How to read a peptide certificate of analysis

A certificate of analysis (COA) is the document that turns a claim into evidence. “≥99% pure” printed on a label is marketing; the same figure backed by a batch-matched COA with the method attached is data. Knowing how to read one is the difference between trusting a material and hoping.

Identity: is it the right molecule?

Purity is meaningless if the peptide isn’t what you think it is. A COA should first establish identity — confirmation that the material matches the expected sequence. This is typically done by mass spectrometry, which compares the measured mass against the theoretical mass for that sequence.

When you read this section, check that the observed mass matches the expected mass for the peptide you ordered. A match here is what lets every downstream number describe the right molecule.

Purity: how much of it is the peptide?

The headline number on most COAs is HPLC purity, usually reported as an area percentage. In plain terms: the sample is separated by high-performance liquid chromatography, the main peak is the peptide, and its area is expressed as a percentage of the total. A result of 98% means the main peak accounts for 98% of the detected material; the remaining 2% is everything else — related sequences, residual reagents, and other minor species.

A few things worth checking:

• Which method? Reversed-phase HPLC is standard. The method conditions (column, gradient, detection wavelength) belong on or with the certificate.
• Area % is not absolute mass. It’s a proportion of what the detector saw at a given wavelength — a sound, standard measure, but understand what it is.
• Higher isn’t automatically necessary. The purity you need depends on your application. Match the grade to the work.

Batch: does this paper describe this vial?

This is the detail most people skip, and it matters most. A COA should be batch-matched — tied to the specific lot the vial came from, with a batch or lot number you can cross-check against the label.

A generic certificate that could apply to any lot tells you what the product usually looks like, not what your material actually is. If the lot number on the vial doesn’t appear on the certificate, you don’t have a COA for that vial — you have a sample document.

The questions a good COA answers

Before you rely on a material, its certificate should let you answer:

1. What is it? Identity confirmed, observed mass matching expected mass.
2. How pure is it? A purity figure, by a named method, at a stated wavelength.
3. Which batch? A lot number that matches the vial in your hand.
4. When? A date, so you know how current the analysis is.

If a certificate can’t answer those four, treat the numbers as provisional and ask. A supplier confident in its material will be glad to show the underlying data.

All materials referenced here are for laboratory research use only and are not intended for human or veterinary use.