The case in front of me: peptide mapping · survodutide.
I would like to define my thresholds before I have a result, for obvious reasons.
I want a plan with explicit stopping rules, not just steps.
What is the minimum version of this that is still defensible?
The underlying point is that 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.
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.
The purity of the reference standard is stated on its certificate, and your content figure is only as good as that purity certificate is.
The caveat is that content assay costs more than purity, so most people do not do it, which is exactly why it is valuable on the first lot from a new supplier.
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 BiochemA 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.
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.
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.
Worth noting that the standard certificate carries its own uncertainty, usually on the order of two to three per cent, which the sample result inherits.
If a supplier gives you content without the standard's purity, ask them to provide it.
Start from what you are trying to know — whether a vial contains what the label claims — and purity does not answer that question.
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.
In practice, the single most common reason for disagreement between a supplier content figure and an independent assay is using different standards.
Where content data have been published from testing services on common peptides, the spread between services on identical material is typically a few per cent.
Ask for both the purity and the content, and do not accept purity alone.
The part that matters: reading a content result requires knowing the purity of the standard against which the sample was quantified, because your result inherits that uncertainty.
For peptides at 214 nanometres the response is roughly proportional to the number of peptide bonds, so truncation impurities have lower response factors and overestimate content.
Pharmacopoeial guidance on quantitative methods specifies validation steps for linearity, range, accuracy and precision that most research-grade work does not claim to meet.
One qualification: a single result from a single vial is a point estimate, and repeating the assay on a second aliquot is worth doing if the first result is surprising.
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.
edited 11 Aug 2026 by tandem_gradient — updated for the 2026 guidance change
The honest answer is that you cannot know for certain what you have without a content assay, and the purity number alone is not enough.
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.
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 limitation is that a quantitative method is only as good as the standard it uses, and a cheap standard is a false economy.
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.
edited 8 Aug 2026 by nine_point_nine — reworded for clarity after a comment
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.