On the subject in the title: Adsorption to container surfaces and to plastics Working notes rather than a conclusion.
I was wrong about adsorption in a thread last spring and I would like to correct it publicly rather than quietly.
The error was in the units, which changed the conclusion by an order of magnitude. Setting out the corrected version, and the way I now check for that class of mistake.
The opening post is the version of this I will quote in future. One addition.
Marking my uncertainty on adsorption explicitly. I am confident about the direction, much less confident about the size, and not confident at all that it generalises past the case in the first post.
Where I part company with the opening post, and it is a narrow parting.
If someone has run adsorption properly I would rather read that than my own reconstruction of it. Posting mine only because the thread has gone quiet.
Narrowing post #3, because the general version has more than one answer.
Benzyl alcohol as a preservative is also a solubiliser for some peptides, which is an incidental property that occasionally matters.
What I would want before treating adsorption as settled: the method, the sample, and whether anyone tried to find the opposite result. Two of the three are usually missing.
Reading back through the adsorption threads from last year, the same three questions come up every time and only one of them has ever been answered properly. That seems like a documentation gap rather than a knowledge gap.
Post #4 put the caveat in the right place and I want to underline it.
Emulsification and suspensions: some formulations are emulsions or suspensions rather than solutions. Semaglutide formulations are solutions; some other compounds are suspensions. The physical form affects stability and administration.
I have no financial interest in anything named in this thread and I want to say so before I comment on adsorption, because it is the sort of subject where it matters.
On post #8 — agreed on the reasoning, with one qualification.
Practical answer on adsorption, since the theoretical one is upthread: do the simplest check first, write down the result, and only then decide whether the complicated explanation is needed. It usually is not.
Excipients and their purposes: mannitol, sucrose, or other sugars in lyophilised formulations protect the peptide during drying and storage. They are not impurities; they are functional components.
The confident version of this sentence would be wrong, so here is the hedged one.
Surfactants such as polysorbate are added to prevent adsorption and aggregation at interfaces. In a bare research preparation there is nothing performing that function.
Narrowing post #11, because the general version has more than one answer.
I read the earlier replies on adsorption twice before writing this, because I had assumed the opposite and wanted to be sure I was disagreeing with what was said rather than what I expected.
Saving this. It is the version I will quote when the question comes round again.
I read post #16 twice before replying, because I had assumed the opposite.
Counter-ion exchange from trifluoroacetate to acetate is a purification step with a cost, and material sold without it retains TFA. That affects mass balance and can affect biological assays.
Post #16 answers the question as asked. The question underneath it is different.
Antioxidants: compounds that prevent oxidation (which damages peptides) are sometimes included. Methionine oxidation is a common degradation pathway and antioxidants help prevent it.
That is what the documentation says. What happens in practice is usually close.
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Coming back to: Solubility limits and what a cloudy solution is telling you
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Coming back to: Adsorption to container surfaces and to plastics
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