What a COA is (and isn’t)

A Certificate of Analysis (COA) is a structured record of what a laboratory measured on a specific sample. It is a snapshot of one tested unit, produced by defined analytical methods, on a defined date. A COA is not a guarantee about a product line, a brand, or the vial in your hand that was never tested. Reading one well means understanding exactly what each field claims — and where each claim stops.

This guide walks through the fields you will typically find on a peptide COA and how to interpret them without over-reading.

Sample and batch identifiers

Near the top you should see identifiers that tie the certificate to a physical sample: a sample ID, a batch or lot number, the date received, and the date reported. These matter because a COA only describes the material that carried those identifiers through the lab. If the lot number on your product does not match the lot on the COA, the certificate is describing a different batch. A QR-verifiable COA lets you confirm the record was issued by the lab and has not been altered — worth checking before you read anything else.

Identity: is it the right molecule?

The identity section answers a single question: is this the compound it claims to be? For peptides, identity is usually confirmed by liquid chromatography–mass spectrometry (LC-MS). The instrument measures the mass of the molecule and the lab compares the observed mass against the theoretical mass calculated from the stated sequence.

You will often see two numbers:

  • Theoretical mass — calculated from the amino-acid sequence. This may be reported as a monoisotopic mass (using the most abundant isotope of each element) or an average mass (using average atomic weights). For small peptides, monoisotopic is common; for larger ones, average mass is often used.
  • Observed mass — what the instrument actually detected.

A close match between observed and theoretical mass supports the identity claim. Mass spectrometry frequently detects multiply charged ions (e.g., [M+2H]²⁺), so the raw instrument reading may look like a fraction of the full mass until it is deconvoluted — a good COA reports the calculated neutral mass so you are comparing like with like. Identity confirmation tells you the correct molecule is present. It does not tell you how pure the sample is.

Purity: how much of what eluted is your peptide?

The purity figure almost always comes from reversed-phase HPLC (RP-HPLC) and is reported as percent area (% area) — the area of the main peak divided by the total area of all detected peaks, typically at a set UV wavelength (often 214 or 220 nm, where the peptide bond absorbs).

Two things are easy to misread here:

  • Purity is relative, not absolute. A 98% area result means the main peak represents 98% of everything the detector saw under those conditions. Substances that do not absorb UV at that wavelength — water, residual salts, counterions, some solvents — may not appear at all.
  • Purity is method-dependent. The same sample can read differently under a different column, gradient, or wavelength. That is why the method should be stated.

Purity answers how pure, not what it is and not how much peptide by weight.

Net peptide content: how much is actually peptide?

Net peptide content (sometimes called peptide content or net content) is a different measurement from chromatographic purity. A lyophilized peptide is rarely 100% peptide by mass — the rest can be water, counterions (like acetate or trifluoroacetate), and residual salts. Net content, often determined by quantitative HPLC against a reference standard or by amino acid analysis (AAA), estimates the fraction of the total mass that is actual peptide.

This is why a vial can be simultaneously high purity and lower net content: the peptide present is clean (few other peaks), but it is only, say, 80% of the powder’s weight. Purity and content are not interchangeable, and a careful COA reports them separately.

Better COAs list impurity peaks rather than only the headline purity number. You may see individual related peaks by retention time and their % area, sometimes flagged as largest single impurity. This detail matters because 2% of impurity spread across many tiny peaks reads differently from 2% concentrated in a single unidentified peak. Where the lab can, it may note likely origins (e.g., a deletion or oxidation product), but an unidentified peak is exactly that — unidentified.

Methods and conditions

A trustworthy certificate states how each number was produced: the instrument type, column, mobile phase and gradient, detection wavelength, and the mass-spec ionization mode. Methods let a second analyst understand and, in principle, reproduce the result. A COA that reports numbers with no methods is asking for trust it hasn’t earned.

Reading the verdict

Many COAs summarize results as a status. Interpret these as statements about the tested sample only:

  • Pass — the sample met the stated criteria (e.g., identity confirmed and purity at or above a threshold).
  • Caution — results are within range but something warrants a closer look, such as a notable single impurity or a borderline value.
  • Fail — the sample did not meet a stated criterion; identity or purity fell short.
  • Inconclusive — the test could not return a reliable result (for example, insufficient sample, a matrix interference, or a signal the method couldn’t resolve). Inconclusive is not a soft fail; it means not measured, and it should prompt a retest, not an assumption.

The one-sentence takeaway

A peptide COA answers three separate questions — is it the right molecule (identity/mass), how pure is it (HPLC % area), and how much of it is peptide (net content) — for one sample, by stated methods, on one date. Read each field for what it measures, check that the lot and the methods are present, and resist the urge to stretch a snapshot into a guarantee.