How to read a Certificate of Analysis

A Certificate of Analysis (COA) records what was measured for one production lot: which tests were run, by which method, against which specification, and what the lot returned. This walks Vitro’s certificate field by field against an annotated specimen.

What a Certificate of Analysis is

A Certificate of Analysis is a per-lot analytical record. It belongs to one production lot, identified by the lot number printed on the vial, not to the product line: two lots of the same catalog number are tested and released separately. Vitro’s certificates close by stating that the analytical data pertain solely to the identified lot.

Four blocks make up the document, only one of them measurement.

An annotated example

The specimen below numbers eight fields in the document’s own order. Callouts 1 and 2 sit in the header block: the lot number and the retest date. Callouts 3 to 8 walk down the analytical-results table in its own row order: appearance, identity, purity, water content, counter-ion content, net content. The document’s table also carries a single max impurity row, not shown on the specimen.

The sections that follow take those fields in the order the questions arise: identity, purity, the three rows that decide how much peptide the vial holds, then appearance and the retest date, and matching the certificate to the vial.

Identity

Identity is measured by ESI-MS, electrospray ionization mass spectrometry, against a specification that the [M+H]⁺ ion is consistent with the structure. The analyzer reports mass-to-charge, so for a small peptide carrying one added proton the reading is the molecular mass plus roughly 1 Da.

Two expected masses can be calculated for the same molecule: the monoisotopic mass uses the lightest isotope of each element, the average weights each element by natural isotopic abundance. They separate by roughly 0.5–1 Da per 1,000 Da, so an observed figure is comparable only once you know which the certificate quotes.

Acceptable deviation is set by the instrument, not the peptide. A unit-resolution analyzer, a single quadrupole or ion trap, reports m/z to about ±0.5 Da, so agreement within half a mass unit is a match. A high-resolution analyzer, time-of-flight or Orbitrap, is quoted in parts per million, commonly under 5 ppm. A mismatch is diagnostic by its size.

A mass matching no plausible variant of the ordered compound means the vial is not that compound. A matching mass is weaker evidence than it looks, since it does not distinguish sequence permutations or residues of equal mass such as leucine and isoleucine, which the written sequence does; see peptide nomenclature. What these instruments resolve is covered in peptide purity by HPLC and MS.

Purity

Purity is reported by RP-HPLC as an area percent at 220 nm, specified on the specimen at ≥ 98.0 % of total peak area: the main peak’s area over the total integrated area. Reversed-phase HPLC separates the sample by hydrophobicity, and the ultraviolet detector at 220 nm responds to the backbone amide bond.

Vitro’s certificate states the conditions behind the number: C18, a 5 to 65 % acetonitrile gradient with 0.1 % TFA over 20 minutes, detection at 220 nm. The number is method-dependent. A shallower gradient pulls apart species that a steeper one merges into the main peak, and a different wavelength changes the relative response of the species present, so a purity number without its method is not comparable.

Two limits sit inside the purity number. Area percent is not mass percent: it weights species by ultraviolet response, not by mass. And only species that elute and absorb at 220 nm are integrated at all, which is why water, salts and the counter-ion, the anion of the acid the peptide is isolated as a salt of, never appear in it.

Single max impurity, the row the specimen figure does not draw, caps the largest individual related species rather than their total; the document specifies ≤ 1.0 % of area. Both describe the lot tested, on the day it was tested.

Net content

Vitro’s certificate uses that phrase twice. The header block carries Net content / vial, a declared value with no method: the labeled fill. The analytical-results table carries Net content, method Gravimetric, the weighed result against a specification of the label claim ± 5 %. The first is the claim, the second the measurement that tests it.

Neither is the mass of peptide. The balance weighs the whole lyophilized solid: peptide plus its counter-ion, the anion of the acid it is isolated as a salt of, plus residual water. Both are reported on the certificate as percentages of the weighed mass, so the correction is arithmetic:

peptide mass = weighed net content × (1 − water fraction − counter-ion fraction)

On the specimen figure’s values, a weighed 50.3 mg at 5.1 % water and 6.8 % counter-ion leaves about 44.3 mg of peptide. Where a stock’s concentration is itself the result, divide that corrected mass by the diluent volume; for routine bench work the labeled figure is close enough. Reconstituting lyophilized peptides covers the concentration arithmetic.

