MS/MS sequence confirmation: what it costs and when it is worth it — what changed since

Quantitation by MS: most quantitation is done by LC-UV detection at 214 nm, not by MS, because extinction coefficients are better known. MS can quantify if an internal standard is used but that requires preparation.

Worth separating two things that post #13 runs together.

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.

Thank you for the correction. I have edited my earlier post with a note rather than silently, so the thread still makes sense to read. The error was mine and it was the kind that comes from remembering a figure instead of looking it up.

Mass accuracy is expressed in parts per million. It is the difference between observed and theoretical mass divided by theoretical mass, multiplied by a million. A high-resolution instrument in good calibration achieves low single-digit ppm on a peptide of this size.

Coming back to post #28, because the follow-up matters more than the original answer.

Common adducts: sodium adds ≈22, potassium adds ≈38 compared to hydrogen. A [M+Na]+ peak is common and its mass is predictable from the base mass.

I read post #35 twice before replying, because I had assumed the opposite.

Desalting before analysis: some samples need desalting to remove salts that suppress the peptide signal. Report whether desalting was used, because it can affect the apparent ionization efficiency and the reported purity.