- HPLC answers 'how much of this is one thing'. MS answers 'is that thing the molecule on the label'. They are different questions.
- Purity by HPLC is an area percentage of the chromatogram — it is blind to anything the detector can't see, including water, salts and residual solvent.
- A synthetic peptide's impurities are mostly near-relatives of the sequence — deletions, truncations, oxidised or deamidated forms — which is exactly why MS is needed to tell them apart.
- 'Purity' and 'net peptide content' are different figures. A vendor quoting only one is telling you half the story.
Two acronyms do most of the work in peptide quality control, and they are routinely confused — including by vendors who print them. This is what each one measures, what the combined result means, and the questions a purity figure leaves unanswered.
HPLC: separating the sample
High-performance liquid chromatography pushes a dissolved sample through a column packed with fine particles under pressure. Different components travel through at different speeds depending on how strongly they interact with the packing, so they emerge from the far end one after another and pass a detector — for peptides, almost always a UV detector reading around 214 nm or 220 nm, where the peptide backbone absorbs.
The output is a chromatogram: signal against time. Each component is a peak. The area under a peak is proportional to how much of that component passed the detector.
Purity by HPLC is the area of the main peak divided by the total area of all peaks, expressed as a percentage. If the main peak accounts for 99.3% of the total area, the certificate says 99.3%.
That is a real, reproducible, useful measurement. It has three limits worth stating plainly:
- It only counts what the detector sees. Water, counter-ions (such as trifluoroacetate from synthesis), inorganic salts and residual solvent are largely invisible at 214 nm and are not in the denominator.
- It assumes every component absorbs light similarly per unit mass. For peptide impurities that share most of the sequence this is a fair approximation; for something chemically unrelated it may not be.
- It tells you that the main peak is one thing. It does not tell you which thing.
MS: confirming the molecule
Mass spectrometry ionises the sample and measures the mass-to-charge ratio of the ions produced. Peptides typically pick up several charges, so the instrument sees a family of peaks that deconvolute back to a single molecular mass.
That measured mass is compared with the theoretical mass of the compound calculated from its sequence. Agreement to within a small tolerance is the identity check: the main peak is not merely pure, it is the right molecule.
MS also does something HPLC can't: it can put a mass to the minor peaks. A synthetic peptide's impurities are overwhelmingly near-relatives — a sequence missing one residue, a sequence with one extra, an oxidised methionine, a deamidated asparagine, an incompletely removed protecting group. Their masses differ from the parent by predictable amounts, so MS can say what the 0.7% probably is.
Why the pairing matters
Put the two side by side:
| Question | HPLC alone | MS alone | HPLC-MS |
|---|---|---|---|
| How much of the sample is one component? | Yes | No | Yes |
| Is that component the molecule on the label? | No | Yes | Yes |
| What are the minor components? | Retention times only | Masses, but not proportions | Both |
| Is there residual salt, water or solvent? | No | No | No |
The final row is the one people miss. Neither method, nor the two together, measures how much peptide by mass is in the vial. That is a separate determination.
Purity versus net peptide content
A lyophilised peptide is never 100% peptide by weight. Bound water and counter-ions can account for 10–30% of the mass on the balance, sometimes more. A vial can therefore be 99% pure by HPLC and contain substantially less peptide than its gross weight would suggest.
Net peptide content (sometimes "peptide content" or "assay") is the figure that addresses this — usually from amino-acid analysis, nitrogen determination or a quantitative UV method against a reference standard. It is a more expensive measurement, and it is the one most often absent from a vendor's certificate.
So when you read a certificate, hold two separate questions in mind: how pure is the peptide (HPLC), and how much peptide is there (content). A vendor quoting only the first is not lying, but they are answering the easier question.
Reading a chromatogram without an instrument
You don't need to run the method to sanity-check the result. On a certificate that shows the chromatogram:
- There should be a peak table — retention time, area, area percentage — not just a picture.
- The main peak should be sharp and symmetrical. A broad, tailing or split main peak suggests a method problem or a mixture, and a good lab would rerun it.
- The baseline before and after the main peak should be flat. A rising baseline can hide small peaks and inflate the apparent purity.
- Small peaks that elute just before or after the main one are the near-relative impurities described above. That's expected. A large peak far from the main one is a different substance, and should be explained.
What the figure on our product page is
Each Biovanta product page carries a purity figure and a test date. Both are lifted from the manufacturer's HPLC-MS certificate for the batch currently on the shelf, and the certificate is sent on request before an order. The testing page describes the process end to end, including the part we're careful about: the figure on the product page is the manufacturer's QC number; where a batch also has an independent Janoshik report, the testing page links it with a key you can verify on Janoshik's own site, and it shows which batches are still waiting for one. Two laboratories rarely report the identical figure — we explain why in the Janoshik article. As with everything on the site, the products are supplied for laboratory research use only.
Sources
Questions this article answers
Is HPLC purity the same as how much peptide is in the vial?
No. HPLC purity is the share of the detected signal that belongs to the main peak. It doesn't account for water, salts or solvent, which are largely invisible to the detector. The amount of peptide by mass is a separate figure — net peptide content — and is not implied by a purity percentage.
Why do peptide certificates rarely show 100%?
Solid-phase synthesis produces a small population of near-identical sequences alongside the target — deletions, truncations, oxidised or deamidated forms. Purification removes most of them, but a few tenths of a percent typically remain and show up as small peaks near the main one.
Does mass spectrometry measure purity?
Not directly. MS confirms identity by measuring molecular mass and can identify what the minor components are. The proportion of each component comes from the chromatography step, which is why the two methods are run together.
Want the certificate for a real batch?
Name a compound and we'll send the manufacturer's HPLC-MS certificate for the batch you would receive — before you order, no account needed.