Reading an HPLC Chromatogram
Go beyond the purity number: learn to read the HPLC trace itself — the main peak, retention time, baseline, and the neighbouring peaks that reveal a peptide's true quality.
A Certificate of Analysis states a purity percentage, but the HPLC chromatogram behind that number is where the real information lives. Learning to read the trace lets you judge quality for yourself rather than taking a single figure on trust — and it is a genuinely useful skill when comparing products or verifying a batch.
This guide is a deeper companion to the certificate-of-analysis overview. There we covered which fields to check; here we focus specifically on interpreting the chromatogram image: axes, the main peak, retention time, baseline behaviour and impurity peaks.
Research use only.
WHAT YOU NEED
Tools
- An HPLC chromatogram from a Certificate of Analysis
STEP-BY-STEP
Read the axes
A chromatogram plots detector response (vertical axis) against time (horizontal axis, in minutes). Each peak is a component of the sample emerging from the column at a characteristic time. Taller and larger-area peaks mean more of that component.
Tip: Purity is calculated from peak area, not peak height — a tall thin peak and a short wide one can carry the same area.
Find the main peak
The dominant peak — the tallest, largest-area one — is the target peptide. In a quality sample it should clearly overshadow everything else on the trace, accounting for 98% or more of the total peak area.
Check the retention time
Retention time is when the main peak emerges. It is a fingerprint for a given compound and method: a consistent retention time across batches, matching the expected value, supports that the right compound is present. A shifted peak can hint at a different or altered molecule.
Inspect the baseline
Between peaks the trace should return to a flat, stable baseline. A drifting, noisy, or lumpy baseline can mask small impurities and suggests a less clean separation. A clean flat baseline is a good sign.
Scrutinise the neighbouring peaks
Small peaks near the main one are impurities — often closely related synthesis by-products. A few tiny, well-separated minor peaks are normal; large or numerous neighbours, or peaks fused into the main peak, are a quality concern.
Confirm identity with mass spec
The chromatogram shows how pure and how consistent the sample is, but not definitively what the molecule is. Cross-check the mass-spectrometry result on the certificate, which confirms the molecular weight matches the named peptide.
A good trace versus a poor one
A good chromatogram shows one sharp, dominant peak at the expected retention time, a flat stable baseline, and at most a few tiny, cleanly separated minor peaks. The reported purity matches what your eye sees.
A poor chromatogram shows a shorter or shouldered main peak, large or numerous neighbouring peaks, a drifting baseline, or peaks fused together so purity cannot be cleanly integrated. Be especially wary if the printed purity figure does not match what the trace visibly shows.
Reading the trace this way turns the CoA from a number you accept into evidence you can evaluate — which is exactly what a certificate is for.
PUT IT INTO PRACTICE
FREQUENTLY ASKED
What does an HPLC chromatogram show?
It plots detector response against time. Each peak is a component of the sample; the dominant peak is the target peptide, and its share of the total peak area is the purity. Neighbouring peaks are impurities.
What is retention time and why does it matter?
Retention time is when a peak emerges from the column. It acts as a fingerprint for a compound under a given method — a consistent retention time matching the expected value supports correct identity, while a shifted peak can signal a different or altered molecule.
What makes a chromatogram look good?
One sharp, dominant peak at the expected retention time, a flat and stable baseline, and at most a few tiny, well-separated minor peaks. The visible trace should agree with the stated purity figure.
Does the chromatogram confirm what the molecule is?
Not on its own — it shows purity and consistency. To confirm identity you also need the mass-spectrometry result, which verifies the molecular weight matches the named peptide.