Numbers first: 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.
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 BiochemContent 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.
On the detail: 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.
The practical summary: if you are ordering from a new supplier, budget for content assay on the first lot.
edited 20 Jan 2026 by k_szabo — added the method parameters
It helps to be literal here: 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.
Ask for both the purity and the content, and do not accept purity alone.
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.