How to Read a Peptide Certificate of Analysis: HPLC, Mass Spec, and Purity Benchmarks
Every reputable peptide vial ships with a certificate of analysis (CoA) — a one-page PDF that summarizes how the batch was tested and what the testing found. Most users glance at the headline percentage, see something like “98.7%”, and file the document away. The CoA is doing a lot more than that, and the extra fields are exactly where the variation between batches lives. This guide walks through what each section actually represents and which numbers matter when you're trying to keep a multi-month protocol interpretable.
What a CoA is — and what it is not
A certificate of analysis is a record of the analytical tests the manufacturer or the contract lab ran on a specific lot of material. It is not a clinical document. It does not certify that the substance is safe, legal in your jurisdiction, or appropriate for any particular use. It does certify that on the day the lot left the lab, the methods listed were run and the results listed were observed. For research peptides the CoA typically covers identity, purity, water content, counter-ion content, and a basic appearance check.
The lot number on the CoA must match the lot number printed on the vial label. If those two numbers don't agree, the CoA describes a different batch and the document is effectively meaningless. This is the first thing to check, and the one most people skip.
HPLC purity: the headline number, in context
High-performance liquid chromatography (HPLC) is the standard purity method for peptides. The lab dissolves the peptide, pushes it through a column, and measures the area under each peak in the resulting chromatogram. The target peptide produces one major peak; any contaminants or related substances produce smaller peaks elsewhere. The headline purity figure is the percentage of total peak area attributable to the target peak.
For research-grade peptides, common purity benchmarks are:
- ≥ 95% — minimum for most credible research applications
- ≥ 98% — common target for compounds intended for chronic dosing in published studies
- ≥ 99% — tighter spec typical of GLP-grade material and some compounding pharmacy stock
Two practical points: first, the detection wavelength matters. HPLC purity at 220 nm captures a broader range of related peptide species than purity at 280 nm, which is selective for aromatic residues. A CoA that reports only 280 nm purity may overstate the headline figure for sequences without tryptophan or tyrosine. Second, the chromatogram itself, if attached, is informative — a clean, symmetric main peak with no shoulders is a different quality signal from the same percentage produced by integration that happens to ignore a co-eluting impurity.
Mass spectrometry: identity confirmation
HPLC tells you how much of the sample is one substance. Mass spectrometry tells you whether that substance is the one you ordered. The CoA should report an observed mass and an expected (theoretical) mass for the sequence. The two should agree to within a few Daltons, and the deviation should be much smaller than the mass of any plausible failure intermediate.
For a typical 30- to 40-residue peptide, expect to see something like “Observed: 3424.9 Da, Expected: 3424.8 Da” on the CoA. Larger discrepancies usually indicate either a synthesis error (missing residue, wrong amino acid, incomplete deprotection) or a confusion between the free peptide and a salt form. If the mass is off by ~60 Da, the lab is probably reporting the acetate salt rather than the free peptide and didn't subtract.
Water content and counter-ion content: the numbers that change your effective dose
Peptides are almost always shipped as a salt — usually the acetate or trifluoroacetate (TFA) salt — and they are hygroscopic, which means they pick up water from the air. The CoA should report both:
- Water content (Karl Fischer titration): commonly 4–10% by mass for lyophilized research peptides
- Counter-ion content (ion chromatography): commonly 6–14% by mass for the acetate salt of a typical sequence
The combined effect is meaningful. A vial labeled “5 mg” that reports 8% water and 10% counter-ion contains roughly 4.1 mg of actual peptide mass; the remainder is solvent and salt. For most peptides this difference is small relative to dose-finding variability and doesn't change the protocol. For tightly titrated compounds — GLP-1 agonists, certain growth-hormone secretagogues — it's the difference between a clean log entry and a confusing one.
If the manufacturer reports a “net peptide content” figure directly, that's the number the reconstitution math should use. If they don't, you can estimate it as labeled mass × (1 − water% − counter-ion%). Our reconstitution calculator can take either input.
The fields that usually round out the document
Most CoAs also include a short list of additional checks:
- Appearance: “white to off-white lyophilized powder.” A pink, yellow, or grey colour is a real concern; ask before reconstituting.
- Solubility: a confirmation that the peptide dissolves in water or in a specified solvent at a stated concentration.
- Sequence: the one-letter or three-letter amino acid sequence, useful for cross-checking against literature.
- Molecular formula and molecular weight: redundant with the mass spec section but useful for an independent calculation.
- Test date and lot expiry: usually 24 months from the lyophilization date for the dry powder.
Red flags on the document itself
A few patterns recur on lower-quality CoAs and are worth knowing:
- No lot number, or a lot number that doesn't match the vial. The CoA describes a specific batch; without the lot number the document is decorative.
- HPLC chromatogram missing or unreadable. A clean PDF will include the chromatogram itself, not just the integrated percentage.
- Mass spec result reported without a method (ESI-MS, MALDI-TOF) or without an expected mass to compare against.
- “Purity > 99%” with no underlying figure. A real HPLC result is a specific number with a decimal, not a threshold.
- No water content or counter-ion content reported at all. Common on the lowest-tier documents, and it means dose math is being done on the labelled mass rather than the peptide mass.
- Generic template that doesn't list the actual analyst, instrument, or test date. A CoA is a record of work performed; it should be specific.
Tracking CoA data alongside the cycle
The point of reading the CoA isn't a one-time qualification check. The numbers it reports — net peptide content, lot, expiry, observed mass — should travel with the vial into the log so that batch-to-batch differences across a cycle are visible. Peptra vial entries include a CoA-attached field so the PDF lives next to the dose schedule; the lot number is captured as a searchable field so that a future batch comparison is one tap rather than a folder hunt.
The practical pay-off is unglamorous: when something shifts mid-cycle — injection-site response changes, side-effect profile drifts, or a subjective marker stops moving — the first question is whether the vial changed. With the CoA captured alongside the doses, the answer is in the same screen as the rest of the protocol.