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.
What mass spectrometry confirms
Mass spectrometry (MS) measures the mass-to-charge ratio of ionised molecules. For a peptide, the observed molecular mass is compared with the theoretical mass calculated from its sequence. A match within the instrument’s tolerance confirms the peptide is the intended sequence; a mismatch points to a wrong sequence, a deletion, or a modification such as oxidation.
MS is an identity test. It does not measure how much of the sample is the target peptide.
Purity vs identity: reading them together
HPLC answers “how much of this sample is one component?” MS answers “is that component the right molecule?” Neither answers both. A lot can be 99% pure by HPLC and still be the wrong peptide, and a correct mass says nothing about the impurities beside it. A complete certificate reports both results for the same lot number.
Frequently asked questions
How do you read a peptide certificate of analysis?
Match the lot number to the vial, then read the HPLC purity (area % of the main peak), the mass-spectrometry identity result (observed vs theoretical mass), the test date and the testing laboratory. Purity and identity should both be reported for the same lot.
What does HPLC purity mean on a COA?
It is the main peak’s area as a percentage of all integrated peak area in the chromatogram. It measures how much of the sample is one component under that method, not what the component is.
Why is mass spectrometry needed if HPLC shows high purity?
Because HPLC cannot confirm identity. A single dominant peak could be the wrong sequence. Mass spectrometry checks that the molecular mass matches the intended peptide.
Can two labs report different purity for the same lot?
Yes. Column chemistry, gradient and the reporting threshold for small peaks all change the calculated area %, so compare purities only between reports that used the same method.
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?