On most research-peptide documentation, the purity figure is not a standalone number — it is derived from a reversed-phase HPLC chromatogram, a plot of detector signal against time. Being able to read that trace is what turns "99.1% by RP-HPLC" from a claim into something you can see the basis for. This guide walks through the parts of a chromatogram — peaks, retention time, baseline, and area percent — and explains how the reported purity figure is produced from them. The framing is analytical throughout; it is not guidance for any use of the material.

What the Plot Is

In reversed-phase HPLC, the dissolved sample is pushed through a column that separates components by how strongly they interact with the column material. As each component leaves (elutes), a detector — typically measuring UV absorbance — registers a signal. The chromatogram plots that signal (the y-axis) against time (the x-axis). A component that elutes produces a peak; the target peptide is normally the largest, and smaller peaks represent other UV-absorbing components. How this fits alongside identity testing is covered in our guide to how identity and purity are verified.

Retention Time

Retention time is the time from injection to the top of a peak — how long that component took to travel through the column under the stated method. It helps establish which peak is the target and lets runs be compared. The essential caveat: retention time is only comparable between runs performed under the same method — the same column, mobile phase, gradient, flow rate, and temperature. A retention time from a different method is not a meaningful comparison, and a matching retention time is a consistency check, not an identity confirmation on its own.

Baseline and Resolution

The baseline is the detector signal when nothing is eluting. A stable, flat baseline is what integration is measured against; a drifting or noisy baseline makes small peaks harder to distinguish and integrate. Resolution describes how cleanly adjacent peaks are separated — well-resolved peaks return to baseline between them, while poorly resolved peaks overlap, which can cause a minor component to be counted as part of the main peak. These are the features an analyst weighs when deciding how trustworthy an integration is.

How Purity Is Calculated: Area Percent

The reported purity figure is almost always an area-percent value:

  • The area under each peak is integrated (the region between the peak and the baseline).
  • The main-peak area is divided by the total area of all integrated peaks.
  • The result is expressed as a percentage — for example, a main peak that is 99.1% of total integrated area gives "99.1% by RP-HPLC."

The precise meaning matters: area percent is the proportion of detected, UV-absorbing material represented by the main peak, under that method. It is not an absolute mass fraction of everything in the vial, and it says nothing about components the detector cannot see. This is why purity is method-bound — a point developed in our guide to what 99% purity actually means.

What a Chromatogram Does Not Show

A chromatogram is analytical evidence about a tested sample under a stated method. Even a clean, high-purity trace does not, on its own, establish:

  • Identity — that is confirmed by mass, not by a peak shape (see reading mass spectrometry data).
  • Net peptide content — area percent is not the same as how much of the vial's mass is peptide (see net peptide content vs total mass).
  • Freedom from every impurity — only components that elute and absorb under the method are seen.
  • Suitability for a specific research workflow, or any conclusion about human or veterinary use.

A Short Reading Checklist

  • Confirm the method is stated — column, mobile phase, gradient, detection wavelength.
  • Identify the main peak and its retention time; check it matches the stated target.
  • Look at the baseline and whether minor peaks are resolved from the main peak.
  • Read the purity figure as area percent under that method, not as an absolute mass fraction.
  • Read it together with the identity (MS) data and net-peptide-content figure, not in isolation.

Key Takeaways

  • An HPLC chromatogram plots detector signal over time; peaks are eluting components, and the main peak is the target peptide.
  • Retention time is only comparable within the same method and is not an identity confirmation on its own.
  • Purity is reported as area percent — main-peak area over total integrated area — and is method-bound.
  • A clean trace does not establish identity, net peptide content, freedom from all impurities, or suitability.
  • Read the chromatogram alongside the mass-spectrometry and net-peptide-content data.