An HPLC chromatogram is the plotted output of high-performance liquid chromatography (HPLC), the laboratory method that measures the purity of a peptide sample. The instrument separates a sample into its component substances and draws each one as a peak on a graph. That graph, not the single percentage a vendor prints on a label, is the actual evidence of what a batch contains. This guide teaches a non-specialist to read the trace so the number stops being something you take on faith.
What is an HPLC chromatogram actually showing?
Think of HPLC as a race. The sample is pushed through a long column packed with fine particles, and different molecules travel through at different speeds depending on their chemistry. As each substance leaves the column, a detector registers it and the software draws a peak. A pure sample produces one tall, dominant peak with little else around it. A mixed or degraded sample produces several peaks of varying size. The chromatogram is simply the record of that separation: one line that rises into peaks and returns to a flat baseline between them. Reading it means understanding what the two axes represent, which peak is your target, and how much of everything else is present. None of that requires chemistry training, only a willingness to look at the trace instead of the summary line beneath it.
What do the two axes on the trace mean?
Every chromatogram uses the same layout. The horizontal axis (x-axis) is time, usually in minutes, measured from the moment the sample is injected. The vertical axis (y-axis) is detector response, often labelled in absorbance units such as mAU, which reflects how much of a substance is passing the detector at that instant. A peak is therefore a substance arriving at a specific time (its horizontal position) in a specific quantity (its height and, more importantly, its area). The flat stretches between peaks are the baseline, where only solvent is passing through. When you first look at a trace, orient yourself: find the baseline, note that time runs left to right, and note that taller and wider peaks mean more material. Once the axes make sense, the rest of the chromatogram is just interpretation, and you can begin to read it the way an analyst does.
How do I find the main peak and its retention time?
The main peak is normally the tallest and largest, and on a clean sample it dominates the trace. Its position on the time axis is the retention time: how many minutes it took that substance to travel through the column under the stated method conditions. Retention time is a fingerprint of behaviour under a specific method, so a laboratory establishes what time the target peptide should elute and checks that the main peak appears there. A word of caution: retention time tells you a peak behaves like the expected compound, but it does not prove identity. Two different molecules can share a retention time. Confirming that the peak truly is the peptide claimed requires mass spectrometry, which is why purity and identity are different questions. HPLC tells you how clean the sample is; it does not, on its own, tell you what the clean substance is.
What are the smaller impurity peaks telling me?
Any peak that is not your target is an impurity peak: a related substance, a synthesis by-product, a degradation product, or a solvent artefact. On a high-purity sample these are small and few; on a poor batch they are larger and more numerous. When reading the trace, scan the whole time window, not just the region around the main peak, because impurities can elute early or late. Look for peaks that are well separated from the target, since a peak sitting right on the shoulder of the main peak can be hidden inside it and understate the true impurity level. You are not expected to identify each impurity by name. What matters for judging transparency is simply that the full trace is shown, that the baseline is stable, and that the impurities are visible and integrated rather than cropped out of the picture. A truncated trace that shows only the main peak is a weaker document than a full one.
How does the target-peak area become a purity percentage?
Purity by HPLC is reported as percentage by area. The software measures the area under every peak, adds them together to get the total area, and expresses the target peak as a fraction of that total. If the target peak is 99 percent of all the peak area, the sample is reported as 99.0 percent pure by area. This is why area, not height, is what counts: a short, wide peak can hold more material than a tall, narrow one. It is also why the printed number is only a summary of the picture above it. One important limit: purity by area describes the proportion of peptide-related material the detector sees, not net peptide content, which accounts for water, salts such as TFA or acetate, and counter-ions and is a separate figure. A sample can be 99 percent pure by area and still contain a meaningful mass of non-peptide matter. The anatomy of a certificate of analysis covers where each of these figures should appear.
Why is the chromatogram the evidence, not the printed number?
A purity percentage is a conclusion; the chromatogram is the data that conclusion is drawn from. A bare number with no trace behind it cannot be checked: you cannot see whether the baseline was stable, whether impurity peaks were integrated or ignored, whether the method separated substances properly, or whether the main peak eluted at the expected time. When the full chromatogram is published, tied to a batch or lot number and a date, and produced by a named, independent laboratory, the number becomes verifiable at the source rather than a marketing claim. Transparency here is a documentary property, not a slogan. Our scoring methodology weights whether vendors publish the trace and its batch context, not merely a headline figure, and you can see how vendors compare on the transparency scoreboard. Peptides sold for research use only deserve the same scrutiny of evidence as any other laboratory material.