How to read a peptide Certificate of Analysis (COA)

A Certificate of Analysis (COA) is the document that links a vial of peptide to the laboratory tests run on its batch. Read well, it tells you what the material is, how clean it is, what else is in the powder, and whether it suits the experiment you have planned. Read quickly, it is easy to look at one headline percentage and miss the rest. This guide walks through the sections you will usually meet, in the order they tend to appear.

What is a Certificate of Analysis?

A COA is issued for a specific production batch. For each quality attribute it should state three things: the test procedure used, the acceptance criterion (the limit the batch must meet), and the value actually measured. Together, the attributes, procedures and acceptance criteria make up a material’s specification. For peptide medicines, pharmacopoeias define these specifications in monographs that cover identification, purity and assay, yet the European Pharmacopoeia and the United States Pharmacopeia still differ considerably in how they test and what limits they set (Vergote et al., 2009). Research-grade peptides are not bound by those monographs, so the useful question is simple: which tests were run, by which method, and against which limit?

Which batch does the certificate describe?

Start at the header. A COA should give the peptide’s name and, ideally, its sequence and molecular formula; a batch or lot number; the date of analysis; and the laboratory that performed the tests, whether the manufacturer’s own quality-control lab or an independent one. The batch number is the anchor for everything else: it should match the label on the vial. A certificate with no batch number, or one that is identical across several batches, cannot be tied to the material in hand.

How is identity confirmed?

Identity is normally shown by mass spectrometry (MS), most often electrospray ionisation (ESI) or matrix-assisted laser desorption/ionisation (MALDI). The certificate should list the theoretical molecular mass of the sequence and the observed mass, and the two should agree within the instrument’s accuracy. With ESI, a peptide often appears as several multiply charged ions, so the spectrum shows mass-to-charge (m/z) values rather than the molecular mass directly; the report should show how the observed mass was derived.

Mass spectrometry confirms mass, not every detail of structure. A peptide in which one amino acid has switched from the L to the D form (racemization, a known side reaction of solid-phase synthesis) has exactly the same mass as the intended product, so it forms a diastereomeric impurity that MS alone cannot distinguish (D’Hondt et al., 2014). That is one reason identity and purity are separate tests.

What does HPLC purity measure?

Purity is usually measured by reversed-phase high-performance liquid chromatography (RP-HPLC) with UV detection at a low wavelength where the peptide bond absorbs (around 214 nm). The sample is separated on a column, each component produces a peak, and purity is reported as the area of the main peak as a percentage of the total peak area.

That figure is only as good as the method behind it. Look for the column type, the gradient and the detection wavelength, and, above all, for the chromatogram itself. An impurity that elutes at the same time as the main peak is counted as product; a component that barely absorbs at the chosen wavelength is not counted at all.

The impurities HPLC is looking for are well described. Solid-phase peptide synthesis can leave deletion sequences (a missing amino acid) and insertion sequences (an extra one), peptides still carrying protecting groups, oxidised side chains, and dimers or larger oligomers; storage and handling add degradation products such as diketopiperazine, pyroglutamate and succinimide forms (D’Hondt et al., 2014). The same review notes that such impurities can distort early functional studies and lead to wrong conclusions, which is why the purity section matters for laboratory work and not only for regulators.

Why is peptide content different from purity?

HPLC purity compares the peptide with other peptide-like material in the sample. It says nothing about substances the method does not detect. A lyophilised peptide is a powder that also holds counter-ions bound to its charged groups and some residual water, so the fraction of the powder’s mass that is actually peptide, its net peptide content, is lower than the purity figure suggests. Peptide content is measured separately, typically by amino acid analysis or by elemental nitrogen analysis.

The practical consequence: weighing out a given mass of powder delivers less peptide than that mass. If you need a solution of known peptide concentration, the peptide content figure, not the purity figure, is the one to use in the calculation. A COA that reports purity but not peptide content leaves that question open.

