Setup, so nobody has to ask: a sterility test · dulaglutide.
I want to know what the trade-off actually is rather than which option is fashionable.
I would rather have a defensible reason than a marginal improvement.
What does each option buy me, and what does it cost me?
The underlying point is that reading a content result requires knowing the purity of the standard against which the sample was quantified, because your result inherits that uncertainty.
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.
| 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 |
If the standard and sample have different absorption coefficients at the detection wavelength, the response factors differ and the inference fails.
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 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.
edited 19 Mar 2025 by kwn_analytical — updated for the 2026 guidance change
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsStart from what you are trying to know — whether a vial contains what the label claims — and purity does not answer that question.
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.
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.
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 a supplier gives you content without the standard's purity, ask them to provide it.
It helps to be literal here: 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.
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.
The part that matters: 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 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.
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.
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.
I would not rely on a content figure from a lab that is not willing to state their standard's purity on request.
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.
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.
The purity of the reference standard is stated on its certificate, and your content figure is only as good as that purity certificate is.
Pharmacopoeial guidance on quantitative methods specifies validation steps for linearity, range, accuracy and precision that most research-grade work does not claim to meet.
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.