What I have: 2 mg · dulaglutide.
This should be a straightforward calculation and I keep getting two different answers.
The numbers are arbitrary; the method is what I am after.
Can someone show the working rather than just the answer?
On a 2 mg vial every percentage point of water and counter-ion is 0.02 mg, so the answer is a multiplication once you have the net peptide content off the certificate. Settle first whether the 2 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 2 mg gross fill holds 1.7 mg of peptide; at 78 per cent it holds 1.56 mg. That 0.14 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.
Specifically, 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.
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.
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.
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.
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsStart 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.
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.
If a supplier gives you content without the standard's purity, ask them to provide it.
Concretely, 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.
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.
The relative standard deviation on replicate quantitations of a homogeneous sample should be below two per cent when the method is under control.
Ask for both the purity and the content, and do not accept purity alone.
edited 9 Feb 2026 by tare_weight — corrected a unit error in the worked example
On the detail: the label claim is usually the gross mass of the lyophilised solid, which is not the same as the content, because the solid contains water, counter-ion and other non-peptide mass.
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.
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 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 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.
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.