How to Read the HPLC Chromatogram on a Peptide COA
Updated 2026-10-09 · 6 min read · Compiled from published literature · Laboratory reference only
The chromatogram on a peptide certificate of analysis is a plot of ultraviolet absorbance (vertical axis) against time (horizontal axis) as the sample passes through an HPLC column. Each peak is material that absorbed UV light as it left the column; the main peak should be the target compound, and a UV purity figure is that peak's share of the total peak area. Reading the trace shows where the main peak sits, what else was detected and how well it was separated, but not what the smaller peaks are, or anything the detector can't see.
The parts of the plot
| Element | What it shows |
|---|---|
| Horizontal axis (min) | Time since injection; a peak's position is its retention time |
| Vertical axis (mAU) | UV absorbance in milli-absorbance units at the stated wavelength |
| Peak labels | Usually the retention time of each detected peak, in minutes |
| Header line | The detector, wavelength or wavelength mode, and the instrument data file |
| Peak table (when printed) | Each peak's retention time, area and area percent |
| Baseline | The detector signal when nothing is eluting; drift and dips show up here |
Retention time
In reversed-phase HPLC, the usual mode for peptides, a sample travels through a column packed with a hydrophobic material while the mobile phase becomes steadily more organic. Mant et al. (2007) describe how peptides are retained largely according to the hydrophobicity of their residues, so related impurities, such as a sequence missing one residue, often elute at a different time from the target. Retention time depends on the column, the gradient, the flow rate and the temperature, so it identifies a peak only within one method; the same compound elutes at different times on different methods.
Material that isn't retained at all comes off at the column's dead time (often written t0). USP <621> defines the retention factor, k, from the retention time and the dead time: the further a peak elutes beyond t0, the more the column has acted on it. A main peak very close to the start of a run may be eluting near the dead time, where separation from other unretained material is weakest; whether it is can't be told without the column, flow rate and dead time.
Peak height, area and the purity figure
A peak's height is its maximum absorbance; its area, the integral of the signal across the peak, is what purity is calculated from. Area-percent purity is the main peak's area divided by the total area of all integrated peaks. Two points follow:
- The plot's scale can hide small peaks. When the main peak runs to the top of the axis, a peak at 0.5% of its area can be a barely visible bump. A peak table, or a zoomed view of the baseline, is what shows them.
- Area isn't amount. Different compounds absorb UV light to different degrees. Kuipers and Gruppen (2007) measured how strongly the peptide bond and each amino acid absorb at 214 nm, with aromatic residues absorbing several times more than the peptide bond, so an impurity that has lost an aromatic residue is under-counted by area. Water, salts and the counterion don't absorb at these wavelengths and never appear in the trace. More in how peptide purity is measured.
Shoulders, tailing and the baseline
- Shoulders. A bump on the leading or trailing edge of the main peak, or a peak that doesn't return to the baseline before the next one starts, suggests two species eluting close together. How they were split between the peaks, if at all, depends on the integration, and a figure computed from merged peaks overstates the main component.
- Tailing. A peak whose back edge falls more slowly than its front edge rose. USP <621> measures this as the symmetry (tailing) factor. Strong tailing can hide a small peak that elutes just after the main one.
- Dips and drift. A brief dip near the start of a run commonly reflects the injection solvent passing the detector, and a gradual rise or fall during a gradient often reflects the changing mobile phase rather than sample components. Neither is integrated as a peak in a typical purity calculation.
- Resolution. Fekete et al. (2012) reviewed how column chemistry, temperature, gradient steepness and the mobile-phase additive change which impurities separate from the main peak. A steep gradient can merge peaks that a shallow one separates, which is why a purity figure belongs with its method.
Wavelength: XWC and TWC traces
Peptides are usually monitored between about 200 and 220 nm, where the peptide bond absorbs. A diode-array detector (DAD) records many wavelengths at once, and the trace on a certificate can be drawn in two ways. An XWC (extracted wavelength chromatogram) shows absorbance at one wavelength, printed in its header. A TWC (total wavelength chromatogram) sums the detector's readings across its whole range, so its peak sizes don't correspond to any single wavelength and its vertical scale can run far higher than a single-wavelength trace.
