Second pass at: What a 2 ppm match establishes, and what it does not

Building on the opening post rather than restating it.

Calibration matters: a high-resolution instrument out of calibration can report mass with ppm error large enough to be uninformative. Check when the instrument was last calibrated before trusting the reported accuracy.

I have written this out at length because the short version keeps being misread.

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.

Having read the whole 2 ppm match thread before replying: the question in the first post has not actually been answered yet, and three of us have answered a nearby one instead.

One more thing on 2 ppm match that took me far too long to see: the two figures people quote are not measuring the same quantity. Once you notice that, the apparent contradiction disappears.

Where I part company with post #23, and it is a narrow parting.

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.

Same conclusion as the reply above, reached differently, which is mildly reassuring.

Two sentences on 2 ppm match and then I will stop, because the rest is speculation and the thread is better without mine.

What is documented is narrow. What is inferred from it is broad. The gap between them is where every argument here lives.

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.

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