For reference: a sterility test · dulaglutide.
This is a planning question. I know what my options are; I do not know how to weigh them.
What I want is the minimum viable version, which I suspect is smaller than what I would design.
What does a sensible plan look like, and what are the decision points?
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.
The single most common reason for disagreement between a supplier content figure and an independent assay is using different standards.
| 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 |
More usefully, 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.
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.
The practical summary: if you are ordering from a new supplier, budget for content assay on the first lot.
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsOn the detail: if a supplier quotes a content figure without stating the purity of their reference standard, they have not told you the uncertainty in the number.
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.
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 a supplier gives you content without the standard's purity, ask them to provide it.
Specifically, reading a content result requires knowing the purity of the standard against which the sample was quantified, because your result inherits that uncertainty.
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.
If the standard and sample have different absorption coefficients at the detection wavelength, the response factors differ and the inference fails.
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 honest answer is that you cannot know for certain what you have without a content assay, and the purity number alone is not enough.
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.
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 underlying point is that content assay requires a reference standard of known purity and traceability, which is why it costs more than purity testing does.
The purity of the reference standard is stated on its certificate, and your content figure is only as good as that purity certificate is.
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 28 Dec 2025 by tare_weight — tightened the wording; no substantive change
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.