Worked examples from our certificates
Two certificates in our certificate library, both issued by AxisPharm, LLC, read as follows: KPV 10 mg, lot KPV10VAS0826, and VIP 10 mg, lot VIP10VAS0426.
| On the certificate | KPV 10 mg | VIP 10 mg |
|---|---|---|
| Trace type | XWC at 200.0 nm | TWC (all wavelengths) |
| Main peak | 0.25 min, axis maximum 973.2 mAU | 2.15 min |
| Other labelled peaks | 0.35, 0.56, 3.68, 3.88, 4.15 min | 0.30, 0.47, 0.81, 1.66 min |
| Purity (results table) | 99.2%, no wavelength printed | >99.3% at 208 nm |
| Peak areas | Not printed | Not printed |
| Mass spectrum window | 0.234 to 0.368 min | 2.187 to 2.225 min |
Three things are worth checking on any certificate, and these two show each of them:
- Does the mass spectrum come from the main UV peak? On both, the time window printed above the mass spectrum falls within the main UV peak (just after its apex on the VIP certificate), so the reported ion and the peak counted as the target are the same material. Reading the ion itself is covered in observed vs theoretical mass.
- Can the purity be read off the trace? Neither certificate prints a peak table, so the area of each labelled peak isn't shown and the purity figure can't be recalculated from the document. On the VIP certificate the figure is reported at 208 nm while the trace is a TWC, so the printed number can't be read directly off the plot.
- Where does the main peak sit? KPV's main peak appears a quarter of a minute into the run. The certificate doesn't state the column, gradient, flow rate or dead time, so whether that is close to the dead time, and how well it was separated from other early material, can't be judged from the document (see the KPV certificate guide). VIP's main peak elutes later, at 2.15 min (see the VIP certificate guide).
What a chromatogram can't show
- What the other peaks are. A UV trace detects material; it doesn't name it. Identity needs mass spectrometry or a reference standard run under the same method.
- Anything that doesn't absorb UV. Water, salts, counterions and many residual solvents are invisible to it, so net peptide content isn't on the trace.
- Species hidden under the main peak. A compound that co-elutes exactly with the target adds to the main peak's area. Isomers often elute close to the parent compound, and only a method that separates them will show them.
- Anything about a particular vial. The trace describes the sample the laboratory tested from one lot. The lot number is what ties a vial to that report (see what a lot number does and doesn't prove), and the lot lookup opens the certificate for a lot.
Regulatory status
A chromatogram is a quality-control record and confers no regulatory standing. Materials described here are not approved by the FDA, are supplied for laboratory research use only, and are not for human or animal consumption; see the research use policy.
Questions about chromatograms
What is retention time on an HPLC chromatogram?
The time between injection and a peak leaving the column, read from the horizontal axis in minutes. It depends on the column, gradient, flow rate and temperature, so it identifies a peak only within one method.
What does mAU mean on a chromatogram?
Milli-absorbance units: the UV absorbance the detector recorded, one-thousandth of an absorbance unit. Peak height is read in mAU; purity is calculated from peak areas, not heights.
Can purity be checked from the chromatogram alone?
Only if the certificate prints a peak table with each peak's area, or the trace is clear enough to integrate. The certificates used as examples here print labelled peaks but no areas, so their purity figures can't be recalculated from the document.
What is the difference between an XWC and a TWC trace?
An XWC (extracted wavelength chromatogram) shows absorbance at one stated wavelength. A TWC (total wavelength chromatogram) sums the detector's readings across its whole wavelength range, so it doesn't correspond to a purity figure reported at one wavelength.
Does a single large peak mean a sample is pure?
It means most of the UV-absorbing material eluted together under that method. Compounds that co-elute with the target, or that don't absorb UV, such as water, salts and counterions, don't show up as separate peaks.
References
- Snyder LR, Kirkland JJ, Dolan JW (2010). Introduction to Modern Liquid Chromatography, 3rd edition. Wiley.
- United States Pharmacopeia. General chapter <621> Chromatography.
- Mant CT, et al. (2007). HPLC analysis and purification of peptides. Methods in Molecular Biology.
- Fekete S, et al. (2012). New trends in reversed-phase liquid chromatographic separations of therapeutic peptides and proteins: theory and applications. Journal of Pharmaceutical and Biomedical Analysis.
- Kuipers BJ & Gruppen H (2007). Prediction of molar extinction coefficients of proteins and peptides using UV absorption of the constituent amino acids at 214 nm to enable quantitative reverse phase high-performance liquid chromatography-mass spectrometry analysis. Journal of Agricultural and Food Chemistry.