Purity is the number everyone looks for and the number almost nobody interrogates. It gets printed in large type on product pages, compared across suppliers as if it were a single standardised measurement, and used to justify price differences. It is none of those things. Purity is the output of a specific method run under specific conditions, and it only means something when you know what that method was.
The two questions analysis answers
Peptide analysis usually addresses two separate questions, and it is easy to conflate them.
- Is this the right compound? That is identity, and it is answered by mass spectrometry.
- How much of what is in the vial is that compound? That is purity, and it is answered by chromatography, normally HPLC.
A material can pass one and fail the other. Correct identity with poor purity means the right peptide is present alongside a significant quantity of something else. High purity with unconfirmed identity means the vial contains one dominant substance, but nothing has established what that substance is. You need both results.
What HPLC actually measures
High performance liquid chromatography pushes a dissolved sample through a packed column under pressure. Different molecules travel through the column at different speeds depending on how strongly they interact with the packing material. A detector at the far end registers each component as it exits, and the result is a chromatogram: a trace with peaks, where each peak is a component and the retention time tells you when it emerged.
Purity by HPLC is calculated by area. The software integrates the area under every peak, then expresses the main peak as a percentage of the total. That is the number on the certificate.
What the detector does not see
Most peptide HPLC uses ultraviolet detection, typically around 214 nm, where the peptide bond absorbs. That works well for peptides. It works poorly for several things that may be present alongside them.
| Component | Visible on a standard UV chromatogram? |
|---|---|
| The target peptide | Yes, the main peak. |
| Truncated or deletion sequences | Yes, usually as peaks close to the main peak. |
| Residual synthesis solvents | Partly, depending on the solvent and wavelength. |
| Residual water in the cake | No. |
| Counter-ions such as acetate or trifluoroacetate | No. |
| Inorganic salts | No. |
This is why a peptide can be reported at a very high chromatographic purity and still have a materially lower net peptide content by weight. The two figures are not in conflict. They are answers to different questions. Chromatographic purity asks what proportion of the peptide-like material is the target. Peptide content asks what proportion of the total mass in the vial is peptide at all.
Reading a chromatogram
You do not need to be an analytical chemist to get value from a chromatogram. Four things are visible to anyone.
- One dominant peak. A clean synthesis produces one tall, well-resolved peak with everything else close to the baseline.
- Peak shape. A sharp, symmetrical peak is a good sign. Broad peaks or pronounced tailing can indicate resolution problems or co-eluting material.
- Baseline behaviour. A flat, quiet baseline suggests a clean sample. A restless or drifting baseline suggests otherwise.
- Shoulders. A bump on the side of the main peak often means a closely related impurity is partly hiding under it, which inflates the reported purity.
A supplier who supplies the chromatogram rather than just the summary figure is giving you the ability to check their arithmetic. That willingness is itself informative.
Why identity needs mass spectrometry
Mass spectrometry ionises the sample and measures the mass-to-charge ratio of the resulting ions. Because a peptide's molecular mass is fully determined by its amino acid sequence, the theoretical mass can be calculated in advance. The analysis then compares the observed mass with that calculated value.
A properly reported identity result shows both the expected and the observed mass. If a certificate simply says "identity confirmed" with no numbers, the confirmation cannot be checked by the reader, which defeats the purpose of documenting it.
Note the limitation: standard MS measures mass, and mass alone cannot always distinguish sequences that happen to share the same composition. For most catalogue peptides this is not a practical concern, but it is why identity and purity are reported together rather than either being treated as sufficient on its own.
What to ask a supplier
- Which HPLC method was used, and at what detection wavelength?
- Can I see the chromatogram for this batch, not a representative example?
- Was identity confirmed by MS, and what were the expected and observed masses?
- Is peptide content by weight reported separately from chromatographic purity?
- Was the analysis performed in-house or by an independent laboratory?
Frequently asked questions
What does HPLC purity mean for a peptide?
It is the area of the main chromatographic peak expressed as a percentage of the total area of all detected peaks. It describes the proportion of detected material that is the target compound, under one specific analytical method.
Is HPLC purity the same as peptide content?
No. HPLC purity is a relative measure of detected material. Peptide content is the proportion of the vial's total mass that is peptide, and it accounts for water, counter-ions and salts that UV detection does not register. The two figures are usually different and are reported separately.
Why do purity figures differ between laboratories?
Because purity depends on the method. Different columns, gradients, run lengths and detection wavelengths resolve impurities differently, so the same material can yield different figures across laboratories without either being wrong.
What does mass spectrometry confirm?
Identity. It measures the molecular mass of the material and compares it with the mass calculated from the expected peptide sequence. It does not measure how much of the vial is that compound, which is why it is paired with chromatography.