How to read a certificate of analysis (COA) for research peptides

A certificate of analysis, usually shortened to COA, is the document a supplier provides to show what testing was run on a specific batch of material and what the results were. For research peptides, it is the closest thing you have to a paper trail: it links a physical vial to a set of measurements taken at a known point in time. Reading one well takes a few minutes and tells you a lot about whether a batch matches what was ordered.

This guide walks through the main sections you will see on a peptide COA and what each number actually means. None of it requires specialist software, just a careful eye and a rough sense of what good looks like.

What a certificate of analysis is

A COA is a batch-specific record. It is not a marketing sheet and it is not a general product description. Each certificate should name the compound, carry a unique lot or batch number, list the tests performed, and state each result against a specification (the pass or fail threshold). If a document gives you a purity figure with no method, no lot number, and no date, regard it as a claim rather than a test result.

You can cross-check a certificate against our records using the lot lookup on our verify page, and browse example documents on our certificates page.

HPLC purity

High-performance liquid chromatography (HPLC) separates the contents of a sample as they pass through a column, then a detector records each component as a peak. Purity is reported as the area of the main peak divided by the total area of all peaks, given as a percentage. A figure of 98% means the main component accounts for 98% of the detected area, with the remaining 2% split across related impurities such as truncated or deletion sequences.

Two practical points. First, HPLC purity is an area measure, not a weight measure, so it describes the proportion of peptide-related material, not how much of the vial is peptide (that is net peptide content, below). Second, look for the detection wavelength (often 214 nm or 220 nm, where peptide bonds absorb) and, if shown, the gradient. A chromatogram with a single dominant peak and a flat baseline is a good sign; a cluster of shoulders next to the main peak points to closely related impurities.

Mass-spec identity

Where HPLC tells you how much of one thing is present, mass spectrometry (MS) tells you what that thing is. The instrument measures the mass-to-charge ratio, and the COA reports an observed mass next to the theoretical (calculated) mass for the sequence. The two should match within a small tolerance, typically a fraction of a percent for the methods used on peptides.

Check that the observed value sits close to the expected value. Peptides often show up at several charge states, so you may see more than one number; the deconvoluted or monoisotopic mass is the one to compare against the theoretical figure. A mass that is off by a consistent amount can indicate a modification, a missing residue, or the wrong counterion being counted.

Net peptide content

Net peptide content answers a question purity does not: of the total powder in the vial, how much is actually peptide? The rest is water, residual salts, and counterions left over from synthesis and purification. A vial can be 98% pure by HPLC and still be, say, 80% peptide by mass, because purity and content measure different things.

Net peptide content is usually determined by amino acid analysis or nitrogen content and reported as a percentage of net weight. It matters whenever you are working from a stated milligram amount, because the labelled weight and the peptide weight are not the same number. If a COA omits net peptide content, the gross weight on the label is all you have.

Salt form and counterion

Synthetic peptides are usually supplied as a salt. The counterion is the ion paired with the peptide, most commonly acetate or trifluoroacetate (TFA), sometimes hydrochloride. The salt form is worth noting for two reasons: it contributes to the total weight (and therefore affects net peptide content), and TFA in particular can interfere with some assays. A COA that names the salt form is giving you information you need to interpret the other numbers correctly.

Appearance

Appearance is the simplest entry and still useful. Most lyophilised (freeze-dried) peptides are described as a white to off-white powder or cake. The COA records what the material looked like at testing. If what arrives does not match (discolouration, obvious moisture, or a collapsed cake where a solid one was described), that mismatch is worth flagging before anything else is done with the vial.

Lot number and test date

The lot number (or batch number) is the identifier that ties every result on the certificate to one specific production run. Two vials of the same compound from different lots can have slightly different purity and content figures, which is exactly why the lot number exists. Keep it with your records so a result can always be traced back to its source. You can confirm a lot against our system on the verify page.

The test date is when the analysis was performed, which is not necessarily when the batch was made or when you received it. It gives the results a point in time. Read it alongside any stated storage conditions, since the figures describe the material as tested, not indefinitely into the future.

For laboratory research use only. Not for human or veterinary use.

Common questions

Does a high HPLC purity mean the vial is mostly peptide?

Not on its own. Purity is the share of the main peak among detected peptide-related material. Net peptide content is what tells you how much of the powder is peptide rather than salt and water. Read the two figures together.

What purity figure is normal for a research peptide?

Many synthetic research peptides are supplied at 95% or higher by HPLC, and short, well-behaved sequences often test higher. The useful check is not a single magic number but whether the reported figure meets the stated specification and is backed by a chromatogram and a named method.

The observed mass differs slightly from the theoretical mass. Is that a problem?

A small difference within the method's stated tolerance is expected and normal. A large or consistent gap can point to a modification, a missing residue, or the counterion being included in the count. If the gap is well outside tolerance, ask the supplier before proceeding.

References

1. Chandrudu S, Simerska P, Toth I. Chemical methods for peptide and protein production. Molecules, 2013.
2. United States Pharmacopeia. General Chapter 621 Chromatography. USP-NF.