Counter-ion form: the field almost nobody reports — one year on posts 121–140
This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
Coming back to post #121, because the follow-up matters more than the original answer.
The most useful thing anyone has posted about Counter-ion form in this category was a table of what had been measured and by whom. That is what I would want again.
A note on scope: what I am saying about Counter-ion form applies to the case in the first post and I would not extend it further without checking.
Peptide content and chromatographic purity are separate determinations and a certificate that reports only one has answered only one question. Content is the one that tells you how much is in the vial.
Collapsed as off-topic by two members at trust level 3 or above
Taking post #125 at face value and following it one step further.
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.
What would change my mind on Counter-ion form is a second dataset collected by someone with no stake in the first. Until then I hold it loosely and I would rather say so than pretend to more.
That is the distinction I keep failing to hold on to. Written down now.
Post #125 describes the usual case. This is about the unusual one.
Before the thread moves on from Counter-ion form — what is the sample size behind the claim? I am not being difficult; I have seen the same figure quoted from an n of four and from an n of four hundred.
Adding the measurement that post #129 says would settle it.
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.
I keep a log of this specifically because memory is unreliable about it.
Post #130 describes the usual case. This is about the unusual one.
The distinction between release testing and characterisation is one industrial documents make and retail ones usually do not. It is the difference between "we tested every lot for this" and "we established this once".
Adding the caveat now so it does not have to be extracted later.
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.
Bookmarking this. I will come back when I have something worth adding.
On post #134 — agreed on the reasoning, with one qualification.
Two questions I would want answered before drawing anything from the Counter-ion form data above: how were the cases selected, and what happened to the ones that dropped out.
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.
Narrowing post #136, because the general version has more than one answer.
The practical version of Counter-ion form is three sentences long. The rigorous version is three pages and reaches the same conclusion with the conditions attached.
This topic was referenced in
- Reused chromatograms across lots: how to spot itAnalytics › COA interpretation · 60 replies
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