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. Short of commissioning your own analysis it is the main 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, make up the remainder, and how much depends on the sequence: trifluoroacetate pairs with basic residues, so a peptide rich in lysine or arginine carries more of it and has a lower net peptide content. 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 supplies the most recent certificate for a batch on request from the product page.
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.
