A practical guide for anyone evaluating research-grade material, written from the analytical side.
In short
- A COA describes one lot. A certificate detached from the lot number it was issued against carries no information.
- HPLC purity is a relative area measurement at one detection wavelength, not an absolute statement of composition.
- Purity and identity are separate questions and need separate methods — chromatography for the first, mass spectrometry for the second.
- Peptide content, water content and counterion content are routinely omitted, and their absence is why net peptide mass in a vial can sit well below the labelled figure.
It certifies a lot, not a product
The single most common misreading of a certificate of analysis is treating it as a property of the compound rather than of one manufactured batch. Synthesis is variable. Two lots of the same sequence from the same facility can differ in impurity profile, water content and net peptide mass. A COA is a record of what was measured on one of them.
So the first thing to check is not a number, it is whether the lot identifier on the certificate matches the lot identifier on the vial in front of you. If it does not, the document is decorative. This is also why certificates are not usefully published on an open catalog page — by the time anyone reads the page, the lot has changed.
What HPLC purity actually measures
Reverse-phase HPLC separates components of a mixture by hydrophobicity and reports each as a peak. Purity is then expressed as that peak's area as a percentage of total integrated area, almost always with UV detection at 214 nm, where the peptide bond itself absorbs, or at 280 nm, where aromatic side chains do.
Two consequences follow, and both are routinely missed. First, the figure is relative: it describes the proportion of what the detector saw, so anything that does not absorb at the chosen wavelength — many salts, residual solvents, water — is invisible to it. A lot can read 99% by HPLC and still be a substantial fraction non-peptide by mass. Second, co-elution is real. A closely related impurity, a deletion sequence or an epimer, can hide under the main peak at one gradient and resolve into a shoulder at another.
A certificate worth reading therefore states the method, not just the result: column chemistry, gradient, flow rate, detection wavelength. The USP general chapter on chromatography, <621>, is the reference point for how those parameters are reported and validated.
Identity is a different question
Chromatography tells you how many things are in the vial and in what proportion. It does not tell you that the main peak is the sequence on the label. For that you need mass spectrometry: ESI-MS or MALDI-TOF, with the observed monoisotopic or average mass compared against the theoretical mass calculated from the sequence.
A match within a few daltons is the baseline. Where it matters, tandem MS fragmentation or Edman sequencing confirms the actual residue order, because isobaric substitutions — swapping two residues of identical combined mass — will pass a simple mass check. Acetate and trifluoroacetate adducts also shift observed masses and should be accounted for rather than explained away.
A certificate that reports purity but not identity has answered the easier of the two questions.
The figures usually missing
Peptide content is the fraction of the total powder mass that is actually peptide, determined by amino acid analysis or nitrogen determination. Lyophilised peptides are hygroscopic and are usually isolated as salts, so a vial labelled 10 mg is 10 mg of powder, and the net peptide mass can be meaningfully less. Water content by Karl Fischer titration or loss on drying quantifies part of the gap; residual counterion content quantifies another.
For any work where the material contacts cells, bacterial endotoxin — by LAL assay, reported in EU/mg — and sterility are the relevant additional lines. Residual solvents from synthesis and cleavage fall under the ICH Q3C framework. Method validation generally, including the parameters an analytical result is expected to be validated against, is covered by ICH Q2(R2) and USP <1225>.
None of this is exotic. It is simply the difference between a certificate that documents a lot and a certificate that decorates it.
A short checklist
Does the lot number match the vial? Is the sequence printed in full, with any modification and the salt form stated? Is purity given with its method and detection wavelength, and is a chromatogram attached rather than only a number? Is identity confirmed by MS, with theoretical and observed mass both shown? Are water content and peptide content reported? Is the assay date recent relative to the retest date, and is a storage condition specified?
Where the answer to several of those is no, the honest reading is that the material is uncharacterised, whatever the headline percentage says.
References
- United States Pharmacopeia. General Chapter <621> Chromatography. USP–NF.
- United States Pharmacopeia. General Chapter <1225> Validation of Compendial Procedures. USP–NF.
- International Council for Harmonisation. ICH Q2(R2) Validation of Analytical Procedures. ICH Harmonised Guideline, 2023.
What this article is, and is not
This is a summary of published research, written for qualified professionals evaluating compounds for laboratory work. Every compound discussed is supplied by strictly for in-vitro and laboratory research use. None is a drug, a dietary supplement, or a cosmetic; none is intended for human or veterinary use; and nothing above is medical advice, a treatment recommendation, or a claim that any compound produces any effect in a person. We publish no dosing or administration guidance of any kind.
