How to Read a Peptide Certificate of Analysis

A certificate of analysis is the document that turns a labelled vial into a known quantity. It is produced at the batch level by the manufacturer, and it reports, at minimum, the sequence identity, the HPLC purity, the mass-spectrometric molecular weight, and the solvent and water content of the specific lot that vial came from. For research peptides, the COA is not optional documentation. It is the analytical basis for every downstream protocol decision, and a researcher who cannot read one cannot tell the difference between a legitimate research-grade peptide and a well-labelled bag of mystery powder.

This guide walks through each section of a typical peptide COA, explains what the analytical methods prove (and don’t), and lists the red flags that should stop a vial from entering your research program.

Why a COA matters more for peptides than for small molecules

Small-molecule chemistry has centuries of infrastructure behind it: defined reference standards, well-characterised impurity profiles, and regulatory frameworks that set floor levels of analytical quality even for research-grade material. Peptide chemistry does not share that depth of infrastructure, and the analytical toolkit for peptides is more specialised.

Solid-phase peptide synthesis (SPPS), the dominant production method, is efficient but not perfect. Deletion sequences (a residue skipped during coupling), truncations (synthesis stopped early), side-chain modifications (oxidation, racemisation), and residual protecting groups are all routine synthesis artefacts that require downstream purification and analytical detection. The analytical question for every lot of a research peptide is: of the material in this vial, how much of it is actually the sequence on the label, and what is the other part?

Gilg and colleagues laid out a canonical framework for this analytical question for pharmaceutical peptides and proteins, using erythropoietin as the worked example ⁵. The principles apply directly to SPPS-produced research peptides: reverse-phase HPLC for purity, mass spectrometry for identity, orthogonal methods for cross-confirmation. A modern research-grade COA is a direct descendant of that framework.

Anatomy of a peptide COA

A well-formed peptide COA has seven standard sections. They may be named differently from vendor to vendor, but the substance is consistent:

1. Identifying information. Product name, sequence (one-letter or three-letter), batch/lot number, synthesis date, analyst, expiration or retest date.
2. Appearance. Colour, form, solubility description.
3. HPLC purity. The headline number. Typically reverse-phase HPLC at one or two wavelengths, reported as area-percent.
4. Mass spectrometry. Identity confirmation. Observed vs. theoretical molecular mass.
5. Solvent and water content. Residual TFA, acetic acid, acetonitrile; water content by Karl Fischer titration.
6. Peptide content. The percentage of the vial that is actual peptide (the rest is counter-ion, water, excipients). Often reported alongside net peptide mass.
7. Signatures or digital equivalents. The analyst’s sign-off, any QA review, release date.

Each section has its own interpretation, and each is worth reading literally rather than skipping to the HPLC headline.

The HPLC purity line: what it actually measures

Reverse-phase HPLC (RP-HPLC) is the standard method for peptide purity assessment. A sample is dissolved, injected onto a C18 column, eluted with a water/acetonitrile gradient containing a counter-ion modifier (typically TFA), and detected by UV absorbance at 214 nm (amide-bond absorbance) and/or 220/280 nm.

The resulting chromatogram is a trace of absorbance over time. Pure peptide appears as a tall main peak; impurities appear as smaller peaks at different retention times. Purity is reported as the area of the main peak divided by the total area of all peaks, expressed as a percentage.

What this measurement proves:

• What fraction of the UV-absorbing material is the main peak. If the main peak is the intended peptide (confirmed separately by mass spec), purity is meaningful.
• That the separation worked. A clean baseline, sharp peak, and sensible retention time are all part of the purity assessment.

What HPLC purity does not prove:

• Identity. A contaminant with similar retention behaviour would coelute and be counted as part of the main peak. Identity confirmation is the mass-spec job.
• Purity of non-UV-absorbing species. Salts, counter-ions, and inorganic contaminants are invisible to UV and are quantified by orthogonal methods (Karl Fischer for water, ion chromatography for counter-ions, etc.).

Research-grade expectations:

• ≥98% by RP-HPLC is the standard benchmark for most modern research peptides. This is achievable for well-established sequences and is the floor you should expect.
• 95–97% is acceptable for historically difficult sequences, natural-source-derived peptides, or peptides with known purification challenges. It is worth a line on the COA explaining why.
• Below 95% is technical-grade, not research-grade. For pharmacology, PK, or any dose-response work, this material is not fit for purpose.

Hettiarachchi and colleagues illustrated the research-level characterisation workflow for biphalin, an opioid peptide with a palindromic sequence. HPLC purity was confirmed alongside complementary analytical methods to support the claim ¹. This is the pattern modern research-grade COAs follow.

The mass spec section: what it proves about identity

Mass spectrometry answers a different question than HPLC. Where HPLC asks how much of the sample is the main peak, mass spec asks is the main peak the molecule on the label.

Standard methods for peptide identity confirmation:

• ESI-MS (electrospray ionisation). Typical for peptides under ~5 kDa. Produces one or more charge states; the reported molecular mass is deconvoluted from the charge envelope.
• MALDI-TOF. Common for larger peptides. Produces a singly-charged ion at [M+H]⁺.
• High-resolution MS (Orbitrap, FT-ICR). Higher precision. Used for reference-standard characterisation and for peptides where isotope resolution matters.

