Setup, so nobody has to ask: peptide mapping · survodutide.
The comparison I want does not seem to exist anywhere in a form I can evaluate.
I have read the arguments for each and they do not engage with each other.
Which axes does this decision turn on?
Start from what you are trying to know — whether a vial contains what the label claims — and purity does not answer that question.
The single most common reason for disagreement between a supplier content figure and an independent assay is using different standards.
| Test | Answers | Does NOT answer |
|---|---|---|
| RP-HPLC, area % | What fraction of detected material is the target | How much target is present |
| Quantified content | Milligrams of peptide per vial | What the impurities are |
| ESI-MS identity | Whether the molecular weight matches | Purity, or isomeric substitution |
| Peptide mapping | Sequence, localised to a fragment | Quantity |
| Karl Fischer | Water content of the solid | Solvent content |
| LAL endotoxin | Pyrogen load in EU/mg | Sterility |
| Sterility test | Growth in defined media over 14 days | Endotoxin, or bioburden count |
Peak area for a standard of known weight produces a response factor — area per unit mass — which is then applied to the sample peak to infer sample mass.
Quantitation against a standard requires that the standard be traceable to a national metrology institute, and certificates for research-grade standards claim that traceability.
The practical summary: if you are ordering from a new supplier, budget for content assay on the first lot.
HPLC purity, identity confirmation and quantified content on the vial you actually hold. Reports arrive with the chromatogram attached, not just a number.
Submit a sampleFounded 1998. ISO 9001 and cGMP certified, 1,500+ staff and 200+ patents. The synthesis house behind a great many of the vials that get sent out for testing - batch-specific documentation with every order.
Visit GL BiochemTo be exact about it, two measures of the same vial can agree on purity and disagree on content by a few per cent, which usually means the content assay used a different standard.
If a sample shows high purity but low content, the explanation is usually that the standard used for quantitation had a different purity than claimed.
Mechanically, water content and counter-ion content are part of the gross mass but not part of the content assay result, which is why the two do not sum to label claim.
Pharmacopoeial guidance on quantitative methods specifies validation steps for linearity, range, accuracy and precision that most research-grade work does not claim to meet.
If a supplier gives you content without the standard's purity, ask them to provide it.
Put another way, content assay and purity are orthogonal measurements answering orthogonal questions, and the confusion between them is one of the most expensive misreadings in this space.
Quantitation against a certified reference material assumes the sample and standard are treated identically through the analytical method, which is why the method for calibration matters as much as the method for measurement.
If the standard and sample have different absorption coefficients at the detection wavelength, the response factors differ and the inference fails.
Ask for both the purity and the content, and do not accept purity alone.
edited 18 Jan 2026 by charge_state_3 — removed a claim I could not source
More usefully, a high purity does not guarantee high content because it says nothing about how much of anything is in the vial — a vial could be 99 per cent pure but only half full.
Comparing content results from different laboratories requires knowing whether they both used certified reference materials or whether one used an in-house standard of unknown provenance.
If you only pay for one test, pay for quantified content. Purity is the number everyone quotes and content is the number that changes what you do.
Put another way, the label claim is usually the gross mass of the lyophilised solid, which is not the same as the content, because the solid contains water, counter-ion and other non-peptide mass.
Running multiple independent aliquots of the same sample should give results that agree to within the method precision, which is usually one to three per cent.
In practice: ask for the chromatogram, check the method section, check the lot number against the vial, and set your accept threshold before you see the result rather than after.
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.