How much of the gross mass of a 5 mg cagrilintide vial is actually peptide?

What I am working with: 5 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.

Is my approach right even if my number is wrong?

Most research-grade certificates report purity without content, which is exactly backwards from what users actually need.

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.

What each test answers

Specifically, if the standard and sample have different absorption coefficients at the detection wavelength, the response factors differ and the inference fails.

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.

Start from what you are trying to know — whether a vial contains what the label claims — and purity does not answer that question.

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.

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.

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

The part that matters: the 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.

Specifically, 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.

The relative standard deviation on replicate quantitations of a homogeneous sample should be below two per cent when the method is under control.

If a supplier gives you content without the standard's purity, ask them to provide it.

edited 26 Jul 2024 by plate_count_9k — added a caveat about sampling

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.

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.

More usefully, 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.

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.