How much of the gross mass of a 5 mg a GLP-1 receptor agonist vial is actually peptide?

Stated plainly: 5 mg · a GLP-1 receptor agonist.

The units are where I keep going wrong, so please be explicit about them.

I have sanity-checked the order of magnitude and it seems right, which is not the same as being right.

Where is my error, and what is the correct working?

The relevant detail is that 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.

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.

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 limitation is that a quantitative method is only as good as the standard it uses, and a cheap standard is a false economy.

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

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

The single most common reason for disagreement between a supplier content figure and an independent assay is using different standards.

To be exact about it, 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.

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.

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

In practice, 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.

The underlying point is that 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.

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