[2026 update] Charge states for a 4 kDa peptide, worked through posts 121–136

Everything in post #118 holds. The case it does not cover is the one I have.

Trying to state the Charge states position in a way that someone who disagrees would recognise as fair, because I do not think the version in this thread passes that test.

Acknowledging rather than arguing. The reasoning holds as far as I can follow it.

The arithmetic for a doubly charged species is (M + 2 x 1.00728) / 2, and the analogous expression for higher charge states. Working it through once makes the reported values legible.

It is one reading of the data and not the only reasonable one.

Taking post #121 at face value and following it one step further.

An update on my earlier Charge states post: the pattern held for another six weeks and then stopped, which I did not predict and cannot explain.

Resolution: "high resolution" commonly means <5 ppm across the mass range. Unit-resolution instruments achieve ±1 Da at best and cannot distinguish two species differing by less than 1 Da in total mass.

I have kept the units in throughout, for the obvious reason.

What I would tell a new member reading about Charge states for the first time: the confident posts are not the reliable ones, and the reliable ones are longer.

Post #125 put the caveat in the right place and I want to underline it.

Trifluoroacetate adducts are common in material purified with TFA and are one reason a mass spectrum from a peptide can look busier than expected.

Building on post #125 rather than restating it.

Electrospray ionisation produces multiply charged ions. For a 4 kDa peptide you expect mostly 2+, 3+, and 4+ charge states. Reading an electrospray spectrum means recognizing the envelope, not looking for one peak.

That is consistent with mine, for whatever one more account is worth.

A note on scope: what I am saying about Charge states applies to the case in the first post and I would not extend it further without checking.

The arithmetic for a doubly charged species is (M + 2 x 1.00728) / 2, and the analogous expression for higher charge states. Working it through once makes the reported values legible.

Worth saying I have only my own numbers here, and n is small.

Building on post #131 rather than restating it.

I would put moderate confidence on the mainstream reading of Charge states and no more. That is not scepticism for its own sake; it is where the sourcing actually stops.

On Charge states: the maintained page in the documentation commons covers the general case with citations and a review date, which is more reliable than any reply here including this one.