Racemisation and why it is invisible to a mass measurement posts 121–128

Answering the question post #119 raises rather than the one it answers.

My position on racemisation is current rather than settled. I have revised it once already and I expect to again, so treat it accordingly.

That matches what I have seen, for whatever a single anecdote is worth.

Deletion sequences (incomplete coupling during synthesis): lower in mass by one residue. Chromatographically they usually elute earlier or later depending on the residue's hydrophobicity. They are the most common impurity in solid-phase synthesis.

This has been discussed before and I could not find the thread, so, again.

The arithmetic on racemisation is the easy part and it is where the errors are, which is an uncomfortable combination. Show your working and someone will catch 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.

Second-hand, so weight it accordingly.

Truncation products are substantially lower in mass and usually well separated chromatographically, which makes them the easiest class to see and the least likely to be missed.

It reads as pedantry until the day it does not.

Building on post #127 rather than restating it.

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.