Reading a Certificate of Analysis (COA): A Walkthrough

A Certificate of Analysis (COA) is the single most useful document a research-compound supplier can provide, and the single most misunderstood one. For peptides in particular, a COA translates a vial of white powder into a set of measurable claims: what the molecule is, how much of it is present, and what else came along for the ride. Researchers who learn to read a COA in detail can distinguish a well-characterized reference material from a loosely-labelled mixture, often within a minute of scanning the page.

What a COA is actually claiming

A peptide COA is a summary of analytical tests performed on a specific lot of material. It is not a general datasheet about the peptide itself, and it is not a marketing document. Each figure on the page refers to the exact batch identified by the lot number printed near the top. If the lot number is missing, or if a single COA is reused across multiple product listings, that alone is a meaningful red flag.

A complete peptide COA typically reports six categories of data: identity (by mass spectrometry), purity (by high-performance liquid chromatography), water content, counter-ion or salt content (usually acetate or trifluoroacetate), bacterial endotoxin, and physical appearance. Some COAs also include sequence confirmation, chiral purity, or residual solvent measurements. The test methods should be named — "HPLC", "MS", "Karl Fischer", "LAL" — and in strong COAs, the instrument model and the specific method reference are included as well.

The document should be dated, signed or initialed by a quality-control analyst, and tied to a manufacturing or testing facility. "Signed" can mean a digital signature or printed name; the absence of any named analyst is unusual for legitimate suppliers. A COA printed on a generic template with no laboratory letterhead, no analyst name, and no lot-specific data is functionally unverifiable.

Identity: mass spectrometry

The identity section answers a narrow question: is the molecule in the vial the molecule on the label. Peptide identity is confirmed by mass spectrometry (MS), most often electrospray ionization (ESI-MS) or matrix-assisted laser desorption/ionization (MALDI-MS). The COA reports a theoretical monoisotopic or average mass calculated from the sequence, and an observed mass measured from the sample. The two should match within a small tolerance — typically under 1 Da for MALDI and under 0.1 Da for higher-resolution ESI instruments.

For a peptide like BPC-157 (sequence GEPPPGKPADDAGLV, theoretical average mass approximately 1419.5 Da), a legitimate COA will show an observed mass in that neighborhood, often with multiple charge states annotated. Retatrutide, a much larger molecule at roughly 4.9 kDa, will show a correspondingly larger observed mass and typically a deconvoluted spectrum rather than a single peak. If the observed mass on a COA is off by more than a few daltons from the theoretical value, and no explanation is given (for example, a known modification or a salt adduct), the identity claim is not supported.

Stronger COAs also include the raw MS spectrum as an embedded image or an attached PDF page. This allows an independent reviewer to see the peak shape, check for obvious secondary peaks that might indicate truncated sequences or deletion products, and confirm the charge-state pattern. A COA that reports only a single numeric mass with no spectrum is acceptable but thinner.

Purity: HPLC and what the percentage means

Purity is reported by high-performance liquid chromatography, almost always reversed-phase HPLC with ultraviolet detection at 214 nm (the peptide bond absorbance) or 220 nm. The headline number — "Purity: 98.4%" — is an area-percent value: the area under the main peak divided by the total area of all peaks detected, excluding the solvent front. It is not a mass percentage and it does not account for anything that does not absorb UV at the detection wavelength, such as residual salts, water, or some small counter-ions.

For research peptides, purity figures in the 95 to 99 percent range are typical of reputable suppliers. Values under 95 percent are not automatically disqualifying, but they should come with an explanation — some long or hydrophobic peptides are genuinely difficult to purify past that threshold. Values reported as "99.9%" or "100%" for a synthesized peptide should be read with skepticism; even well-run GMP facilities rarely report three-nines purity on a routine COA, because the HPLC method itself has a detection floor and typical area-percent rounding conventions.

The HPLC chromatogram — the visual trace of the run — is where most of the real information lives. A clean chromatogram shows one dominant peak with a symmetric shape and a flat baseline on either side. Secondary peaks, if present, are small and well-separated. Red flags include a shoulder on the main peak (often indicating a closely-related impurity such as a deamidated or oxidized form), a drifting baseline, or a main peak that elutes at an unexpected retention time relative to a reference standard. A COA that reports a purity number with no chromatogram attached provides the claim without the evidence.

Gradient and column detail

The method section of a strong COA specifies the column (for example, a C18 reversed-phase column with a stated particle size and dimensions), the mobile phase composition (typically water and acetonitrile with 0.1% trifluoroacetic acid), the gradient profile, the flow rate, and the injection volume. These details matter because purity is method-dependent: a peptide that looks 99% pure on a fast gradient may resolve into 96% plus several co-eluting impurities on a slower, better-resolved method. Reputable suppliers publish enough method detail that an independent lab could, in principle, reproduce the analysis.

