How to read a peptide certificate of analysis (COA)

A peptide certificate of analysis (COA) is the document that tells you what is actually inside a vial. It lists the tests a laboratory performed on a specific batch and the results of those tests. A COA is the only objective evidence that the powder matches the label. This guide explains what a good COA contains, how to interpret each value, the difference between purity and net peptide content, and which signs point to a document that you should not trust.

What a peptide certificate of analysis contains

A COA is always tied to one batch, also called a lot. Different batches of the same peptide have different results, so the document must name the batch number that is printed on your vial. A complete COA for a research peptide usually shows the following items:

• Product name and sequence: the amino acid sequence or at least the name and molecular formula

• Batch or lot number: must match the vial label

• Theoretical and observed molecular weight: from mass spectrometry

• Purity: from reversed-phase HPLC, given as a percentage

• Appearance: usually white lyophilised powder

• Counter-ion: trifluoroacetate (TFA) or acetate

• Water content and net peptide content: if determined

• Endotoxin or bioburden: on some certificates

• Date of analysis and name of the testing laboratory

A COA with identity and purity is the minimum. A COA with net content and counter-ion is better, because those values change how you calculate concentrations when you reconstitute peptides.

Identity by mass spectrometry

Mass spectrometry (MS) measures the mass of the molecule. The COA lists a theoretical molecular weight, calculated from the sequence, and an observed molecular weight, measured by the instrument. The two values should agree within the tolerance of the method. For ESI-MS on a small peptide, a difference of less than 1 Da is normal. A larger difference means that the molecule carries a modification such as an extra oxygen atom or a missing amino acid.

Many certificates show the raw spectrum with several peaks. This is expected. A peptide with molecular weight 1419 Da appears at 1420 (singly charged, [M+H]+), at 710.5 (doubly charged) and at 474 (triply charged). These are the same molecule with different numbers of protons attached.

MS confirms what the molecule is. It does not tell you how much of the sample is that molecule. That is the job of HPLC, which we explain in our article on peptide purity, HPLC and mass spectrometry.

Purity by HPLC on a peptide certificate of analysis

Reversed-phase HPLC separates the sample into its components and detects them by UV absorbance, usually at 214 or 220 nm. The result is a chromatogram with peaks. The purity value on the COA is the area of the main peak divided by the total area of all peaks. A purity of 98.5% means that 98.5% of the UV-absorbing material eluted as the main product. Check three things:

• The wavelength. 214 nm detects the peptide bond and sees almost all impurities. 280 nm detects only tryptophan and tyrosine and can hide impurities. A purity measured at 280 nm is less informative.

• The chromatogram itself. A good COA shows the trace. One sharp main peak and a flat baseline is the picture you want. Several peaks close to the main peak point to deletion sequences or oxidised forms.

• The method. Column, gradient and run time should be listed. A very short run can merge impurities into the main peak and make purity look better than it is.

Net peptide content versus purity

This is the point where most readers go wrong. Purity tells you what fraction of the peptide material is the correct sequence. Net peptide content tells you what fraction of the total powder mass is peptide at all. The two numbers are independent.

A lyophilised peptide powder is never 100% peptide. It also contains bound water, counter-ions and sometimes residual salts. A typical synthetic peptide with 98% purity may have a net peptide content of 70 to 85%. If a vial is labelled 10 mg and the net content is 80%, the vial contains 8 mg of peptide. If you need an exact concentration, calculate with the net content, not with the label weight. Net content is usually determined by amino acid analysis or nitrogen analysis. If the COA does not state it, assume the label shows the gross weight.

Counter-ions: TFA and acetate

Peptides are purified by HPLC with an acid in the mobile phase. The most common acid is trifluoroacetic acid (TFA). After purification, the TFA stays bound to the basic groups of the peptide as a counter-ion. A peptide with several basic residues can carry 10 to 20% of its weight as TFA salt. Roux and colleagues reviewed the methods to remove or exchange TFA in the Journal of Peptide Science in 2008 (DOI 10.1002/psc.951). Acetate is the usual alternative. It is exchanged in an extra step and costs more.

The counter-ion matters for two reasons. First, it changes the net peptide content and therefore the real amount in the vial. Second, TFA is not inert in cell-based assays. Cornish and colleagues showed in 1999 in the American Journal of Physiology that trifluoroacetate left over from purification changed the growth of cultured osteoblasts and chondrocytes (DOI 10.1152/ajpendo.1999.277.5.E779). For cell culture work, an acetate salt or a TFA removal step is the better choice. A COA that does not name the counter-ion leaves you guessing.

Water content, endotoxin and other tests

Water content is measured by Karl Fischer titration and usually lies between 3 and 10% for a lyophilised peptide. It feeds into net content. Endotoxin is tested with the LAL assay and reported in endotoxin units per milligram. This test is relevant for cell-based assays where bacterial contaminants would distort the result. Residual solvents such as acetonitrile appear on pharmaceutical-grade certificates but rarely on research-grade ones.

Third-party versus in-house testing

A COA can come from the manufacturer, from the reseller or from an independent laboratory. In-house testing is not wrong, but it has a conflict of interest. A third-party COA from a named, accredited laboratory with its own report number is stronger evidence. The best case is a supplier that publishes both: the manufacturer certificate and an independent re-test of the same batch. Our lab-tested peptides collection lists the products for which batch testing is available, for example BPC-157 10 mg. Check that the laboratory exists and that the batch number matches your vial. A reputable testing lab will confirm a report number on request.

Red flags on a peptide certificate of analysis

Some patterns appear on certificates that are reused, edited or invented. Treat the following as warning signs:

• No batch number, or a batch number that does not match the vial

• A purity of exactly 99.0% or 99.9% on every product and every batch

• No chromatogram or spectrum, only a number in a table

• Observed molecular weight that is identical to the theoretical value to four decimal places

• No date of analysis, or a date that is older than the stated shelf life

• Blurred or cropped images that hide the laboratory name

None of these alone proves that a product is bad. Together they should lower your confidence. Correct storage after delivery decides whether the batch still matches the certificate months later, which we cover in our guide on how to store peptides.

Frequently asked questions about reading a peptide COA

Sources

• Roux S, Zékri E, Rousseau B, Paternostre M, Cintrat JC, Fay N. Elimination and exchange of trifluoroacetate counter-ion from cationic peptides: a critical evaluation of different approaches. Journal of Peptide Science. 2008. DOI 10.1002/psc.951

• Cornish J, Callon KE, Lin CQ, Xiao CL, Mulvey TB, Cooper GJ, Reid IR. Trifluoroacetate, a contaminant in purified proteins, inhibits proliferation of osteoblasts and chondrocytes. American Journal of Physiology-Endocrinology and Metabolism. 1999. DOI 10.1152/ajpendo.1999.277.5.E779

This article describes laboratory handling of research compounds. All products on this site are sold for laboratory research use only and are not intended for human or veterinary use.