Water content

Water content is measured by Karl Fischer titration and reported as a percentage by weight, specified on the specimen at ≤ 8.0 %. The titration is specific to water: iodine reacts with water in the presence of sulfur dioxide and a base, one iodine per water. The certificate names the coulometric variant, which generates the iodine electrically and suits small samples.

Water content matters twice. As a mass contributor, residual water is part of the weighed solid and not part of the peptide, so it comes out of the net peptide mass along with the counter-ion, the anion of the acid the peptide is isolated as a salt of. As a hydrolysis substrate, it is the reagent in backbone amide hydrolysis and in deamidation of the asparagine and glutamine side-chain amides, and it gives an amorphous solid the mobility those reactions need.

The value describes the vial as sealed at release: a lyophilized cake is hygroscopic, and a vial opened cold takes up moisture that no later handling removes. The conditions that hold the figure are in storing research peptides.

Counter-ion content

Vitro’s analytical-results table carries a row labeled Counter-ion (acetate), measured by HPLC, whose specification reads Report: recorded rather than passed or failed, hence the dash in its status column. Certificates carry a counter-ion row; the specimen shows acetate, and trifluoroacetate (TFA) is the other common form.

A synthetic peptide carries one because of how it is purified. Reversed-phase purification runs in a mobile phase acidified with about 0.1 % trifluoroacetic acid; at that pH the N-terminus and the basic side chains, arginine, lysine and histidine, are protonated, so the peptide leaves the column as a salt of that acid. TFA is the default outcome; an acetate salt takes a deliberate exchange step, so read the salt form off the certificate.

Counter-ion content is no part of the purity measurement, which is why it has a row of its own. Acetate and trifluoroacetate do not absorb meaningfully at 220 nm, so neither is integrated into the area percent: on the specimen figure a counter-ion row of 6.8 % by weight sits beside a purity of 99.2 % by area. The salt also shifts the pH of an unbuffered solution, and residual trifluoroacetate is cytotoxic to some cell lines at concentrations acetate is not; choosing a diluent is covered in reconstituting lyophilized peptides.

Appearance and retest date

Appearance is the first analytical row, method Visual, specification a white to off-white lyophilized powder, result Conforms: someone looked at the vial before release and the cake matched that description. It is not an instrumental measurement.

It is the only row a recipient can repeat without equipment. A cake that arrives discolored, slumped, shrunk away from the glass, or glassy rather than powdery no longer matches the description the lot passed on; moisture ingress or a thermal excursion in transit are the usual reasons, and that is grounds to query the lot rather than a specification failure. What produces each of those appearances, and why a film or a puck are both normal, is covered in lyophilization.

The retest date is the date by which the lot should be re-analyzed to confirm it still meets its specification. It is not the date the material stops being usable, and it is not an expiry date, which belongs to a finished pharmaceutical product and comes from formal stability studies. The claim is conditional too. Vitro’s storage line defers to the vial label, gives the lot’s own condition, and states that the material is stable to the retest date under stated conditions, so it covers material held that way and says nothing about a vial that was not. Storing research peptides covers those conditions.

Matching the lot to your vial

The lot number on the certificate and the lot printed on the vial label must be the same string. A certificate for a different lot of the same catalog number describes different material; none of its results carry across. The lot string appears four times on Vitro’s certificate, in the Lot № field, in the document reference in the release block, in the verify panel beside the QR code, and in the page footer. It is that string, not necessarily the whole field around it, that must match the label each time: the document reference and the footer wrap it inside a longer code rather than repeating the bare lot number.

Check that the document is released rather than draft: the release block states a disposition, reading Released for a lot cleared for supply, with a QC reviewer, a date released and a signature.

To pull a lot’s certificate from our records, scan the QR code on the document or enter the lot number at verify. Each vial’s label lot resolves to exactly one certificate. A lot that resolves to nothing, or to the wrong product, is a question to settle before the vial is opened. Vial sizes per compound are listed in the catalog.

For laboratory research use only. Not for human or veterinary use, and not intended to diagnose, treat, cure, or prevent any disease.