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How to Read a Certificate of Analysis (COA)

A Certificate of Analysis (COA) is the analytical fingerprint that travels with a research compound. For a synthetic peptide intended strictly for in-vitro and laboratory-research use, it is the document that answers two separate questions: is this material actually the molecule named on the label, and how much of the sample is that molecule versus everything else. Those are the identity question and the purity question, and a competent COA reports them with different instruments and different numbers. This guide walks through the sections a peptide COA typically contains, what each analytical method measures, and how the figures relate to the structural facts a peptide carries on paper, such as its sequence, molecular formula, and calculated mass. The framing throughout is laboratory and bench-science only: a COA characterizes a chemical reference material so that an investigator can interpret a receptor-signaling or cell-culture experiment, not anything beyond that. Wherever specific peptides appear below, the structural values are drawn from a curated reference dataset and used purely to illustrate how catalog identity data lines up against what an instrument reports.

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Key takeaways

A COA answers two distinct questions with two different methods: HPLC reports purity (how clean), while mass spectrometry confirms identity (what it is).
HPLC purity is a relative peak-area percentage under one UV detection method, not an absolute mass measurement, and it does not by itself prove the molecule's identity.
Mass spectrometry confirms identity by matching an observed deconvoluted mass to the compound's calculated molecular weight, for example tesamorelin's 5135.9 g/mol versus the GHK tripeptide's 340.38 g/mol.
Salt form and counter-ions change the expected mass: TB-500 reads near 889.0 g/mol as free base but near 949.1 g/mol as the acetate salt, so results must be read against the correct form.
Sequence and specific modifications are the true identity backbone; near-isobaric analogues like semaglutide (C187H291N45O59, 4113.58 g/mol) are distinguished by defined substitutions, not mass alone.
Net peptide content, water (Karl Fischer), residual solvents, and lot/date traceability complete the picture; a sheet missing batch identity or method detail is hard to trust.

Section 01

What a COA Actually Certifies

A COA is a batch-specific analytical report, not a marketing sheet.

A COA is a batch-specific analytical report, not a marketing sheet. It ties a particular lot of material to a set of measurements taken on that lot. Read in order, a useful peptide COA establishes three things. First, identity: the analysis is consistent with the named compound and not a different sequence or a degradation product. Second, purity: the fraction of the sample that is the target peptide, with the balance being synthesis-related impurities, salts, water, or counter-ions. Third, traceability: a lot or batch number, a manufacture or test date, the analytical methods used, and ideally the name of the testing party. The header carries the catalog claims the lab is checking against, typically the compound name, the stated sequence, the molecular formula, and the calculated molecular weight. For BPC-157 the catalog reference is the pentadecapeptide GEPPPGKPADDAGLV with formula C62H98N16O22 and a calculated mass near 1419.5 g/mol; the COA's job is to demonstrate that the powder in the vial is consistent with those fixed values. Identity and purity are reported separately because they are measured separately. A sample can be unambiguously the right molecule (identity confirmed) yet only modestly pure, or highly pure yet, in a flawed assay, the wrong molecule entirely.

Section 02

HPLC: Reading the Purity Number

The detector records absorbance over time, producing a chromatogram of peaks.

High-performance liquid chromatography (HPLC) is the workhorse for the purity figure on a peptide COA, most commonly reversed-phase HPLC with UV detection. The principle is separation: the sample is pushed through a column whose packing retains molecules according to hydrophobicity, so the target peptide and any structurally similar impurities exit at different times. The detector records absorbance over time, producing a chromatogram of peaks. Purity is reported as the area of the main peak divided by the total area of all peaks, expressed as a percentage, so a figure such as 98 percent means the main component accounts for that share of the integrated UV signal. When reading this section, check the wavelength (peptides are often monitored near 214 nm, where the peptide bond absorbs, sometimes also 220 or 280 nm), the gradient or method, and whether the main peak is sharp and well-resolved from its neighbors. Closely eluting shoulder peaks often represent the hardest-to-remove impurities: truncated sequences missing a residue, or chemically modified variants. Crucially, an HPLC purity percentage is a relative-area measurement under one detection method, not an absolute statement of mass, and it does not by itself confirm the molecule's identity. It tells you how clean the sample is, not what it is.

Section 03

Mass Spectrometry and Identity Confirmation

Where HPLC quantifies cleanliness, mass spectrometry (MS) addresses identity.

Where HPLC quantifies cleanliness, mass spectrometry (MS) addresses identity. The instrument ionizes the molecule and measures its mass-to-charge ratio, returning an observed mass that should match the compound's calculated molecular weight within the method's tolerance. This is the single most direct identity check on a peptide COA: the measured mass either agrees with the structure or it does not. Peptides are routinely analyzed by electrospray ionization, which tends to produce multiply charged ions, so a raw spectrum may show several peaks that software deconvolutes back to one neutral mass. That deconvoluted value is what you compare against the catalog figure. The expected target differs sharply by molecule, which is exactly why MS is diagnostic: the tripeptide GHK in GHK-Cu has a free-peptide mass of 340.38 g/mol (formula C14H24N6O4), whereas the GHRH-analog tesamorelin is calculated at 5135.9 g/mol with formula C221H366N72O67S. A mass landing far from the labeled value signals a wrong sequence, an unexpected modification, or a salt-form mismatch. Many COAs pair HPLC and MS in a single LC-MS run, which both separates components and identifies the main peak, linking the purity peak directly to a confirmed mass.

