On a COA, what is the difference between "peptide content", "net peptide" and "gross weight"?

I have a COA in front of me with all of the following on it, and I cannot work out which of these numbers is the one I care about:

• Appearance: white lyophilised powder
• Purity (HPLC): 99.34%
• Peptide content: 84.2%
• Net peptide: 8.42 mg
• Total weight: 10.00 mg
• Water content: 7.1%
• Acetate: 8.3%

The label on the vial says 10 mg. The COA says total weight 10.00 mg. So on the face of it the vendor has delivered what they said. But net peptide is 8.42 mg and purity is 99.34%, which would make the actual semaglutide 8.36 mg, which is 84% of what I assumed I was buying.

Is this vendor being honest and I misunderstood what "10 mg" means, or is this a standard way of dressing up a 16% shortfall? Genuinely unsure which, because unlike most COAs I have seen this one gives me every number I would need, which does not feel like the behaviour of someone hiding something.

This vendor is being technically honest and commercially misleading, and the COA is unusually good. Both things are true at once. Here are the definitions, then the judgement.

The four quantities

Gross weight (also "total weight", "fill weight"): the mass of everything in the vial as weighed. Peptide plus water plus counter-ion plus residual reagents. This is what a filling line actually controls, because it is what a balance reads.

Peptide content (%): the mass fraction of the gross weight that is peptide backbone — that is, gross minus water, counter-ion and inorganic residue, expressed as a percentage of gross. Your COA says 84.2%. This is the standard pharmacopoeial concept and it is a property of the powder, not of the fill.

Net peptide (mg): gross weight multiplied by peptide content. 10.00 x 0.842 = 8.42 mg. This is the mass of peptide material in the vial, but it includes the related substances — the deletion sequences, the aspartimide, the oxidised forms. They are peptide; they are just not the peptide you want.

Purity (area %): the fraction of the peptide material that is the target molecule. 99.34%.

So the mass of the actual target is 8.42 x 0.9934 = 8.36 mg, exactly as you calculated. Your arithmetic is right.

Which number do you care about?

8.36 mg. That is the only number with physical meaning for anything you would do with the vial. Everything else is an intermediate.

Is 10 mg on the label defensible?

Here is the honest state of the field. In pharmaceutical practice, a label claim on a peptide drug product refers to the active — the anhydrous, salt-free peptide. A vial labelled 10 mg is expected to deliver 10 mg of peptide, and the manufacturer fills to whatever gross mass achieves that after correcting for assay, water and counter-ion. That correction is called overage and it is routine and documented.

In the research-chemical trade, a substantial fraction of suppliers label on gross weight, because gross weight is what they weighed. There is no regulation compelling either convention. So "10 mg" is genuinely ambiguous and the only way to resolve it is to look at whether the COA reports content.

This vendor reported it. They told you, in the document, that the vial holds 8.42 mg of peptide of which 99.34% is target. They did not hide anything. They also chose the labelling convention that makes their vial look 20% bigger than it is, and they know which convention the buyer assumes. That is a marketing decision, not a fraud.

How to use this going forward

Ask one question before ordering: is the label claim gross fill weight or net peptide? The answer sorts vendors into three groups.

• Answers "net peptide, we fill with overage" and can show a COA with content on it: you can compare their price per mg against anyone else's directly.
• Answers "gross weight" plainly: honest, and you multiply their price per mg by about 1.18 to compare against the first group. On a 10 mg vial at 84.2% content and 99.3% purity, you are paying for 8.36 mg, so the effective price is 1 / 0.836 = 1.196 times the headline.
• Does not understand the question, or answers "our purity is 99%": no information available, and the purity answer specifically means they have conflated the two, which is the failure mode that produces genuinely bad vials.

A COA like the one you posted, with water, acetate, content and purity all present and internally consistent, is in the top few percent of documents in this market. Compare it to the usual offering, which is a purity number and a mass spectrum. I would rather buy an 8.36 mg vial from someone who told me it was 8.36 mg than a "10 mg" vial from someone who has no idea.

A wrinkle on the definitions that matters if you ever compare COAs from a Chinese synthesis house against one written to a Western pharmacopoeial template.

"Peptide content" is not measured the same way everywhere. There are two routes and they give different numbers on the same powder:

1. Mass balance / subtraction. Measure water (Karl Fischer), counter-ion (ion chromatography), residual solvents (GC headspace) and inorganic residue (residue on ignition), and subtract all of it from 100%. This is what most COAs mean. It is cheap, it is indirect, and it inherits the error of four separate assays. It also silently attributes anything you did not measure to the peptide.
2. Direct quantitation against a reference standard. Dissolve a known mass, inject, compare peak area against a standard of known assigned purity. This gives you peptide content directly and its accuracy is limited by the standard.

Route 1 reads high when there is an unmeasured non-peptide component — an excipient, a residual coupling reagent, a silica fine from a column. Route 2 reads whatever the standard tells it to read, which is a different problem.

Note also that some COAs use "peptide content" to mean something else entirely: the fraction of the gross mass that is target peptide, i.e. content multiplied by purity. On your COA that would be 83.6% rather than 84.2%. The difference is small here because purity is high, but on a 92%-pure batch the two definitions differ by seven points and a careless reader loses track of a percentage they already accounted for.

So when you see a content figure, the question after "what does it mean" is "how was it obtained". A COA that says "peptide content 84.2% (by mass balance)" or "(by HPLC against USP reference standard)" is telling you something. One that just says 84.2% is telling you a number.

Reading the same COA slightly less charitably than the answer above, on one specific point: the water content.

7.1% water is at the upper end of normal, and combined with 8.3% acetate it gives a total non-peptide burden of 15.4%, which the stated 84.2% content matches (100 - 15.4 = 84.6, close enough that they presumably also counted a small residue term). Internally consistent. Fine.

But high residual water is not just a mass accounting issue, it is a stability prediction. Water is the reactant in the two dominant solid-state degradation pathways for these peptides: hydrolysis of the C-terminal amide and of labile Asp-Xaa bonds, and it also mobilises the amorphous cake enough to permit aspartimide chemistry to keep running slowly at storage temperature. Empirically, lyophilised peptides held above about 5% water degrade measurably faster than the same peptide at 2 to 3%.

The practical implication: a batch at 7.1% water will not hold its purity as long as a batch at 3%, all else equal. If you are buying to hold for a year, water content is a number worth caring about in its own right and not just as a subtraction term. And if a vendor's COA reports water at manufacture and you buy the vial eight months later, the current water content is higher than the COA says, because the stopper is not a perfect barrier.

None of that makes your vial bad. It makes it a vial to use sooner rather than later, stored cold and dry, and it is a reason to prefer the batch with the lower water figure when you have a choice.