What I have: 2 mg · orforglipron.
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.
Start from what you are trying to know — whether a vial contains what the label claims — and purity does not answer that question.
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 single most common reason for disagreement between a supplier content figure and an independent assay is using different standards.
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 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.
edited 31 Jul 2024 by Dr_Otto_Lindqvist — expanded the table to cover the lower concentration
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsContent assay requires a reference standard of known purity and traceability, which is why it costs more than purity testing does.
If the standard and sample have different absorption coefficients at the detection wavelength, the response factors differ and the inference fails.
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.
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.
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.
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.
The practical summary: if you are ordering from a new supplier, budget for content assay on the first lot.
A high purity does not guarantee high content because it says nothing about how much of anything is in the vial — a vial could be 99 per cent pure but only half full.
System suitability for a quantitative method is stricter than for purity because a small systematic error in the standard directly translates into an error in the sample result.
The relative standard deviation on replicate quantitations of a homogeneous sample should be below two per cent when the method is under control.
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.
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.
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.
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.
In practice: ask for the chromatogram, check the method section, check the lot number against the vial, and set your accept threshold before you see the result rather than after.
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.