What I am working with: 5 mg · mazdutide.
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.
How many significant figures are actually justified here?
On a 5 mg vial every percentage point of water and counter-ion is 0.05 mg, so the answer is a multiplication once you have the net peptide content off the certificate. Settle first whether the 5 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 5 mg gross fill holds 4.25 mg of peptide; at 78 per cent it holds 3.9 mg. That 0.35 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.
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.
If the standard and sample have different absorption coefficients at the detection wavelength, the response factors differ and the inference fails.
| Test | Answers | Does NOT answer |
|---|---|---|
| RP-HPLC, area % | What fraction of detected material is the target | How much target is present |
| Quantified content | Milligrams of peptide per vial | What the impurities are |
| ESI-MS identity | Whether the molecular weight matches | Purity, or isomeric substitution |
| Peptide mapping | Sequence, localised to a fragment | Quantity |
| Karl Fischer | Water content of the solid | Solvent content |
| LAL endotoxin | Pyrogen load in EU/mg | Sterility |
| Sterility test | Growth in defined media over 14 days | Endotoxin, or bioburden count |
The underlying point is that the purity of the reference standard is stated on its certificate, and your content figure is only as good as that purity certificate is.
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.
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.
Ask for both the purity and the content, and do not accept purity alone.
edited 10 Jun 2024 by birk_nordahl — clarified the distinction between purity and content
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsThe short answer is that purity says "what fraction of detected material is the target" while content says "how many milligrams of the target are present," and those are two different things.
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.
In practice, 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 limitation is that a quantitative method is only as good as the standard it uses, and a cheap standard is a false economy.
If a supplier gives you content without the standard's purity, ask them to provide it.
edited 10 Jun 2024 by Dr_Bram_Verhoeven — added the citation requested in comments
To be exact about it, most research-grade certificates report purity without content, which is exactly backwards from what users actually need.
Comparing content results from different laboratories requires knowing whether they both used certified reference materials or whether one used an in-house standard of unknown provenance.
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.
Pharmacopoeial guidance on quantitative methods specifies validation steps for linearity, range, accuracy and precision that most research-grade work does not claim to meet.
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.
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.
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 relative standard deviation on replicate quantitations of a homogeneous sample should be below two per cent when the method is under control.
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.
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.
Quantitation against a standard requires that the standard be traceable to a national metrology institute, and certificates for research-grade standards claim that traceability.
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.