This is a continuation of a long topic, addressed by post number rather than by page. Start at post 1.
Related substances: compounds chemically related to the target peptide but not the target peptide itself. The standard method separates them and reports them as area percent. How related they can be before they exceed specification is a regulatory question.
Coming back to post #90, because the follow-up matters more than the original answer.
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.
This follows post #92 rather than contradicting it.
Where an impurity is identified rather than merely counted, the certificate is telling you the manufacturer has characterised its own process. That is a meaningful difference in documentation quality.
Adding the caveat now so it does not have to be extracted later.
Good question, well framed, and I would like to see it answered properly.
Post #92 answers the question as asked. The question underneath it is different.
Deletion sequences arise from incomplete coupling and are lower by one residue mass. Where they elute depends on the hydrophobicity of the residue that is missing, so they can appear on either side of the main peak.
A weak preference rather than a position.
I read post #95 twice before replying, because I had assumed the opposite.
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 matches what I was told, which is not the same as knowing it.
Acetate content: counter-ion content. Trifluoroacetate or acetate from the salt form of the peptide. Affects mass calculations and should be stated on a complete certificate.
Not the whole picture, but the part of it I can speak to.
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.
On balance I think that is right, and I would not bet much on it.
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.
That is the version I would defend. It is not the version I started with.
Comparing impurity profiles across suppliers is much more informative than comparing purity figures, and almost nobody has the documents to do it.
I have separated what I observed from what I concluded, which does not always happen.
Fair, and the limits you put on it are the part I will remember.
Freeze-drying itself does not generate impurities in any significant way. Handling before and after it does, which is why transit conditions belong in the discussion.
The reasoning is more useful than the number, which is why I have shown it.
Post #102 is the version of this I will quote in future. One addition.
Oxidation at methionine and tryptophan: adds 16 per oxygen. Usually elutes earlier. Oxidation is common in storage, especially if the solution is exposed to light or if antioxidants are not present.
This is the sort of thing that ought to be settled and apparently is not.
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.
Related substances: compounds chemically related to the target peptide but not the target peptide itself. The standard method separates them and reports them as area percent. How related they can be before they exceed specification is a regulatory question.
The short version is the first sentence; the rest is why.
Disulfide formation: if a peptide contains cysteine, it can form disulfide bonds with itself or with other molecules. Under oxidising conditions multiple species appear. Reducing conditions (like DTT) convert them back.
Picking up post #106: that is the part I would want checked first.
Deamidation at asparagine and glutamine: adds 1 approximately. Frequently appears as a close-eluting pair. It is a chemical modification that occurs during storage.
The step people skip is the one I have spelled out.
Incomplete deprotection leaves a protecting group attached, raising the mass substantially and usually pushing retention much later. A late-eluting peak on a peptide chromatogram is worth asking about.
Adding this to the thread rather than to the wiki, because I am not confident enough for the wiki.
Incomplete deprotection: mass higher by the protecting group mass. Usually markedly later eluting. A synthesis artifact from incomplete removal of protecting groups.
Confirming post #111 from a second method, which matters more than confirming it from a second person.
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.
It is one reading of the data and not the only reasonable one.
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.
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.
Posted with less confidence than the sentence structure implies.
Coming back to post #115, because the follow-up matters more than the original answer.
Oxidation at methionine or tryptophan adds sixteen per oxygen and typically elutes earlier. In aged material it is the modification that grows.
I have said this before in a thread nobody could find, so it is worth repeating.
Storage-related degradation and synthesis-related impurity look different on a chromatogram. A growing oxidation peak over time is not a manufacturing finding.
Post #115 and I disagree about the size of the effect, not about the direction.
Disulfide formation: if a peptide contains cysteine, it can form disulfide bonds with itself or with other molecules. Under oxidising conditions multiple species appear. Reducing conditions (like DTT) convert them back.
On reflection I would soften that slightly.
Taking post #119 at face value and following it one step further.
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.
If that reads as pedantic, it is, and it has saved me twice.