How much of the gross mass of a 10 mg liraglutide vial is actually peptide?

The case in front of me: 10 mg · liraglutide.

I want the working, not the result — I need to be able to redo it with different numbers.

I care about the precision as well as the value — I want to know how many figures are real.

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

On a 10 mg vial every percentage point of water and counter-ion is 0.1 mg, so the answer is a multiplication once you have the net peptide content off the certificate. Settle first whether the 10 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 10 mg gross fill holds 8.5 mg of peptide; at 78 per cent it holds 7.8 mg. That 0.7 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.

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

Specifically, 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.

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.

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

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

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.

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

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

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.

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 underlying point is that 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.

Pharmacopoeial guidance on quantitative methods specifies validation steps for linearity, range, accuracy and precision that most research-grade work does not claim to meet.

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.

Put another way, 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.

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.

Quantitation against a standard requires that the standard be traceable to a national metrology institute, and certificates for research-grade standards claim that traceability.

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.