Coming back to: Purity assay versus content assay: two different numbers posts 31–60

An itemised related-substances table with retention times says considerably more about the synthesis than a single total. A total tells you how much is not the main peak and nothing about what it is.

That distinction has done more work for me than anything else in this category.

The number people quote for Purity assay versus content assay is a central estimate presented without its interval, and the interval is wide enough that the estimate is nearly uninformative on its own.

Bookmarking this. I will come back when I have something worth adding.

Worth separating two things that post #32 runs together.

Purity and content are different measurements: purity tells you what proportion of the detected material is the intended species. Content tells you how much of the intended species is present in the container. A lyophilised vial can be 99% pure and contain considerably less than the label claims because the remainder is water, counter-ion and excipient.

The disagreement above is smaller than it looks once the terms are fixed.

Whatever the answer on Purity assay versus content assay turns out to be, the method for getting there is the same: state the assumption, do the arithmetic in public, invite the correction.

Counter-ion form is crucial: peptides are isolated as salts, most often trifluoroacetate from reversed-phase purification or acetate after salt exchange. The counter-ion is part of the mass in the vial and is not part of the peptide. Two vials of the same purity, one as TFA and one as acetate, contain different amounts of peptide per milligram.

Picking up post #34: that is the part I would want checked first.

Where two documents for the same lot disagree, the useful question is which was measured and which was transcribed. Most such disagreements are the second.

I am confident about the direction and much less about the magnitude.

That matches what I have seen, for whatever a single anecdote is worth.

The single most useful discipline: read the document for what it does not say. The omissions are more consistent between suppliers than the numbers are.

I had written a reply contradicting post #36 and deleted it. Here is what survived.

Amino acid analysis gives content: it hydrolyses the peptide and quantifies the residues. It measures the peptide content independent of chromatographic purity. Very few supplier certificates report it.

Answering the question post #37 raises rather than the one it answers.

I changed my mind about Purity assay versus content assay after someone here asked me for the source and I could not produce one. That is worth saying out loud because it is the ordinary way it happens.

The arithmetic in post #41 is right; the assumption feeding it is the part to check.

Two claims get bundled together under Purity assay versus content assay and they need separating. The descriptive one — this is what was observed — is usually well supported. The causal one — this is why — usually is not.

Almost every disagreement in threads like this one dissolves once you say which of the two you are making.

Purity and content are different measurements: purity tells you what proportion of the detected material is the intended species. Content tells you how much of the intended species is present in the container. A lyophilised vial can be 99% pure and contain considerably less than the label claims because the remainder is water, counter-ion and excipient.

Second-hand, so weight it accordingly.

Counter-ion form belongs on the document and almost never is. Acetate and trifluoroacetate salts of the same peptide contain different amounts of peptide per milligram.

Scoping that to what I have actually seen rather than what I have read.