Water, acetate, and what the vial actually contains

Two numbers that often surprise researchers new to COAs are water content and counter-ion content. A peptide that reports 98% HPLC purity may still be only 80% peptide by mass, because the remaining 20% of the vial is water and acetate salt from the purification process.

Water content is measured by Karl Fischer titration and reported as a percentage by mass. Lyophilized peptides typically carry 2 to 8 percent residual water, sometimes more for hygroscopic sequences. Values above roughly 10 percent suggest incomplete lyophilization or moisture uptake during storage, both of which shorten shelf life.

Acetate content (or trifluoroacetate, depending on the purification conditions) is the counter-ion associated with basic residues in the peptide. It is typically measured by ion chromatography or a dedicated HPLC method and reported as a percentage. Acetate fractions of 5 to 15 percent are routine; TFA content, when reported, is usually lower because many suppliers perform an acetate salt exchange specifically to reduce TFA burden, which has its own biological effects in research settings.

The practical consequence is that reconstitution math should be done against the peptide content, not the gross vial mass. A 10 mg vial labelled at 98% HPLC purity with 6% water and 10% acetate contains roughly 8.4 mg of actual peptide. Strong COAs make this explicit with a "net peptide content" line; weaker ones leave the calculation to the reader.

Endotoxin, sterility, and appearance

Bacterial endotoxin is measured by the Limulus amebocyte lysate (LAL) assay and reported in endotoxin units per milligram (EU/mg) or per milliliter. For research-use material, a typical specification is less than 5 EU/mg, though some suppliers target tighter limits. Endotoxin data is particularly relevant for peptides intended for in vivo research in rodent or cell-culture models, where even low-level contamination can confound inflammatory endpoints.

Sterility testing, when present, is reported as a pass/fail result from a defined method, most often a membrane filtration or direct inoculation protocol per a pharmacopeia reference. It is not a substitute for endotoxin testing — a sterile vial can still contain endotoxin from lysed bacteria upstream in the process.

Appearance is the simplest field on the page and the easiest to skip. It should read something like "white to off-white lyophilized powder" or "white amorphous solid". Deviations — yellow tint, oily residue, visible particulates — are worth noting because they sometimes indicate oxidation, incomplete drying, or container contamination. Appearance does not override the instrumental data, but it is a free sanity check.

Spotting a fake or recycled COA

A meaningful fraction of COAs circulating on research-compound marketplaces are fabricated, edited, or recycled across unrelated lots. A few patterns recur often enough to be worth memorizing.

• Identical COAs across different lots. If two vials with different lot numbers carry byte-for-byte identical HPLC traces and MS spectra, at least one is fake. Legitimate lots produce visually different chromatograms even when the numeric results are similar.
• Mismatched theoretical mass. The theoretical monoisotopic and average masses of most research peptides are published or easily calculated from the sequence. A COA that reports a theoretical mass inconsistent with the labelled peptide is either mislabelled or fabricated.
• Missing method details. Purity "98%" with no column, no gradient, no detection wavelength, and no chromatogram is a number without evidence.
• No lot number, no date, no analyst. These are the three fields most often stripped when a COA is reused as generic marketing material.
• Impossible precision. Purity reported as "99.98%" or endotoxin as "0.000 EU/mg" suggests a template with placeholder values that were never replaced with real data.
• Third-party testing claims without the third-party report. If a supplier states that an independent lab verified the material, the independent lab's own report — on its own letterhead, with its own analyst signature — should be available. "Third-party tested" as a bare claim is not verification.

A useful habit is to request the raw chromatogram and MS spectrum as a separate attachment rather than relying on the summary PDF. Suppliers that can produce these files on request, with matching timestamps and instrument metadata, are operating at a different tier from those that cannot.

Open questions

Even a well-constructed COA leaves open questions. It reports what was measured on the day of testing, not what is in the vial after six months of shipping and storage. It does not address batch-to-batch consistency across multiple lots, which is a separate analysis requiring comparison of several COAs over time. It does not report on stereochemistry unless a specific chiral method is run — a meaningful concern for peptides with D-amino acid substitutions, where racemization during synthesis can go undetected on a standard reversed-phase HPLC method.

For researchers building a long-term record, the most robust practice is to archive every COA received, record the lot numbers against any experimental data generated from that material, and periodically request retest data on older lots. Where the research context warrants it, independent verification through a commercial analytical lab remains the only way to confirm a COA's claims rather than trust them — and the cost of that verification has dropped substantially as contract peptide-analysis services have expanded.