What are counter-ions, and why does TFA matter?

Peptides carry positively charged groups (the N-terminus and the side chains of lysine, arginine and histidine), and each is paired with a negatively charged counter-ion. Which counter-ion depends on the last purification step. Because trifluoroacetic acid is a standard component of the mobile phase in reversed-phase HPLC, peptides purified this way are often isolated as trifluoroacetate (TFA) salts (Cornish et al., 1999). TFA can also originate from the synthesis itself (D’Hondt et al., 2014): in Fmoc solid-phase synthesis it is the usual reagent for cleaving the finished peptide from the resin. Other common salt forms are acetate and hydrochloride.

For cell-based work this is more than bookkeeping. In fetal rat osteoblast cultures, TFA at concentrations of 10⁻⁸ to 10⁻⁷ M reduced cell numbers and thymidine incorporation within 24 hours, with similar findings in articular chondrocyte and neonatal mouse calvaria cultures. When the TFA and hydrochloride salts of the same peptides were compared in osteoblasts, cell proliferation was consistently lower with the TFA salts, enough to hide a proliferative effect or to suggest an antiproliferative one that was not there. The authors recommended converting peptides to a hydrochloride or another biologically equivalent salt before assessing their biological effects (Cornish et al., 1999). A useful COA therefore states the salt form, and some also report the TFA content directly, for example by ion chromatography.

What does the water content tell you?

Lyophilised peptides are hygroscopic: they take up moisture from the air. Water content is commonly measured by Karl Fischer titration and reported as a percentage of the powder’s mass. Together with the counter-ion content, it accounts for most of the gap between the powder’s weight and its peptide content. A water figure also gives a baseline: material stored with poor protection from moisture can take up more over time than the certificate records.

What is endotoxin testing, and when does it matter?

Endotoxins are lipopolysaccharides (LPS) from the outer membrane of Gram-negative bacteria. They are heat-stable, so they can survive steps that kill the bacteria themselves, and they can enter a product through water, reagents or equipment. The standard assay is the limulus amebocyte lysate (LAL) test, in gel-clot, turbidimetric or chromogenic formats, and the value is usually reported in endotoxin units (EU) per milligram of material.

Whether this matters depends on the experiment. A study of commercially available recombinant proteins found endotoxin levels mostly in the range stated on their data sheets, but some higher. When the authors exposed cultured immune cells to very low concentrations of LPS, primary CD1c+ dendritic cells in particular were activated by amounts equivalent to the contamination found in some of those proteins. They concluded that even low endotoxin contamination can generate erroneous data in LPS-sensitive cell systems, and recommended screening with the LAL test or comparable fluorescence- or reporter-based assays (Schwarz et al., 2014). That study concerned recombinant proteins, but the lesson carries over to any material added to immune-cell cultures: if your cells respond to LPS, an endotoxin figure on the COA is worth looking for.

What about appearance and solubility?

Most certificates include a visual description, typically a white to off-white lyophilised powder, and a solubility statement naming a solvent. These are simple checks, but they are the ones you can repeat yourself on arrival. A powder that looks different from the description, or a solubility statement with no solvent named, is worth querying.

What does a missing section signal?

A short certificate is not proof of a problem, but each gap is a question to put to the supplier:

• No mass spectrum or observed mass: identity has not been shown.
• A purity percentage with no chromatogram or method: there is no way to judge how the figure was obtained.
• No batch or lot number, or no date: the document cannot be linked to the material.
• No laboratory named: you cannot tell who did the testing, or whether it was independent.
• No salt form or peptide content: you cannot work out how much peptide a weighed amount of powder contains.
• No endotoxin value: acceptable for many chemical and biochemical experiments, but a gap if the material is going into immune-cell or other LPS-sensitive cultures.

To practise on real documents, compare the sections above with a published certificate, such as those in the EuroPeps COA library. Reading one or two certificates line by line, against this list, is the quickest way to learn what a complete one looks like.