How to read the chromatogram on a certificate of analysis

A certificate of analysis usually opens with a number — 99.1%, say — and then, if you are lucky, prints the trace that number came from. The trace is the actual evidence. The number is a summary of it, produced by an integration routine that made several decisions on your behalf. This is a guide to reading the trace well enough to know whether you agree with those decisions.

What the axes are

Time runs along the x-axis: minutes from injection. A compound's retention time under a stated gradient is a property of the compound and the method together, which is why a retention time quoted without the gradient and column is not information.

The y-axis is detector response — for a UV detector, absorbance. Two wavelengths matter for peptides:

• 210–220 nm detects the peptide bond itself. Every peptide absorbs here, so this is the wavelength that sees everything, including impurities with no aromatic residues. Most peptide purity figures are quoted at 214 or 220 nm for that reason.
• 280 nm detects tryptophan and tyrosine (and weakly phenylalanine). A peptide with none of those residues is close to invisible at 280 nm. A purity figure quoted only at 280 nm for such a sequence is measuring almost nothing.

So the first check on any chromatogram is whether the stated detection wavelength is one at which your compound and its likely impurities both absorb.

Area percent is not mass percent

Integration gives the area under each peak, and purity is reported as your peak's area divided by total integrated area. That conversion assumes every species has the same response factor — that a microgram of impurity absorbs as much as a microgram of your peptide. It usually does not. At 214 nm the response tracks the number of peptide bonds, so a small fragment under-reports relative to its mass and a dimer over-reports.

The practical consequence is that area percent is a good comparator between lots run by the same method, and a poor absolute statement about composition. It is also the reason net peptide content is a separate assay rather than something you can read off the chromatogram.

One peak is not proof of one compound

This is the point that matters most and is stated least often. A single, symmetrical, well-shaped peak is consistent with a pure compound. It is not evidence of one. Two species with similar hydrophobicity — a peptide and its deletion sequence, most commonly — can co-elute completely, and the summed peak looks entirely normal.

The established way to interrogate this is to use spectral information across the peak rather than the peak shape alone. With a diode-array detector, the absorbance ratio between two wavelengths is constant across a peak that contains one compound and varies across a peak that contains two; photodiode-array detection was adopted for exactly this purpose — verifying peak homogeneity rather than merely peak shape — and the strategies for doing it are a settled part of pharmaceutical analysis.^[1][2] Combining diode-array data with mass spectrometric detection, and treating the peak as a multivariate object rather than a shape, extends that further and is what makes a co-elution visible rather than assumed absent.^[3] Mass spectrometric detection adds an orthogonal dimension to the same question.^[4]

What this means when you are looking at a printed trace: a purity figure derived from a single-wavelength trace with no orthogonal check is an upper bound. It cannot be lower than the truth, and it can be considerably higher. Purity supported by a mass spectrum of the collected peak is a much stronger statement, which is why MS identity belongs on the certificate rather than in a separate document.

Whether the method could see a problem at all

A purity method is only meaningful if it resolves the compound from the things it turns into. In pharmaceutical analysis this is the definition of a stability-indicating method: the sample is deliberately stressed — acid, base, oxidation, heat, light — and the assay is shown to separate the parent from every degradation product formed.^[5] A method never challenged that way may simply be co-eluting the degradants it was supposed to detect.

You will rarely see forced-degradation data on a research-grade certificate, and that is not by itself a red flag. But it does bound how much the number is worth: it says the material looked clean under one gradient on one day, not that the method would notice if it were not.

A reading checklist

• Is the method stated? Column chemistry and dimensions, gradient, flow rate, detection wavelength, injection amount. Without these the trace cannot be reproduced or compared.
• Is the y-axis scaled honestly? A trace scaled to the top of the main peak flattens 0.3% impurities into the baseline. A useful certificate often shows a second, expanded view.
• Is the baseline visible before and after the peak? A trace that starts and stops at the peak has been cropped, and cropping hides late-eluting material.
• Is there anything at the injection front? Salts, TFA and unretained material elute in the void volume and are frequently excluded from integration. That exclusion should be stated.
• What was the integration threshold? Peaks below the reporting limit are not counted. The limit belongs on the certificate.
• Does a mass spectrum accompany it? Identity and purity are different questions; the chromatogram alone answers only the second, and only partly.
• Does the lot number on the certificate match the vial? The most common failure is not a bad chromatogram. It is a good chromatogram belonging to a different batch.

What should make you stop

An image with no axes or no scale. A purity figure with no method attached. A trace whose lot number does not appear anywhere on it. A certificate that is a screenshot of a number. And — the one that is easiest to miss — a supplier who publishes a purity figure for a lot but cannot produce the underlying chromatogram when asked, which means the figure was transcribed rather than measured.

References

1. Evaluation of a photodiode array detector for the verification of peak-homogeneity in high-performance liquid chromatography. PMID 2094426
2. Strategies for peak-purity assessment in liquid chromatography. PMID 8466954
3. Assessment of chromatographic peak purity of drugs by multivariate analysis of diode-array and mass spectrometric data. PMID 1298391
4. Peak purity assessment in liquid chromatography-mass spectrometry. PMID 11358261
5. LC and LC-MS/MS study of forced decomposition behavior of anastrozole and establishment of validated stability-indicating analytical method for impurities estimation in low dose anastrozole tablets. PMID 19541446

This article describes the chemistry of peptide degradation and laboratory handling practice. It is not guidance for use in humans or animals. X Factor Peptides supplies research reference standards for in-vitro and non-clinical research only.