The particulars: an endotoxin test · cagrilintide.
Both of these get recommended confidently by different people, which suggests neither is obviously right.
My constraints are cost, measurement resolution and how much handling I am prepared to do — in roughly that order.
Is there a defensible reason to prefer one, or is this a coin flip?
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.
If the standard and sample have different absorption coefficients at the detection wavelength, the response factors differ and the inference fails.
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.
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.
If a supplier gives you content without the standard's purity, ask them to provide it.
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsThe 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.
The single most common reason for disagreement between a supplier content figure and an independent assay is using different standards.
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.
Pharmacopoeial guidance on quantitative methods specifies validation steps for linearity, range, accuracy and precision that most research-grade work does not claim to meet.
The practical summary: if you are ordering from a new supplier, budget for content assay on the first lot.
edited 27 Aug 2025 by Dr_Ingrid_Baumgartner — updated for the 2026 guidance change
More usefully, reading a content result requires knowing the purity of the standard against which the sample was quantified, because your result inherits that uncertainty.
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.
Worth being precise here: 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.
The relative standard deviation on replicate quantitations of a homogeneous sample should be below two per cent when the method is under control.
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.
Ask for both the purity and the content, and do not accept purity alone.
Mechanically, content assay requires a reference standard of known purity and traceability, which is why it costs more than purity testing does.
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.
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.
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.
To 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.
The ICH Q3A and Q3B thresholds for reporting, identification and qualification of impurities are the framework the pharmaceutical industry works to, and they are worth reading even though nothing in the research-grade supply chain is obliged to meet them, because they tell you which numbers a competent analyst would consider worth reporting at all.
I would not rely on a content figure from a lab that is not willing to state their standard's purity on request.
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.