Section 04

Sequence, Formula, and Counter-Ion: The Identity Backbone

Identity is anchored by fixed structural facts that the COA either restates or verifies.

Identity is anchored by fixed structural facts that the COA either restates or verifies. The amino-acid sequence is the definitive description of a peptide, and several entries in the reference dataset show how much detail this carries. Semaglutide is a 31-residue GLP-1(7-37) analogue whose identity depends on an Aib8 substitution, an Arg34 change, and a C18 fatty-diacid acylation on Lys26 via a gamma-Glu/AEEA linker, with formula C187H291N45O59 and a calculated mass of 4113.58 g/mol. A naked mass number alone could not distinguish such an analogue from a near-isobaric variant, which is why the sequence and the specific modifications matter. The COA should also be explicit about salt form and counter-ions, because these change the measured mass and the effective content. TB-500 illustrates the point: the dataset lists a free-base mass near 889.0 g/mol (C38H68N10O14) but an acetate-salt form near 949.1 g/mol. A purity or mass result must be read against the correct form. Where a peptide is a defined metal complex, identity includes the coordinating ion; GHK-Cu is specified as the 1:1 Cu(II) complex C14H22CuN6O4 at 401.91 g/mol, distinct from the free tripeptide. CAS registry numbers, where present, give an additional cross-check on chemical identity.

Section 05

The Rest of the Sheet: Content, Water, and Appearance

Some COAs report this separately, occasionally via amino-acid analysis or nitrogen content.

Beyond the headline HPLC and MS results, a thorough COA characterizes the physical reality of the powder so an investigator can prepare reference solutions correctly. Net peptide content is a frequently misread line: HPLC purity describes the fraction of organic material that is the target peptide, but a lyophilized powder also contains water and counter-ion salts, so the actual peptide mass per milligram of powder can be lower than the purity percentage implies. Some COAs report this separately, occasionally via amino-acid analysis or nitrogen content. Water content, often by Karl Fischer titration, and residual-solvent or acetate-content figures round out the mass balance. Appearance (typically a white to off-white powder), solubility notes, and storage conditions support correct handling at the bench. For molecules whose identity hinges on subtle structural features, these auxiliary sections matter: retatrutide is defined by non-coded residues such as Aib and alpha-methyl-leucine plus a C20 fatty-diacid acylation, structural elements that drive its calculated 4731.33 g/mol mass and that a complete characterization package should account for. Finally, traceability fields, the lot number, test date, and method references, let a result be reproduced or audited. A COA missing batch identity or method detail is difficult to trust, however clean its purity figure appears.

Straight answers

Frequently asked questions

What is the difference between identity and purity on a COA?

Identity confirms the sample is the named molecule and not a different sequence or a degradation product; it is established chiefly by mass spectrometry matching the observed mass to the calculated molecular weight. Purity reports what fraction of the sample is that target molecule versus impurities, usually as an HPLC main-peak area percentage. They are separate measurements: a sample can be the correct molecule yet only moderately pure, so both sections should be read together.

Why is peptide purity usually measured by HPLC?

Reversed-phase HPLC separates a peptide from structurally similar impurities such as truncated sequences by passing the sample through a column that retains molecules by hydrophobicity. A UV detector records each component as a peak, and purity is reported as the main-peak area divided by total peak area. This makes HPLC well suited to resolving and quantifying the closely related byproducts that arise during peptide synthesis.

What does the mass spectrometry result tell me?

It gives an observed molecular mass that should match the compound's calculated molecular weight within the method's tolerance, which is the most direct confirmation of identity. For peptides, electrospray ionization often produces multiply charged ions that software deconvolutes into one neutral mass. That value is compared to the catalog figure; a large discrepancy points to a wrong sequence, an unexpected modification, or a salt-form mismatch.

Does a high HPLC purity number mean the peptide is the right molecule?

No. HPLC purity only describes how much of the sample is the dominant component under one detection method; it says nothing definitive about that component's chemical identity. A sample could be uniform and clean yet be the wrong sequence. Identity must be confirmed separately, typically by mass spectrometry, which is why thorough COAs pair the two, often in a single LC-MS run that links the main purity peak to a confirmed mass.

Why do molecular weight and salt form matter when reading a COA?

The calculated molecular weight is the reference value mass spectrometry is checked against, and salt form shifts it. For example, the reference data lists TB-500 near 889.0 g/mol as a free base but near 949.1 g/mol as the acetate salt, and GHK-Cu as the 1:1 copper complex at 401.91 g/mol versus 340.38 g/mol for the free tripeptide. A purity or mass result is only interpretable against the correct molecular form stated on the sheet.

What else should a complete COA include besides HPLC and MS?

A thorough sheet adds net peptide content (the actual peptide mass per milligram of powder, which is lower than HPLC purity once water and salts are counted), water content by Karl Fischer titration, residual-solvent or counter-ion figures, appearance and solubility notes, and storage conditions. It should also carry traceability fields: a lot or batch number, test date, and the analytical methods used, so a result can be audited or reproduced.

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For in-vitro laboratory research use only. Not for human or animal consumption. Educational content, not medical advice; not intended to diagnose, treat, cure, or prevent any disease. Not evaluated by the FDA.