A rising related-substance total over six months: degradation or method drift? posts 31–43

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.

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

Taking post #29 at face value and following it one step further.

Deamidation at asparagine and glutamine: adds 1 approximately. Frequently appears as a close-eluting pair. It is a chemical modification that occurs during storage.

Nothing above should be read as advice about what anyone else should do.

Adding a null result on rising related-substance. I looked, carefully, and found nothing, and null results deserve posting precisely because they never are.

Rising related-substance would be much easier to settle if anyone reported the denominator. Almost nobody reports the denominator.

No disagreement from me. Posting only so the question does not look ignored.

Building on post #33 rather than restating it.

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.

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

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.

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.

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

I would rather this thread reach "we do not know" about rising related-substance than reach a confident answer that nobody can support when asked.

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.

Adding the measurement that post #38 says would settle it.

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.

That is where I would start, not where I would stop.

Post #40 describes the usual case. This is about the unusual one.

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.

Happy to be the one who is wrong here if it settles the question.

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.

I am aware this is the third time this month I have made this point.