High-performance liquid chromatography (HPLC) is the most common analytical method used to report peptide purity on Certificates of Analysis (COAs). This guide explains what the key fields mean so you can interpret a report in context — without assuming that a single number tells the whole story.
What HPLC measures
HPLC separates components in a sample by passing them through a column under controlled flow and solvent conditions. A detector (typically UV absorbance at 214 nm for peptides) records signal over time, producing a chromatogram: a plot of detector response versus retention time.
The largest peak is usually assigned as the target peptide. Smaller peaks represent related substances — synthesis by-products, deletion sequences, oxidized variants, or aggregates — that elute at different times.
Key fields on a typical COA
| Field | What it means |
|---|---|
| Sample ID / Lot number | Links the report to a specific production batch. Always match this to the vial label. |
| Method reference | The validated SOP or pharmacopeial method used. Methods differ in sensitivity and selectivity. |
| Column & conditions | Stationary phase, gradient, flow rate, and temperature. Different conditions resolve different impurities. |
| Main peak RT | Retention time of the principal peak — useful for batch-to-batch comparison under identical methods. |
| Area % / Purity | Peak area of the main component divided by total integrated area, expressed as a percentage. |
| Impurity peaks | Individual peaks above the reporting threshold, sometimes listed with relative retention times. |
Reading the chromatogram
- Identify the main peak. It should be clearly dominant for a high-purity lot. Verify the retention time is consistent with prior lots using the same method.
- Scan for impurity peaks. Early-eluting peaks often indicate hydrophilic deletion sequences; late peaks may be hydrophobic variants or dimers.
- Check integration limits. Peaks below the method’s reporting threshold may be excluded from the purity calculation — two labs can report different purities on the same material if thresholds differ.
- Compare method to method. A ≥99% result on one gradient does not guarantee the same result on another column chemistry.
What HPLC does not tell you
- Identity. Purity alone does not confirm you have the intended sequence. Mass spectrometry (MS) or amino-acid analysis provides orthogonal identity data.
- Biological activity. Chromatographic purity does not predict folding, receptor binding, or sterility.
- Endotoxin, bioburden, or residual solvents. These require separate assays not shown on a standard HPLC COA.
Research context: Purity specifications exist to support reproducible laboratory work. They are not clinical quality standards and do not, by themselves, establish suitability for any use beyond qualified research applications.
Questions to ask when reviewing a COA
- Was the same HPLC method used for previous lots I compared?
- Is there a corresponding MS or identity report for this lot?
- Are impurity peaks named or only reported as total impurities?
- Was the analysis performed by an independent third-party lab or in-house?