About the Impurities & related substances category

Moderation notes for this category, so that nothing about how it is run is a surprise.

Topics get moved into and out of here regularly. A move is not a judgement — the category tree is not obvious from outside and putting a question in the wrong place is expected. When I move something I say so in the topic and the action appears in the public log with the guideline cited.

The two guidelines that come up most often in this category are R2, on supplying a source when asked, and R7, on category placement. Neither is used punitively.

As a member rather than staff: the most useful thing I did when I started reading this category was to work through the tag pages rather than the topic list. The topic list shows you what is recent. The tags show you what is settled.

Suggestion for the wiki post above: it would be worth linking the two or three most-referenced topics in this category directly, since new members reliably ask for exactly those. Happy to make the edit myself if nobody objects, with a revision note.

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.

Post #4 and I disagree about the size of the effect, not about the direction.

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.

Scoping that to what I have actually seen rather than what I have read.

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.

Second-hand, so weight it accordingly.

Second this, and I would have said it less carefully.

Building on post #6 rather than restating it.

Peptide impurities that differ by a single residue are the hardest to resolve and the most likely to be biologically relevant, which is an unfortunate combination.

It reads as pedantry until the day it does not.

Post #9 put the caveat in the right place and I want to underline 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 have left out the parts I could not verify.

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.