How to Read a Peptide COA and HPLC Purity Report
How to read a peptide COA: what an HPLC chromatogram shows, how purity is calculated under USP 〈621〉, and why mass spectrometry is needed alongside it.
A Certificate of Analysis (COA) is the document that tells you what is actually in a vial of research peptide. At its core is usually a High-Performance Liquid Chromatography (HPLC) purity report. Learning to read one is the single most useful skill for evaluating any supplier — and the findings below are summarised here in full, so you should not need to leave this page to understand what a chromatogram is telling you.
What HPLC measures
USP General Chapter 〈621〉 describes chromatography as a multistage separation in which the components of a sample distribute between two phases — one stationary, one mobile — and defines a peak as the portion of the chromatogram recording the detector response as a single component (or several unresolved ones) elutes from the column. For a peptide, the main peak is the target molecule; the smaller peaks are impurities such as truncated or deletion sequences and synthesis by-products.
What makes peptides separate at all is hydrophobicity. Mant and colleagues, in a detailed 2007 methods review of peptide HPLC, describe how peptides elute from a reversed-phase column in order of increasing overall hydrophobicity — the more water-repelling the sequence, the longer it holds onto the column. The same review explains why the detector is set where it is: the peptide bond itself absorbs strongly in the far ultraviolet around 220 nm, which is what makes a peptide visible to a UV detector regardless of its sequence.
How the purity percentage is actually calculated
USP 〈621〉 sets out the normalisation procedure: the percentage content of a component is calculated as the area of its peak expressed as a percentage of the total area of all the peaks. Two details matter, and most supplier marketing omits both. First, the calculation explicitly excludes peaks arising from solvents, reagents, the mobile phase or the sample matrix, and any peak at or below the stated disregard limit or reporting threshold — so "total area" is not simply everything the detector drew. Second, the chapter states the procedure applies provided linearity of the peaks has been demonstrated. A purity figure produced without those conditions is not wrong so much as undefined.
The numbers that matter
Peak area (%) — the purity figure; the main peak's area as a percentage of total integrated area, per USP 〈621〉 normalisation Detection wavelength — typically 214–220 nm, where the peptide bond absorbs Retention time — only interpretable against a reference standard run under identical conditions (see below) Baseline — a clean, flat baseline indicates a well-resolved sample Why retention time does not prove identity
It is tempting to treat a familiar retention time as confirmation that the right compound is in the vial. It is not. ICH Q6A — the guideline regulators actually adopt for drug-substance specifications — states plainly that identification solely by a single chromatographic retention time is not regarded as being specific. Two different molecules can elute at the same time; retention time shifts with column age, temperature and mobile-phase preparation. It is meaningful only relative to a reference standard run under identical conditions, which is why 〈621〉 defines relative retention rather than treating the raw number as an identity.
Confirming identity: mass spectrometry
HPLC tells you how pure a sample is, not what it is. ICH Q6A resolves this directly: it accepts two chromatographic procedures separating on different principles, or a combined technique such as HPLC/MS or HPLC/UV diode array, as specific identification. Mass spectrometry confirms the molecular weight; MS/MS data are needed to confirm the actual sequence, as the 2016 Clinical Chemistry consensus recommendations (Hoofnagle et al.) note — MS of the peaks in the UV trace is what establishes which peak is the desired peptide and which are impurities. This is why a complete COA pairs an HPLC purity figure with an MS identity confirmation, and why a purity number on its own is only half a document. If you want to check a reported molecular weight against the sequence yourself, our free peptide molecular weight calculator does the arithmetic.
Red flags of a weak or fabricated report
No batch number linking the report to the vial you received A purity figure stated with no chromatogram image No mass-spec identity confirmation — purity without identity is half a COA No stated detection wavelength, gradient or column conditions A single generic report reused across different products References to standards bodies without a specific chapter or method — a citation to an organisation is not a citation to a method How Vivera handles this
Every Vivera batch is tested by an independent US-based third-party laboratory. We provide the batch-specific Certificate of Analysis on request, so you can verify identity and purity for the exact material you receive. All products are supplied for in-vitro laboratory research use only.
Frequently Asked Questions How is HPLC purity percentage calculated? USP General Chapter 〈621〉 sets out a normalisation procedure in which the percentage content of a component is the area of its peak expressed as a percentage of the total area of all peaks. The calculation explicitly excludes peaks from solvents, reagents, the mobile phase or the sample matrix, and any peak at or below the stated disregard limit or reporting threshold. It also applies only provided linearity of the peaks has been demonstrated.
Does HPLC retention time prove a peptide’s identity? No. ICH Q6A states that identification solely by a single chromatographic retention time is not regarded as being specific. Two different molecules can elute at the same time, and retention time shifts with column age, temperature and mobile-phase preparation. It is meaningful only relative to a reference standard run under identical conditions.
Why does a complete COA need mass spectrometry as well as HPLC? HPLC tells you how pure a sample is, not what it is. ICH Q6A accepts a combined technique such as HPLC/MS as specific identification, where mass spectrometry confirms the molecular weight and MS/MS data confirm the sequence. This is why a complete COA pairs an HPLC purity figure with an MS identity confirmation, and why a purity number on its own is only half a document. If you want to check a reported molecular weight against the sequence yourself, the peptide molecular weight calculator does the arithmetic.
What are the red flags of a fabricated HPLC report? Warning signs include no batch number linking the report to the vial you received, a purity figure stated with no chromatogram image, and no mass-spec identity confirmation. Also watch for no stated detection wavelength, gradient or column conditions, a single generic report reused across products, and references to standards bodies without a specific chapter or method.
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For in-vitro laboratory research use only. Not for human or veterinary use, consumption, or therapeutic application. No medical claims are made.