Interpreting the mass spec line on a COA:

• Observed vs. theoretical mass. A reported mass within ~0.1% of the theoretical monoisotopic mass is a strong identity match for most sequences. For higher-resolution MS, the expected deviation is much tighter.
• Monoisotopic vs. average mass. Modern COAs typically report monoisotopic mass for small peptides and average mass for larger ones. They should be within 0.5–1 Da of each other for short sequences and more for long ones. The COA should state which it is using.
• Single clean peak. The spectrum should show one dominant ion in the expected charge-state envelope. Multiple unexpected peaks suggest a mixture.

Vázquez-Leyva and colleagues demonstrated a modern application: structural and mass-mobility orthogonal analysis for identity profiling of complex peptide mixtures ². The principle generalises: orthogonal methods give more confidence than any single technique. A COA that reports both HPLC and mass spec is doing the baseline job; one that reports only HPLC is asking you to trust identity on the strength of retention time alone, which is not enough.

Residual solvents and water content

Two small but important lines on the COA:

Residual solvents are typically TFA (from RP-HPLC purification), acetic acid (from acetate-form salt exchange), and acetonitrile (residual mobile phase). Reported as percent by weight.

• TFA. Common, usually ≤3% by weight. For most research protocols, tolerated. For cell-based assays where TFA salt form matters, specify acetate form on the order.
• Acetic acid. Common for peptides supplied as acetate salt, which is the physiological counter-ion for most peptide therapeutics and the preferred form for in vivo work.
• Acetonitrile. Should be at or below trace levels (< 0.1%). Higher levels suggest incomplete lyophilisation.

Water content (Karl Fischer) is typically 2–8% for lyophilised peptides. The Karl Fischer titration is the standard method because it measures water directly rather than weight-loss-on-drying, which would also capture residual volatile solvents.

These lines matter for two reasons:

1. Net peptide mass. The vial label says “5 mg.” If the peptide content is 85% (counter-ions plus water make up the rest), the vial contains 4.25 mg of actual peptide. Concentration math must account for this.
2. Counter-ion form. For some research protocols, particularly those involving electrophysiology, ion-channel binding, or sensitive cell-based assays, the counter-ion matters pharmacologically.

Impurity profile and immunogenicity risk

The impurity profile is the list of minor peaks and what they are, to the extent that the manufacturer has characterised them. Most research-grade COAs do not individually identify every minor peak (that is reserved for GMP-grade material), but they do at minimum report the total impurity percentage and may flag any single impurity exceeding a threshold (typically 0.5% or 1%).

For research, the impurity profile matters because:

• Different impurities carry different pharmacological risks. Deletion sequences (one residue missing) may still bind the target; side-chain-modified variants may bind unrelated targets. Summed 2% impurity is not the same as 2% of one well-characterised truncated sequence.
• Immunogenicity can track with specific impurity classes. Roberts and colleagues assessed the immunogenicity risk of salmon calcitonin impurities using combined in silico and in vitro methods, showing that the class and structure of the impurity drove risk independently of the summed percentage ⁴. The same principle applies to any research peptide entering an immunologically competent system.

A COA that characterises the top 2–3 impurities by name or by mass is more useful than one that reports only a summed percentage. Neither is disqualifying; both are common.

Batch number, dates, and traceability

The boring section of the COA is also the section that distinguishes legitimate research-grade material from its rougher cousins.

A well-formed COA includes:

• Batch or lot number that matches the sticker on the vial.
• Synthesis date.
• Retest or expiration date. Typically 12 or 24 months from synthesis for lyophilised material stored at −20 °C.
• Analyst’s name or initials (or digital equivalent) and a QA sign-off.
• Document date on the COA itself.

Krug and colleagues analysed black-market growth-promoting peptide products and documented systematic mismatch between labelled content and actual content, including products that contained either the wrong peptide or less peptide than labelled ³. A COA that is missing a batch number, a synthesis date, or an analyst sign-off is not a COA that enables traceability. That is what the analytical process is supposed to produce.

Red flags

A non-exhaustive list of reasons to reject a vial:

• HPLC purity below 95% on a compound that the research literature produces routinely at ≥98%.
• No mass-spec identity confirmation. HPLC-only COAs provide purity against an unverified target; they do not confirm sequence identity.
• No batch number on the COA or on the vial. COA lot number does not match vial lot number.
• No synthesis date. “Recently synthesised” is not a date. Request the COA with the date, or decline.
• No named analyst or QA signature. A COA without accountability is a templated claim.
• Water content above 10% on lyophilised material. Suggests incomplete freeze-drying and likely reduced shelf stability.
• Unexplained anomalies. Unusual counter-ion reported, atypical retention time, mass outside expected tolerance. Ask first; reject if the explanation is not satisfactory.
• Compounds repeatedly flagged by black-market-product analyses ³ where the manufacturer has no independently verifiable batch history.

How Thailand Peptides handles this

Every compound on this site is supplied against a batch-level COA from the manufacturing partner, covering the sections described above: HPLC purity, mass spec identity, residual solvents, water content, peptide content, and batch traceability. COAs are available on request via the Bangkok research desk for any active research program.

The research desk does not ship a vial without a matching COA, and researchers ordering compounds for primary data collection should request the COA proactively and keep it with their protocol documentation. For any compound where the COA raises a question (reported purity lower than expected, unusual counter-ion, anomalies in the mass-spec section), the research desk will provide the analytical method and, where applicable, the raw chromatogram files.

The reconstitution guide and the storage and handling guide are the natural next reads once a vial’s COA is cleared and it is in hand.