How to Read a Peptide Certificate of Analysis

What a proper certificate contains

A certificate of analysis, or CoA, is a laboratory report on one production lot. It should let someone who has never seen the vial understand what was tested, how, when and with what result.

• Identity of the product. The peptide name, its sequence or a reference to it, the molecular formula and the theoretical molecular weight.
• Lot number and date. A lot or batch number that matches the vial label, and the date of analysis.
• HPLC purity with its chromatogram. The purity figure, the chromatogram behind it, a peak table, and the method: column, gradient and detection wavelength.
• Mass spectrometry. The observed mass beside the theoretical mass, ideally with the spectrum itself.
• Appearance. Usually a white or off-white lyophilised powder; copper peptides such as GHK-Cu are blue.
• Laboratory and approval. Who carried out the analysis, and who reviewed and signed off the result.

Fuller certificates add water content, usually by Karl Fischer titration, counter-ion content and net peptide content. Research-grade documents do not always include them, but when they do they answer a question HPLC alone cannot.

How HPLC measures purity

High-performance liquid chromatography separates the components of a sample so each can be measured on its own. Peptides are analysed by reverse-phase HPLC: the sample is pumped through a steel column packed with silica particles carrying C18 chains, long hydrocarbon tails that make the surface water-repellent. The liquid phase starts as mostly water and grows steadily richer in acetonitrile, usually with a little trifluoroacetic acid to sharpen the peaks. Each peptide clings to the C18 surface according to how hydrophobic it is and leaves the column at its own point in the gradient, its retention time.

A UV detector at the outlet records absorbance, usually at 214–220 nm, where the peptide bond itself absorbs light. Because every peptide contains peptide bonds, related impurities show up whatever their sequence. Purity is calculated as area-percent: the area of the main peak divided by the total area of all integrated peaks, multiplied by 100. A result of 99.2% means that 99.2% of the UV signal at that wavelength came from the main peak.

That definition has limits. It assumes impurities absorb about as strongly as the target peptide, a fair approximation at 214 nm. It cannot see anything that does not absorb there, such as water or salts, and it cannot resolve an impurity that elutes at the same moment as the main peak. A convincing chromatogram shows a sharp, symmetrical main peak, a flat baseline and small side peaks that have been integrated rather than ignored.

Purity is not peptide content

HPLC purity describes the peptide material in the vial, not the powder as a whole. A lyophilised peptide is a salt. Its basic groups, the free N-terminus and the side chains of lysine, arginine and histidine, carry positive charges, each balanced by a negatively charged counter-ion. After synthesis and purification with TFA that counter-ion is usually trifluoroacetate; some manufacturers exchange it for acetate or chloride. The powder also holds water, because lyophilised peptides draw moisture from the air.

Net peptide content is the share of the powder weight that is peptide. It is measured by amino acid analysis or elemental nitrogen analysis, not by HPLC, and in practice it is always below 100%. Peptides rich in basic residues carry more counter-ions and so have a lower net content. Suppose a vial holds 10 mg of powder with an HPLC purity of 99% and a net peptide content of 80%. It then contains 8 mg of peptide, and a concentration calculated from the 10 mg figure would overstate the real one by 25%.

Labels differ on whether the stated amount refers to the powder or to the peptide inside. The certificate or product page should say which; where neither does, treat it as the weight of the powder. Our guide to storing and reconstituting research peptides carries the calculation through to a stock solution.

Mass spectrometry confirms identity

HPLC shows how pure a sample is, not what it is: a perfectly clean vial of the wrong peptide would still give a single sharp peak. Identity comes from mass spectrometry, which weighs the molecules themselves. Peptides are usually ionised by electrospray ionisation (ESI), often coupled directly to an HPLC system as LC-MS, or by matrix-assisted laser desorption/ionisation (MALDI) with a time-of-flight analyser.

The two methods give different-looking spectra. MALDI mostly forms singly charged ions, so the main signal sits at the molecular weight plus one proton, written [M+H]⁺. ESI forms multiply charged ions, [M+nH]ⁿ⁺, appearing as a series of peaks at mass-to-charge ratios (m/z) well below the molecular weight. Semaglutide, at 4113.6 Da, turns up at about m/z 1372.2 carrying three protons and 1029.4 carrying four. Software converts the series back into a single mass, a step called deconvolution.

