How a chromatogram is actually read

A peptide HPLC chromatogram is a plot of detector signal against time. The sample is pushed through a column, usually a C18 reversed-phase column, and the molecules come off in an order set by how tightly they stuck to the packing. What you are looking at is a separation, not a portrait. A single tall peak means most of the ultraviolet-absorbing material left the column together. It does not, by itself, prove that the material is the sequence printed on the label.

In short. The picture shows when things came off the column. A tall peak means most of the material arrived together. It does not spell the sequence.

The horizontal axis is retention time, in minutes. The vertical axis, for almost every peptide certificate, is ultraviolet absorbance. Peptide bonds absorb strongly near 214 nanometres, so 214 nm is the usual wavelength, with 280 nm added when the chain contains tryptophan or tyrosine and you want a second look. The area under a peak is proportional to how much of that absorbing material was there, if the detector was in its linear range and the wavelength suits the molecule. Percent purity on a certificate is that main-peak area divided by the sum of the peak areas the method chose to count. ≥98% HPLC means the main peak was at least ninety-eight percent of the counted area. It is an area ratio. It is not a molar guarantee, and it is not a mass.

In short. Time runs along the bottom. Height is how strongly the sample absorbed ultraviolet light. Ninety-eight percent means the main peak held almost all of that area. It is not a weighing of the sequence.

Retention time is a behaviour, not an identity card. Change the column, the temperature, the acetonitrile gradient, or the ion-pairing acid — trifluoroacetic acid is the usual one — and the same peptide can arrive a minute earlier or later. A matching retention time against a reference, run in the same method, is useful. A number on its own, copied from a different laboratory's method, is not the name of the chain.

In short. The minute a peak appears depends on the method. The same peptide can move if you change the column or the solvent. A time is not a name.

The dangerous impurity in a synthetic peptide is often an N-1 deletion: the chain is missing one amino acid because a coupling failed and the growing chain was not properly capped. That shorter chain is still a peptide. It still absorbs at 214 nm. If the missing residue was not very hydrophobic, it can sit under the main peak or on its shoulder, and a quick glance at a pretty trace will call the vial clean. Area at one wavelength cannot see a missing residue. Mass spectrometry can, because the mass is short by that residue's weight. This is why a certificate that only shows a UV trace has answered a purity question and ducked an identity question.

In short. A chain missing one amino acid still absorbs ultraviolet light, and it can hide under the main peak. Only the mass tells you a letter is gone.

The same trap exists for a deletion, an insertion, a racemised residue, or a truncated chain that happens to have a similar hydrophobicity. Diastereomers — the same sequence with one amino acid in the wrong handedness — are a particular nuisance, because the mass is identical and only the shape, and sometimes the retention, has changed. HPLC can separate some of them. Mass spectrometry cannot, because the mass did not move. The two methods are not substitutes. They close different doors. HPLC-MS on one certificate is the UV separation and the mass of what came off, taken together.

In short. Ultraviolet light asks how much came off together. The mass asks what it weighs. A wrong-handed amino acid weighs the same, so you need both.

Start with the baseline. A drifting or noisy baseline makes small peaks look larger or smaller than they are, and the area percent moves with it. Then look at the main peak's shape. A shoulder, a split top, or a tail that does not come back to baseline is a second species trying to hide, or a column that is tired. Integration matters: where the software drew the start and the end of the peak changes the percent. A certificate that prints 98.4% without showing the trace is asking you to trust the arithmetic you cannot see.

In short. Look at the floor of the trace first, then at whether the main peak is clean or has a shoulder. A percentage without the picture is an arithmetic you cannot check.

Then ask what was not counted. Some methods ignore peaks below a threshold. Solvent, the counter-ion — trifluoroacetate is common on a synthetic peptide — and water do not show as peptide peaks at 214 nm in the way people hope, so a beautiful chromatogram can sit on a cake that is only seventy percent peptide by weight. That is net peptide content, a different number from chromatographic purity. Weighing the cake and calling it all peptide is how a concentration in a dish becomes fiction. The HPLC percent and the peptide content are both real. They are not the same sentence.

In short. A clean trace can still sit on a cake that is part salt and water. Purity of the peak is not the same as how much of the powder is peptide.

If you are comparing research peptides in the UK, read the certificate as two claims. Chromatographic purity is the area of the main peak. Identity is the mass, ideally taken on that peak, matching the calculated mass of the named sequence. A lot number, a date, and the method belong on the same page, because a percent without a method is a number from nowhere. The longer account of how a chain is built, residue by residue, is the synthesis essay. This page is only how to look at the picture that comes back.

In short. Purity is the size of the main peak. Identity is the mass of what that peak actually is. Both should be on the certificate, with the method and the lot.

The vials on this site are lyophilised research reagents, for laboratory use only, not for human consumption. A clean chromatogram is a description of a solid. It is not a permission.

In short. A clean trace describes the powder. It is not an instruction to take it.