Setup, so nobody has to ask: 60 mg · cagrilintide.
I have worked this out and I would like someone to find the error, because I suspect there is one.
My working so far, for the record, is below, and I am fairly sure the error is in the unit conversion rather than the algebra.
How many significant figures are actually justified here?
On a 60 mg vial every percentage point of water and counter-ion is 0.6 mg, so the answer is a multiplication once you have the net peptide content off the certificate. Settle first whether the 60 mg on the label is gross fill or net peptide, because the two differ by exactly the quantity being asked about. If the certificate quotes net peptide content of 85 per cent, a 60 mg gross fill holds 51 mg of peptide; at 78 per cent it holds 46.8 mg. That 4.2 mg gap is 7 per cent of the label, larger than any purity difference anybody argues about, and it is invisible to a purity figure because water and acetate are not impurity peaks. Amino acid analysis or a nitrogen determination gives you the number; an HPLC area per cent never will.
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.
Worth being precise here: 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.
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.
Ask for both the purity and the content, and do not accept purity alone.
edited 14 Mar 2026 by kwn_analytical — tightened the wording; no substantive change
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 BiochemMechanically, reading a content result requires knowing the purity of the standard against which the sample was quantified, because your result inherits that uncertainty.
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 relevant detail is that 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 practical summary: if you are ordering from a new supplier, budget for content assay on the first lot.
The relevant detail is that quantified content is how many milligrams of peptide are actually in the vial, measured against a calibrated reference standard, and it is the only measurement that directly answers that question.
The single most common reason for disagreement between a supplier content figure and an independent assay is using different standards.
Specifically, 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.
Pharmacopoeial guidance on quantitative methods specifies validation steps for linearity, range, accuracy and precision that most research-grade work does not claim to meet.
If a supplier gives you content without the standard's purity, ask them to provide it.
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.
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 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.
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.
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.
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.
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.