What HPLC and mass spectrometry each establish on a peptide COA: purity versus identity, what each cannot show, and why one method alone is half an answer.
Almost every research peptide certificate of analysis carries a purity figure, usually a number above 98 percent, usually from HPLC. Far fewer carry a mass spectrometry result. Suppliers present the purity figure as if it settles the question of what is in the vial. It does not, because purity and identity are different questions answered by different instruments. This guide explains, in analytical terms only, what each method measures, what each cannot show, and why a certificate that reports only one of them has answered half of what a buyer is asking.
Two questions, two methods
A certificate of analysis for a peptide is trying to answer two separate questions about the sample the laboratory received.
The first is identity: is the main component of this sample the peptide named on the label. The second is purity: how much of the sample is that main component, as opposed to other material. Confirming identity without purity tells you the right molecule is present but not how much else is there. Confirming purity without identity tells you the sample is mostly one thing but not what that thing is.
| HPLC | Mass spectrometry | |
|---|---|---|
| Question answered | How much of the detected signal is the main component | What the molecular mass of the main component is |
| Output on a certificate | A chromatogram and a purity percentage | A mass spectrum and an observed mass, compared to a theoretical mass |
| Establishes identity | No; retention time alone is not identity | Yes, when the observed mass matches the expected mass within the instrument’s tolerance |
| Establishes purity | Yes, as a relative area percentage | No; peak intensity is not a quantitative purity measure |
| Common gaps | Does not detect material that does not absorb at the detection wavelength; purity is relative, not absolute | Does not quantify; does not distinguish between sequence variants with the same mass without further work |
| Typical certificate shortfall | Reported alone, with no identity confirmation | Reported alone, with no purity figure, or reported as a single mass with no spectrum shown |
What HPLC establishes
High-performance liquid chromatography pushes a dissolved sample through a column under pressure. Components separate because they interact differently with the column material and emerge at different times. A detector, most often ultraviolet absorbance at a fixed wavelength, records a signal as each component passes. The result is a chromatogram: a trace of signal against time, with peaks for each separated component.
The purity figure on a certificate is the area of the main peak divided by the total area of all peaks, expressed as a percentage. A result of 99.2 percent means that 99.2 percent of the signal the detector saw came from one component.
Three limits follow from that definition.
Purity by HPLC is relative. It compares the main peak to the other peaks the detector saw. It says nothing about material the detector did not see: anything that does not absorb at the chosen wavelength, that did not dissolve, or that did not elute from the column within the run time.
Purity by HPLC is not identity. The main peak is a component with a particular retention time under particular conditions. Different molecules can share a retention time. A chromatogram alone cannot tell you the main peak is the peptide named on the label rather than a related sequence, a degradation product or something else entirely.
Purity by HPLC is not content. It does not tell you how many milligrams of peptide are in the vial. A vial could contain a highly pure peptide at a fraction of the labeled quantity and still show a 99 percent purity figure.
What mass spectrometry establishes
A mass spectrometer ionizes the sample and measures the mass-to-charge ratio of the ions. For a peptide, the result is a spectrum showing the observed mass of the main component, which the laboratory compares to the mass calculated from the peptide’s sequence. If the observed and theoretical masses agree within the instrument’s stated tolerance, the lab reports identity confirmed.
Mass spectrometry is the method that answers the identity question. It also has limits that a certificate reader should understand.
A mass match is not a sequence proof. Two peptides with the same amino acids in a different order have the same mass. Distinguishing them requires fragmentation data or a separate sequencing method, which standard supplier certificates do not include. For most purposes a mass match is accepted as sufficient identity, but it is a match, not a structural determination.
Mass spectrometry does not quantify purity. The relative heights of peaks in a spectrum depend on how readily each component ionizes, not only on how much of it is present. A spectrum can show a clean main signal from a sample that is far from pure by HPLC.
A single reported number is weaker than a spectrum. Some certificates print “MS: confirmed” or an observed mass with no spectrum attached. Without the spectrum, a reader cannot see whether the match was clean or whether other significant masses were present.
Why one method is half an answer
Put the two sets of limits side by side and the gap is obvious.
A certificate with HPLC only tells you the sample was mostly one component, and leaves open what that component was. A certificate with mass spectrometry only tells you a component of the expected mass was present, and leaves open how much of the sample it made up. Neither certificate, on its own, supports the sentence suppliers most want to write: this vial contains the named peptide at the stated purity.
In our supplier records, the testing section tracks HPLC and mass spectrometry as separate fields. Many certificates in the category report HPLC alone. A smaller number report both. A certificate that reports both, with the chromatogram and spectrum shown rather than summarized, is the minimum document that answers both questions a buyer is asking. Our public COA list gives credit on that basis.
Other measurements that sometimes appear
Several further tests appear on more complete certificates. None of them substitutes for identity or purity; each answers a different question.
- Net peptide content, by amino acid analysis or a nitrogen method, reports what share of the vial’s mass is peptide as opposed to water and counter-ions. It is the figure that connects purity to quantity, and it is rarely reported.
- Water content, by Karl Fischer titration, reports residual moisture.
- Endotoxin, by a limulus amebocyte lysate method, reports bacterial endotoxin levels in the sample.
- Sterility testing reports whether viable microorganisms were detected.
- Heavy metals, by inductively coupled plasma methods, reports metal content.
A certificate listing these alongside HPLC and mass spectrometry is more complete. A certificate listing them instead of identity or purity has skipped the fundamentals.
Reading the certificate, not the headline number
When you open a supplier’s certificate, look past the purity percentage and check three things. Is there a chromatogram, not just a number, and does the method section state the column, mobile phase, wavelength and run time. Is there a mass spectrum, with observed and theoretical masses stated and compared. Does the lot number on the certificate match the listing.
If the answer to the first two is yes, the certificate has addressed both questions. If only one is present, the supplier has published half a document, and the missing half is the one you should ask for. If neither method is shown in full and the certificate is a single summary page, our guide on fake COA warning signs explains what else to check before relying on it as evidence. And whichever methods appear, the certificate is a claim by the supplier until the issuing laboratory confirms the report number, which is the subject of our step-by-step verification guide.
The purity figure is the number that sells. The mass spectrum is the one that tells you what you are looking at. A buyer who understands the difference asks for both.