How to Read a Peptide COA: A Plain Guide for UK Researchers
A certificate of analysis, or COA, is the report that shows what was found when a batch of peptide was tested. It usually lists a few numbers, and each one answers a different question. Mixing them up is the most common mistake when reading one. Here is what the main entries mean. Real examples are on our lab reports page, which is a useful companion while you read.
What Does HPLC Purity Mean?
HPLC separates a sample into its parts as it passes through a column, and a UV detector records each part as a peak. Peptide bonds, the links between amino acids (see our introduction to peptide research for the basics), absorb UV light at about 210 to 220 nm, so that is the range used. Purity is the area of the main peak as a percentage of the area of all peaks together. That makes it a share of what the detector could see, not an absolute amount.
Impurities that come out at the same time as the main peak are hard to spot, and anything that does not absorb UV light does not show up at all. A good report also lists the wavelength, the column, the solvents (commonly water and acetonitrile with 0.05 to 0.1% trifluoroacetic acid) and the gradient.
What Does Mass Spectrometry Confirm?
It weighs the molecules. The result is compared with the weight calculated from the peptide's sequence, so a match confirms identity. Two methods are common. MALDI mostly produces ions carrying a single charge, while electrospray ionisation (ESI) produces ions with several charges, which allows instruments to measure larger molecules, as described in this NLM review of the two methods.
A matching weight does not show purity, and it cannot tell apart molecules with the same weight, such as mirror-image versions of the same peptide.
What Is Net Peptide Content?
Net peptide content (NPC) is the share of the powder that is actually peptide, as opposed to water and counterions, which are salts that come along with the peptide. It is not the same as purity. A peptide can look very pure on HPLC and still contain a lot of trifluoroacetate or acetate plus some moisture, because neither shows on a UV trace.
Peptides usually arrive as trifluoroacetic acid (TFA) salts, because TFA is used both to release the peptide from the synthesis resin and in the HPLC solvents. A 2025 study measured TFA at up to about 35% of the salt weight, and found that swapping it for chloride with HCl and then freeze-drying did not change purity. NPC is measured by amino acid analysis or CHN elemental analysis, and water by Karl Fischer titration.
What Else Should a COA Include?
Common practice is to include the lot number, the test date, the appearance (freeze-dried peptides, such as those in our lyophilised powder range, are usually white powders), the methods used and the name of the testing laboratory. This is general practice, not a fixed standard. When comparing peptides from different UK suppliers, or across a range such as our peptides collection, it is worth checking that each batch has its own report.
The European Medicines Agency guideline on synthetic peptides, effective from June 2026, is a useful benchmark for what can be tested. It lists residual TFA or counterion content, water content and bacterial endotoxins among the items a specification should include. It describes peptide-related impurities as deletion, truncated, insertion and stereoisomer types, and it treats purity as one of the most important quality attributes.
Important: This article is for scientific information only. The peptides discussed are intended exclusively for scientific and laboratory research and are not intended for human or veterinary consumption, administration, diagnosis, treatment or therapeutic use.