How to Read a Peptide Certificate of Analysis

A certificate of analysis (CoA) is the document that turns “trust us” into evidence. For research peptides it should let you answer three questions before the vial is opened: is this the right molecule, how pure is it, and how much active peptide is actually in the vial? This guide walks through every field on a typical lot CoA and what each value should look like.

Header fields: traceability

The top of a CoA carries the lot number, manufacture date, ship date, and expiration / re-test date. The lot number is the key that ties the document to the vial in your hand. If a CoA has no lot number, it is marketing, not documentation. BluGen lots are verifiable in the CoA Library.

Identity: mass spectrometry

Look for a calculated mass and an observed mass, usually from ESI-MS. The observed mass should match the calculated monoisotopic mass for the labeled sequence within the method tolerance. For modified peptides, the modification appears as a defined mass shift (for example, an acetate counterion or a palmitoyl group). A CoA that lists only a molecular weight with no observed-mass measurement has not actually confirmed identity.

Purity: HPLC area percent

Reverse-phase HPLC at 214 nm reports purity as an area percentage of the main peak. A research-grade lot is typically ≥98–99%. Two cautions: purity is relative to what the method can see, and a single high number does not describe the impurity profile. Qualified buyers can request the chromatogram to see the actual peak shape and any shoulder peaks.

The field most people miss: net peptide content

This is the difference between a careful supplier and a careless one. HPLC purity is not mass-on-vial. A 10 mg vial at 99% HPLC purity may contain noticeably less than 10 mg of peptide, because the gross mass includes water and counterion. Net peptide content corrects for this. Use it — not the label weight — when you calculate molarity. The reconstitution calculator takes net content into account.

Water content: Karl Fischer

Residual water from lyophilization is measured by Karl Fischer titration. High water content reduces shelf stability and inflates gross mass relative to active peptide. It is reported as a percentage.

Residual solvents

Synthesis solvents (acetonitrile, TFA, DMF, methanol) should be quantified against ICH Q3C class-3 limits by gas chromatography. The CoA should show the panel passed.

Red flags on a CoA

No lot number, or a lot number that does not match the vial.
Identity stated as a molecular weight only, with no observed mass.
Purity quoted with no method (HPLC wavelength, gradient) named.
No net peptide content — you cannot calculate molarity reliably without it.
A “CoA” that is identical across every lot (a real CoA is lot-specific).

FAQ

Is 99% purity always better than 98%?

Higher is generally better, but a 99% lot with no net-peptide value is less useful than a 98% lot with full documentation. Read the whole certificate.

How do I verify a BluGen lot?

Search the lot number in the CoA Library, or request the current-lot CoA from the procurement desk before ordering.