Every check below can be made before money changes hands, from documents a supplier either has or does not. None of them requires trusting anyone, and none of them is about a particular company: they are properties of a document, and a document either carries them or it does not.

1. Does the batch number on the paper match the vial?

This is the first check because everything else depends on it. A certificate describes one batch. If the number printed on the label does not match the number on the document, the document describes someone else's material, however good the figures on it look.

Ask for the certificate for the specific batch you will receive, not a sample certificate. A supplier who can only produce a generic document is telling you the testing is not batch-specific. Our note on reading a certificate covers the fields in detail.

2. Is a method named next to every number?

"Purity: 99%" is a statement. "Purity: 99.2% by RP-HPLC, C18, 0.1% TFA, gradient 20-60% acetonitrile over 30 min, detection at 214 nm" is a result, because someone else could repeat it.

The conditions are not decoration. The column, the gradient and the detection wavelength all change the number that comes out, and the same batch analysed two ways can return two honest figures. Guidance on validating analytical procedures exists precisely because a result is inseparable from the procedure that produced it. (Reference: Swartz & Krull, 2012) Our note on what a purity figure measures covers why.

3. Is there an analysis date?

A result without a date is not tied to a moment in that batch's life. Peptides degrade by known routes, and all of them proceed with time and temperature, so a measurement made eighteen months ago describes the material as it was then. (Reference: Manning et al., 2010)

The date also lets you notice something that a missing date hides: whether the document was produced for this batch or reused.

4. Who signed it, and do they sell the material?

A certificate is a claim made by whoever issued it. Independent testing matters because it separates the party making the claim from the party being paid, and laboratories working to recognised competence standards are assessed against defined criteria for competence and impartiality.

What the number says is one question. Who produced it, and under which method, is the other. A document with no laboratory named and no signature leaves nobody answerable for it.

5. Does it report identity as well as purity?

These answer different questions and a document with only one is half a document. Purity from chromatography reports what share of the detected material is one main component. It has no opinion about what that component is.

Identity comes from mass spectrometry, comparing a measured mass against the mass calculated from the intended sequence. A sample can be 99 percent pure and be 99 percent of the wrong molecule, and a chromatogram alone would look identical.

6. Does it separate purity from net content?

Chromatographic purity asks about the peaks: of everything the method detected, what share was the target? Net peptide content asks about the powder: of what you weighed, how much is peptide and how much is counterion and water?

The gap is real. Material purified by reversed-phase chromatography is isolated as a salt, so part of the powder is counterion and part is absorbed water. A batch can be 99 percent pure by area and well under 99 percent peptide by mass, with no contradiction between the figures. A certificate reporting only one is incomplete, which is a different thing from wrong. Our note on what else is in the vial covers the rest of the mass.

7. If the trace is included, does it hold up?

One tall symmetrical peak on a flat baseline is the picture you want. Small peaks close to the main one are common in synthesis and usually correspond to closely related species such as deletion sequences, which differ from the target by a single residue. A shoulder means the method barely resolved something.

An image too low in resolution to read the axes is not evidence. Neither is a chromatogram with no scale on it.

What the absence of a line means

Some fields are optional for good reason. Endotoxin content matters for work with cultured cells and is irrelevant to a chemistry experiment, so its absence from a document is not automatically a gap. (Reference: Raetz & Whitfield, 2002)

The honest description of an unmeasured parameter is that it was not measured, not that the material is free of it. A supplier who says so plainly is easier to trust than one whose document implies a measurement that never happened.

A specification is not a result

Worth stating separately because the two look alike on a page. "≥99% by HPLC" states the threshold a batch was required to meet. "99.2%, measured 14 March, batch CDX-1017" is a measurement of one batch on one day.

Both have their place. Only one tells you about the material you are buying, and the batch number is the thread that connects a claim to a measurement you can check.

What this checklist is not

It is not a judgement about any supplier. Every item describes a property of a document, and any supplier can satisfy all seven by producing better documents. That is the point: the checks are about what can be verified, not about who is selling.

Research use only

All products sold by Codex Research are strictly for laboratory research and development. They are not for human or animal consumption and are not intended to diagnose, treat, cure, or prevent any disease.

Frequently asked questions

What should I ask a peptide supplier before buying?

Whether the batch number on the certificate matches the vial, whether a method and a date sit next to every number, who signed the document, whether it reports identity as well as purity, and whether net peptide content is separated from chromatographic purity.

Why does the batch number matter most?

Because a certificate describes one batch. If the number on the document does not match the number on the label, the document describes different material, however good its figures look.

What turns a purity figure into a result instead of a claim?

A named method beside it. The column, the gradient and the detection wavelength all change the number, so a percentage quoted without its conditions cannot be repeated by anyone else.

Why is purity alone not enough?

Purity reports what share of the detected material is one main component, without identifying it. A sample could be 99 percent pure and be 99 percent of the wrong molecule. Identity comes from a mass measurement.

What is the difference between purity and net peptide content?

Purity is the target peak as a share of total detected peak area. Net content is the share of the weighed powder that is peptide, once counterion and absorbed water are set aside. A batch can be 99 percent pure by area and well below that by mass.

Is a missing endotoxin line a red flag?

Not necessarily. Endotoxin matters for work with cultured cells and is irrelevant to many chemistry experiments. What matters is that an unmeasured parameter is described as unmeasured, not implied to be absent.

References

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