This is the sort of exchange that makes the archive worth searching.
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Analytics · Impurities & related substances · continued
Aggregates and why a purity assay may miss them entirely posts 31–60
This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
PharmNotes_Whitfield, post #18: The honest reading of a purity figure: it is an upper bound on how much of what the method could see was the intended species, under one integration convention, on one sample. Caveat: everything above assumes the paperwork is what it says it is. Go to post
Building on post #32 rather than restating it.
Aggregates may be a multiple of the monomer mass and may not elute at all under a standard method. What does not come off the column does not appear in the area percentage.
Noting that I have skin in this question and have tried to discount for it.
Post #32 put the caveat in the right place and I want to underline it.
The strongest argument against my own position on aggregates, stated as well as I can state it, since nobody else has yet.
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Where the aggregates reasoning breaks down for me is the step from the group result to the individual case. That step is almost never argued for.
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Trifluoroacetate is a counter-ion rather than an impurity, and it appears in the mass balance rather than in the chromatogram. Conflating the two accounts for several confused threads here.
Not the whole picture, but the part of it I can speak to.
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dr_okonkwo, post #16: Coming back to post #12, because the follow-up matters more than the original answer. The number people quote for aggregates is a central estimate presented without its interval, and the interval is wide enough that the estimate is nearly uninformative on its own. Go to post
A supplier that can tell you what its principal impurity is has answered a harder question than one that can tell you its purity, and the answer is more useful.
Speaking for myself and not for anyone else who has posted here.
Collapsed as off-topic by two members at trust level 3 or above
Where I part company with post #34, and it is a narrow parting.
Residual solvents: traces of solvents used in purification. These are usually tested by gas chromatography, not by HPLC. A specification for residual solvents should be stated separately from the purity.
The variance between people here is larger than the effect being discussed.
Adding the measurement that post #36 says would settle it.
That last point is the ceiling on what any purity figure can claim. A method that cannot see a species cannot exclude it, and no certificate says which species its method cannot see.
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Post #39 describes the usual case. This is about the unusual one.
On aggregates, the part that usually goes wrong is that the question is asked as though it has one answer. It has a range, and the width of the range is the interesting bit.
If you can post the two or three numbers you are working from, several people here will check the arithmetic rather than argue about the conclusion.
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Two claims get bundled together under aggregates 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.
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Post #39 answers the question as asked. The question underneath it is different.
The related-substances total is only as informative as the method that generated it. Two per cent by a method that resolves everything is a different statement from two per cent by a method that resolves little.
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Fairweather, post #10: Off-target structures: if the sequence synthesis goes wrong, a completely different amino acid can be incorporated. The resulting off-target peptide is a structural isomer with the same mass but a different sequence. No chromatographic purity method detects this without a reference standard. Go to post
The most useful thing anyone has posted about aggregates in this category was a table of what had been measured and by whom. That is what I would want again.
Post #43 is right about the mechanism and I think understates the practical bit.
An impurity profile that changes between lots is more informative than the total. A stable profile suggests a controlled process; a shifting one suggests something is varying.
Not the answer, but possibly the question that gets there.
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n.szabo, post #26: Picking up post #24: that is the part I would want checked first. My understanding of aggregates is a few years old and may have been superseded. If it has been, I would genuinely like to know rather than keep repeating it. Go to post
What would change my mind on aggregates 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.
dr_okonkwo, post #16: Coming back to post #12, because the follow-up matters more than the original answer. The number people quote for aggregates is a central estimate presented without its interval, and the interval is wide enough that the estimate is nearly uninformative on its own. Go to post
I changed my mind about aggregates 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.
Answering the question post #47 raises rather than the one it answers.
Truncation products: fragments from incomplete synthesis or from degradation. They elute quite differently from the intact peptide because they are much smaller and have different hydrophobicity. They are usually well separated.
Posting it because the silence on this was starting to look like agreement.
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Aggregates: multiples of the monomer mass. May not elute at all under a standard reversed-phase method. A species that does not come off the column does not appear in the area percentage.
Second-hand, so weight it accordingly.
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Collapsed as off-topic by two members at trust level 3 or above
I have no financial interest in anything named in this thread and I want to say so before I comment on aggregates, because it is the sort of subject where it matters.
v.milanovi, post #8: Narrowing post #5, because the general version has more than one answer. Relative response factors mean impurities are not detected in proportion to how much of them is present. A one per cent peak is not one per cent by mass unless the response factors happen to match. I would treat that as a working assumption and revisit it. Go to post
This follows post #50 rather than contradicting it.
Racemisation produces a diastereomer that is mass-identical and chromatographically resolvable only on a method chosen for the purpose. Standard reversed-phase frequently will not separate it.
Happy to be corrected if someone holds better data than mine.
slow_titrator, post #20: Answering the question post #16 raises rather than the one it answers. A definition problem is doing most of the work in this aggregates discussion. Once the term is pinned down I suspect the disagreement mostly goes away and what is left is small. Go to post
The honest reading of a purity figure: it is an upper bound on how much of what the method could see was the intended species, under one integration convention, on one sample.
That is the practical version. The rigorous version is longer and says the same thing.
Adding a reference point for aggregates. Mine is a single case, collected without controls, and I am posting the method alongside it so it can be discounted appropriately.
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Everything in post #53 holds. The case it does not cover is the one I have.
Reporting thresholds matter: below a stated threshold, peaks are usually not reported at all. A clean-looking table may reflect a high threshold rather than a clean synthesis.
A guess, clearly labelled as one.
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Residual solvents: traces of solvents used in purification. These are usually tested by gas chromatography, not by HPLC. A specification for residual solvents should be stated separately from the purity.
For what it is worth, the same held on the two occasions I checked.
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v.klausen, post #27: Scavengers and cleavage-cocktail residues can persist and appear as small early-eluting peaks. They are process-related rather than sequence-related, which is a useful distinction when reading a profile. Go to post
Saving this. It is the version I will quote when the question comes round again.
The arithmetic in post #57 is right; the assumption feeding it is the part to check.
Off-target structures: if the sequence synthesis goes wrong, a completely different amino acid can be incorporated. The resulting off-target peptide is a structural isomer with the same mass but a different sequence. No chromatographic purity method detects this without a reference standard.
Posted with less confidence than the sentence structure implies.
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Answering the question post #56 raises rather than the one it answers.
Comparing impurity profiles across suppliers is much more informative than comparing purity figures, and almost nobody has the documents to do it.
The short version is the first sentence; the rest is why.
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