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How to Read an HPLC Chromatogram: Peaks, Shoulders and Baselines
Most peptide reports show a single headline number such as 99 percent purity. That number is not measured directly. It is calculated from a picture called a chromatogram, and the picture contains much more information than the number does. Knowing how to read it makes any certificate of analysis more useful.
Key takeaways
- The x-axis is time (retention time) and the y-axis is detector signal.
- Purity is usually the main peak area divided by the total area of all peaks.
- Shoulders and split peaks can hide close relatives of the main peptide.
- A purity figure means little without the method: wavelength, gradient and integration rules.
The axes
In reversed-phase HPLC, the peptide mixture is pushed through a column packed with hydrophobic particles. A gradient of increasing organic solvent, commonly acetonitrile in water with a small amount of an acid such as TFA, gradually washes components off. Polar molecules leave early and more hydrophobic molecules leave later. The detector records signal against time, and each component appears as a peak. Peptides are usually detected by ultraviolet light at about 214 nm, where the peptide bond absorbs, sometimes alongside 280 nm, which picks up tryptophan and tyrosine.
What the peaks mean
- The tall main peak is the target peptide in a good sample. Its position, the retention time, should be reproducible between batches when the method is unchanged.
- Peaks near the very start are often the solvent front, salts or other unretained material, and many methods exclude them from the calculation. Check whether the report says so.
- Small peaks before the main peak are typically more polar relatives, such as truncated sequences or deamidated forms (see deamidation).
- Small peaks after the main peak are more hydrophobic: peptides that kept a protecting group, dimers, or oxidized and modified forms depending on the sequence.
Shoulders and split peaks
A shoulder is a bump on the side of the main peak. It usually means a second compound is eluting almost at the same time. Common culprits are diastereomers (see chiral purity), deamidation products and conformational forms of the same peptide. Because the integrator may fold a shoulder into the main peak, the purity number can look better than the chemistry really is. Using a different gradient or a second column type is the usual way to pull them apart.
Baseline and gradient artifacts
As the organic fraction rises, the baseline often drifts upward because the solvent itself absorbs a little at 214 nm. A blank injection, running the same gradient with no sample, shows which wiggles come from the system rather than the peptide. A reliable report either includes the blank or states that one was run.
Questions worth asking about any trace
- What wavelength was used?
- What column and gradient were used, and how long was the run?
- Was the whole run integrated, or only the window around the main peak?
- What was the integration threshold for small peaks?
- Does the identity of the main peak come from mass spectrometry on the same batch?
What HPLC cannot tell you
UV detection sees compounds that absorb light. It does not see water, many salts, or counterions, so a 99 percent HPLC purity is not the same as 99 percent of the vial being peptide. That difference is the subject of net peptide content versus purity.
The bottom line
Treat the purity number as the summary of a picture, then look at the picture. Clean baselines, a single sharp main peak and a stated method are what a trustworthy chromatogram looks like. Our basic guide to reading a COA shows where this trace sits in the full report.
Related reading
- HPLC and Mass Spectrometry Explained: How Peptide Purity Is Tested
- How to Read a Certificate of Analysis (COA) for Research Peptides
- TFA vs. Acetate Counterions in Research Peptides
- Browse the full Research Library
For laboratory research use only. Not for human or veterinary use, and not intended to diagnose, treat, cure or prevent any condition. This article is educational and is not medical advice.