (for identity). Without both, you cannot know what's in the vial.

2. Reading the HPLC chromatogram

High-Performance Liquid Chromatography separates the peptide from residual synthesis byproducts, deletion sequences, and truncated fragments. The output is a chromatogram — a plot of detector signal (usually UV absorbance at 214 nm) versus retention time.

What each part means

• Main peak. The tallest peak, integrated for area. Its retention time should match the reference standard for that peptide on the same column and gradient.
• Purity %. Reported as area-under-the-curve of the main peak divided by total area of all integrated peaks. A COA claiming ≥99% purity should show the main peak dominating; adjacent minor peaks should be small and few.
• Impurity peaks. Small peaks flanking the main peak are typically deletion sequences (missing one amino acid) or oxidation products. A reputable COA lists them individually with their retention time and area %.
• Method block. Column (e.g. C18, 4.6×250mm, 5μm), mobile phase (typically water/acetonitrile with 0.1% TFA), gradient, flow rate, and detection wavelength. Missing method details is a red flag.

What "≥99% purity" actually means: the peptide of interest accounts for at least 99% of the UV-absorbing material at 214 nm. It does not measure water, counter-ion (acetate/TFA), or non-UV-active impurities — which is why net peptide content is reported separately on high-quality COAs.

3. Reading the mass spectrometry report

HPLC tells you how pure the sample is. Mass spectrometry tells you what it is. The MS report confirms that the dominant HPLC peak is actually the target peptide by measuring its molecular mass.

What to check

• Theoretical mass vs. observed mass. The COA should list the calculated monoisotopic (or average) mass for the peptide and the observed [M+H]⁺, [M+2H]²⁺, or [M+Na]⁺ ion. They should agree within instrument tolerance — typically ±0.1 Da for high-resolution instruments, or ±1 Da for lower-res.
• Charge states. ESI-MS of peptides commonly produces multiply-charged ions. A 5000 Da peptide often appears as [M+2H]²⁺ at ~2501 m/z and [M+3H]³⁺ at ~1668 m/z. Both should deconvolute to the same molecular mass.
• Absence of major mass adducts. Small peaks at +16 Da (oxidation), +22 Da (sodium adduct), or −18 Da (dehydration) can appear in every spectrum. A large unexplained peak at a different mass is a red flag.
• Instrument and ionization method. Usually ESI-TOF or ESI-Orbitrap. LC-MS coupling (running MS after HPLC in the same run) provides the strongest identity confirmation because it ties mass directly to retention time.

4. Batch, lot, and traceability fields

A COA that isn't tied to a specific batch is essentially a specification sheet. Look for:

• • Batch / lot number printed on both the vial and the COA.
• • Manufacturing date and, when applicable, retest or expiration date.
• • Peptide sequence in one-letter or three-letter code, including any modifications (acetylation, amidation, disulfide bridges).
• • Molecular formula and molecular weight.
• • Appearance and solubility.
• • Laboratory name, address, and signatory — the analytical chemist who released the batch.

5. Red flags on a suspicious COA

• No chromatogram or spectrum image. Just a table of pass/fail values with no underlying data.
• Identical COA across multiple batches.Real analyses produce slightly different retention times and impurity profiles run to run. Byte-identical PDFs indicate a template, not a test.
• Missing method details. No column, no gradient, no wavelength, no instrument model.
• Purity reported without an area breakdown."99.8% pure" with no peak table is a marketing claim, not a measurement.
• Mass mismatch. The reported observed mass doesn't reconcile with the peptide's known molecular formula.
• No batch traceability. Vial label doesn't reference the batch ID on the COA.

6. How to independently verify a COA

1. Match the batch number on the vial to the batch number on the COA. They must be identical.
2. Recompute the theoretical monoisotopic mass of the peptide sequence with a free tool (e.g. an online peptide mass calculator) and confirm it matches the "calculated mass" on the COA.
3. Check that the observed MS ions on the report reconcile to that mass through their charge states.
4. Confirm the issuing lab is real: search their name, look for an ISO 17025 scope of accreditation, and — for suppliers you use repeatedly — request contact details for the analytical chemist who signed the release.
5. Where the stakes justify it, submit an aliquot to an independent lab for a confirmatory HPLC and MS run.