For reference: an HPLC purity test · retatrutide.
I would rather over-plan the first cycle and simplify later.
I am prepared to do the work if someone can tell me which work matters.
What does a sensible plan look like, and what are the decision points?
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.
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 a sample shows high purity but low content, the explanation is usually that the standard used for quantitation had a different purity than claimed.
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.
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsContent assay requires a reference standard of known purity and traceability, which is why it costs more than purity testing does.
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.
Mechanically, a content assay is always paired with a purity assay because purity tells you what fraction of the measured mass is the target and content tells you the total measured mass.
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.
Ask for both the purity and the content, and do not accept purity alone.
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.
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.
The purity of the reference standard is stated on its certificate, and your content figure is only as good as that purity certificate is.
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.
I would not rely on a content figure from a lab that is not willing to state their standard's purity on request.
If a supplier gives you content without the standard's purity, ask them to provide it.
The short answer is that purity says "what fraction of detected material is the target" while content says "how many milligrams of the target are present," and those are two different things.
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.
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 20 Jan 2026 by petra_hovland — added the method parameters
Start from what you are trying to know — whether a vial contains what the label claims — and purity does not answer that question.
System suitability for a quantitative method is stricter than for purity because a small systematic error in the standard directly translates into an error in the sample result.
The relative standard deviation on replicate quantitations of a homogeneous sample should be below two per cent when the method is under control.
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.
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.