Every research peptide should arrive with a certificate of analysis (CoA): a report from an analytical laboratory describing the tests run on a specific batch and the results. It is the only objective evidence of identity and purity a researcher has, so knowing how to read one is a core skill. This guide covers the three measurements that matter most and the details that show whether a certificate can be trusted.
Purity by HPLC
High-performance liquid chromatography separates the components of a sample by pushing it through a column under pressure. Each component leaves the column at a characteristic time and registers as a peak on a chromatogram. Purity is the area of the main peak divided by the total area of all peaks, expressed as a percentage. A result of 99% means that 99% of the ultraviolet-absorbing material detected was the target sequence; the remaining 1% is typically deletion sequences (a missing residue), truncated chains, or oxidation products.
Two details decide how much a purity figure means. The first is the method: the column, gradient and wavelength should be stated, because an unsuitable gradient can hide impurities under the main peak. The second is the chromatogram itself. A certificate that shows the trace lets you see the peak shape and any shoulders; one that only prints a number asks you to take it on trust.
Identity by mass spectrometry
Purity says how much of the sample is one thing; mass spectrometry says what that thing is. The instrument ionises the peptide and measures its mass-to-charge ratio, and the observed mass is compared with the mass calculated from the intended sequence. Agreement to within a fraction of a dalton confirms the sequence has the right composition. A purity figure without a mass result is incomplete: a sample can be 99% pure and still be the wrong peptide.
Net peptide content and water
Lyophilised peptides are not 100% peptide by weight. Bound water and counter-ions from purification (usually trifluoroacetate) typically make up 10–30% of the mass. Net peptide content, measured by amino-acid analysis or nitrogen determination, states how much of the powder's weight is actually peptide. It matters for concentration calculations: a 10 mg vial at 80% net peptide content contains 8 mg of peptide. Not every supplier reports it; when it is absent, calculations should assume the label mass is gross weight.
What a credible certificate includes
- The batch or lot number, matching the vial label, and the date of analysis.
- The name and location of the testing laboratory, ideally independent of the seller.
- The method for each test, with enough detail to repeat it.
- The actual data: a chromatogram for HPLC and a spectrum or observed mass for MS, not just a pass/fail.
- Acceptance criteria alongside results, so you can see the specification the batch was tested against.
Red flags
Be cautious of certificates with no batch number, a purity of exactly 99.9% or 100% with no trace shown, a generic laboratory name that cannot be found, results that never vary between batches, or a document that reads as marketing rather than as a lab report. None of these proves a problem, but each removes a reason for confidence. Where a certificate is unclear, the right response is to ask the supplier for the underlying data or to commission an independent test. PepChat publishes the certificate for each batch in the CoA library and supplies the full report on request.
Key takeaways
- HPLC purity is the share of the chromatographic signal from the intended sequence; the trace should be shown, not just the number.
- Mass spectrometry confirms identity; purity without identity is incomplete.
- Net peptide content tells you how much of the powder's weight is peptide and affects every concentration calculation.
- A credible certificate names the lab, the batch, the method and the acceptance criteria, and shows the data.
