How much of the gross mass of a 20 mg a GLP-1 receptor agonist vial is actually peptide?

Setup, so nobody has to ask: 20 mg · a GLP-1 receptor agonist.

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?

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.

More usefully, the purity of the reference standard is stated on its certificate, and your content figure is only as good as that purity certificate is.

I would not rely on a content figure from a lab that is not willing to state their standard's purity on request.

The practical summary: if you are ordering from a new supplier, budget for content assay on the first lot.

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.

Mechanically, 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.

Ask for both the purity and the content, and do not accept purity alone.

edited 6 Oct 2024 by Dr_Yusuf_Adeyemi — fixed an arithmetic slip in the third paragraph

Put another way, most research-grade certificates report purity without content, which is exactly backwards from what users actually need.

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.

On the detail: 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.

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.

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.