observe electron mass in positive and negative ionization mode [improvement]

Hi, this is for version plotms 5.1.

In positive and negative ionization mode the electron mass should be observed. Reason is that modern accurate mass spectrometers are "accurate" enough to measure such differences. While they are calibrated themselves, it is important for software to report correct masses of ion species. This is relevant for fragments and precursors, also differs in positive and negative ionization mode. This would require plotms to obtain information about the used ionization mode.

Mass of electron: m(e) = 0.0005485799092(4) u.
positive mode (electron lost): example [C9H22NO3]+: (192.15996600 - 0.0005485799092) = 192.1594174200908 u
negative mode (electron added): example [C9H22NO3]-: (192.15996600 + 0.0005485799092) = 192.1605145799092 u
neutral mass (wrong for ions): example C9H22NO3 = 192.15996600 u

| Element  | Mass        | Count | Final        |
| -------- | ----------- | ----- | ------------ |
| carbon   | 12.00000000 | 9     | 108.00000000 |
| hydrogen | 1.00782500  | 22    | 22.17215000  |
| nitrogen | 14.00307400 | 1     | 14.00307400  |
| oxygen   | 15.99491400 | 3     | 47.98474200  |
| sum      |             |       | 192.15996600 |

Calculated from values in plotms 5.1 code

Also see reference below: "We have to point out that it is possible to measure these small differences in mass with a benchtop TOF mass spectrometer", that was basically 15 years ago, and newer instruments in common labs do that even more accurate. So that certainly applies to real life usage, even for QTOF/Orbitrap benchtop instruments, but also to FT-ICR of course.

Source: Importance of the electron mass in the calculations of exact mass by time-of-flight mass spectrometry https://analyticalsciencejournals.onlinelibrary.wiley.com/doi/10.1002/rcm.3102

Measuring the Mass of an Electron by LC/TOF-MS: A Study of “Twin Ions” https://pubs.acs.org/doi/10.1021/ac0485942

New determination of the electron's mass https://pubmed.ncbi.nlm.nih.gov/11800934/

qcxms / PlotMS

observe electron mass in positive and negative ionization mode [improvement] #4