The certificate should list the observed mass next to the theoretical one, and on a routine instrument the two should agree to within a dalton or so. Two details explain most small discrepancies. High-resolution instruments often report the monoisotopic mass, calculated from the commonest isotope of each element, which for a peptide of around 4 kDa is roughly 2.5 Da lower than the average mass on our profiles. And ions can carry sodium or potassium in place of a proton, adding about 22 or 38 Da.

Expected masses for peptides on this site

The table lists the formulas and average molecular weights used in our profiles; a certificate should report an observed mass that matches.

Three entries repay a closer look. TB-500 is a trade name: depending on the supplier it means full-length synthetic thymosin beta-4 or a short fragment around the actin-binding motif, such as Ac-LKKTETQ, which weighs under 900 Da. The mass tells you which is in the vial, as the TB-500 profile explains. For CJC-1295, a mass near 3647 Da means the DAC linker is present and one near 3368 Da means it is not, a difference that changes the half-life from days to minutes. Melanotan II and PT-141 differ by only about 1 Da, so telling them apart calls for a well-calibrated spectrum and, ideally, the retention time.

Typical impurities and their signatures

Most impurities in a synthetic peptide are close relatives of the target, which is why they appear as small peaks near the main one and why mass spectrometry can often name them.

• Deletion sequences. A coupling step that did not go to completion leaves chains missing one amino acid, lighter by that residue: 57 Da for glycine, 97 Da for proline.
• Truncated sequences. Chains that stopped growing part-way are much shorter and elute well away from the main peak.
• Oxidation. Methionine becomes methionine sulfoxide and tryptophan oxidises as well, each adding 16 Da per oxygen atom. MOTS-c, with two methionines and a tryptophan, is an obvious candidate.
• Deamidation. Asparagine and glutamine side chains can lose their amide group and turn acidic, a shift of just under 1 Da that a low-resolution spectrum can miss.
• Racemisation. An amino acid flips from the L to the D form during synthesis. The mass does not change, so only a separation method, usually HPLC, reveals it.
• Incomplete deprotection. Leftover protecting groups add characteristic masses, such as 56 Da for a tert-butyl group.

Deletion, truncation and leftover protecting groups come from the synthesis and should be cleared by purification. Oxidation and deamidation can develop later, during storage, which is why a certificate describes a lot at the time of analysis rather than for ever. The chemistry behind them is set out in our primer on what research peptides are.

Red flags on a certificate

Some problems are visible at a glance. Treat any of these as a reason to ask questions first.

• No lot number, or one that does not match the vial. Without a matching number the document cannot be tied to what you received.
• A generic or recycled certificate. The same document, date or chromatogram turning up for different lots, fill sizes or peptides.
• No chromatogram. A purity figure without the trace behind it cannot be checked.
• Masses that do not match. An observed mass more than a dalton or two from the theoretical value, or a theoretical mass that is wrong for the sequence.
• A round 100.00% purity. Real syntheses leave traces of related impurities; a perfect figure usually means small peaks were not integrated, or were edited out.
• No method or laboratory details. A result that does not say which column, wavelength or laboratory produced it cannot be repeated or questioned.

What King Peptides provides

King Peptides, the shop our order links go to, states 99%+ HPLC purity for its peptides and supplies a lot-specific certificate of analysis covering both HPLC and mass spectrometry. Orders are dispatched from the Netherlands, inside the EU, and every product is sold for research use only. Those are the shop claims; this guide is how a buyer confirms them for one vial. Match the lot number, read the chromatogram rather than the headline figure, and compare the observed mass with the table above. What else to check before choosing a supplier is in our guide to buying research peptides in Europe.

Frequently asked questions

What purity does a research peptide need? It depends on the experiment. For quantitative work a high HPLC purity matters, but so do a confirmed mass and a known net peptide content. A 99% figure attached to the wrong mass is worth nothing.

Why does my certificate show a slightly different mass from the table? Small differences come from monoisotopic versus average mass, rounding or instrument accuracy. Particular shifts have meanings: 16 Da for an oxidation, about 22 Da for a sodium adduct. A gap of hundreds of daltons points to a different compound.

Is a certificate from the supplier laboratory acceptable? In-house analysis is common in this trade. What matters is that the certificate is lot-specific, complete and internally consistent; an outside laboratory adds accountability only if the document is complete too.

Does a high HPLC purity mean the peptide is sterile? No. HPLC and mass spectrometry say nothing about sterility or endotoxins, which need separate tests.