How Peptide Purity Is Tested

What a purity figure actually means

Purity is not a measure of quality, safety or correctness. It is a ratio: of everything the instrument detected in the sample, what proportion was the target compound.

A result of 99.1% means that of all the material the detector registered, 99.1% of it behaved like the peptide being tested and 0.9% behaved like something else. That something else is usually related — fragments of the same peptide, sequences that fell a residue short during synthesis, leftover reagent.

Three things that figure does not tell you, and this is the part most people get wrong:

HPLC: separating what is in the vial

HPLC stands for high-performance liquid chromatography, and the principle is simpler than the name. The sample is dissolved and pushed under pressure through a packed column. Some molecules stick to the packing more than others, so they emerge at different times. A detector at the far end records what comes out and when.

The output is a chart with a time axis and a series of peaks. Each peak is a distinct substance. The area under a peak is proportional to how much of that substance there was.

Purity is then arithmetic: the area of the main peak, divided by the total area of all peaks. That is where 99.1% comes from — it is a measured ratio of areas, not a grade someone assigned.

Peak    Retention (min)    Area        Area %
1       2.14               18,402      0.41
2       8.76               4,112,990    99.10
3       9.31               21,870      0.49
                                      ———————
                           Total       100.00

What HPLC is good at

Separation. If the vial holds the target peptide plus three synthesis by-products, HPLC will usually resolve all four and tell you the proportions. That is exactly the question purity asks.

What HPLC cannot do

It cannot identify anything. A peak is a time and an area — the instrument knows something arrived at 8.76 minutes, not what it was. Two different compounds can emerge at nearly the same time and be read as one peak. HPLC alone will happily report 99% purity on the wrong substance entirely.

Mass spectrometry: confirming identity

Mass spectrometry answers the question HPLC cannot. The sample is ionised and the instrument measures the mass-to-charge ratio of what it finds. Because every peptide has a calculated molecular weight derived from its sequence, the measured mass can be compared against the expected one.

Molecular formula       C62H98N16O22
Theoretical mass        1419.56 Da
Observed mass           1419.61 Da
Difference              0.05 Da

Agreement within a small tolerance means the material has the mass the stated sequence predicts. That is strong evidence it is the right compound. A mismatch means it is not, whatever the label says and whatever the purity figure claims.

What mass spec cannot do

Quantify a mixture reliably. Different molecules ionise with different efficiencies, so peak heights in a mass spectrum are not proportional to how much of each substance is present. It tells you what is there, not how much.

Why one test is not enough

The two instruments answer different questions, and each is blind where the other sees:

                      HPLC        Mass spec
How much of one thing?    yes          no
Which thing is it?        no           yes

Run HPLC alone and you can certify 99% purity on a vial of the wrong peptide. Run mass spec alone and you can confirm the right peptide is present without knowing whether it is most of the vial or a tenth of it.

This is why a certificate reporting both is worth meaningfully more than one reporting either. It is also why the method line is the most informative field on the page — and the one almost nobody reads.

The tests nobody advertises

Purity and identity are the two headline tests. Several others exist, are routine in pharmaceutical analysis, and are almost universally absent from research-market certificates.

• Peptide content. How much of the vial's weight is actually peptide, as opposed to salt and bound water. Lyophilised peptides are frequently 70–85% peptide by mass, with the balance counting as neither impurity nor target. A 10mg vial at 99% purity can hold substantially less than 10mg of peptide, and both figures are honest.
• Water content. Measured by Karl Fischer titration. Lyophilised material is hygroscopic and pulls moisture from the air.
• Residual solvents. Traces of what was used during synthesis and purification. Measured by gas chromatography.
• Endotoxin and bioburden. Bacterial contamination. Standard for pharmaceutical material; rare on a research certificate.
• Heavy metals. Catalyst and reagent residues from the synthesis route.

None of these being listed does not mean a batch failed them. It usually means they were never run — the research market prices for two tests, not seven. Worth knowing before you read “99% pure” as a complete description of what is in a vial.

Reading a purity number properly

Four things to look at, in order. None of them require you to understand chemistry — only to check whether the document is internally consistent.

That last check is the one that catches most problems, and it is covered properly in the certificate-reading guide.

Every batch we have sold has its certificate published in the COA Library, readable without an account and before you order anything. Run the four checks above on any of them — including ours.