# is this the reason why time of flight mass spectrometers produce a mass spectagram with mass over charge on the x axis?

Is this the reason why time of flight mass spectrometers produce a mass spectagram with mass over charge on the x axis?

I haven't used one but from what I understand, they have an electron gun and it is meant to knock one electron off an atom, but on rare occasions it may knock more off(probably one more, so knocking two electrons off). The machine isn't aware of how many get knocked off but in its calculation, it assumes one got knocked off 'cos really only one is meant to be knocked off. KE=1/2 M V^2  M=2KE/V^2 If two get knocked off, giving a charge of 2+, then the kinetic energy is doubled, and likewise if three electrons get knocked off then it gives a charge of 3+ and the kinetic energy is tripled. So on rare occasions where more than one electron is knocked off, mass ends up too much, by a factor of charge.

There's no complete way to always correct for that as we don't know the charge when the gun messed up. But my understanding is that we label the x axis as "mass over charge", so at least that is true to what has been actually plotted on the graph. We might adjust for any errors that look not right , if we can. And we look at it as mass. (though bearing in mind that some results could be off a bit due to the charge issue).

Is that the reason and thinking behind why the x axis is labelled as "mass over charge"?

• What else but mass over charge would you expect there? You cannot distinguish twice the charge and half the mass. Commented Jan 11, 2022 at 18:42
• mass over charge comes into play in many situations involving electrostatic accelerators and beam deflecting magnets. It is a fact of life, and how many things get multiply charged or not depends a lot on the particular experiment (thinking well beyond mass spec). Commented Jan 11, 2022 at 19:25
• @Poutnik I did not suggest that you'd get something other than mass over charge there. But perhaps you could if you had a way of detecting the number of electrons in an atom and used that method before and after the gun had hit the atom, then you would know whether the charge of the atom was +1 or +2 or +3 Or, a way of measuring how much energy is produced by the gun accurately enough to know whether it'd knock one electron off or two. Ways of knowing how many electrons will get knocked off an atom and how many atoms an electron has. Commented Jan 11, 2022 at 19:51
• @Poutnik or this alternative to a mass spectrometer maybe this doesn't have that problem as doesn't use an electron gun internetchemie.info/news/2008/jul08/nanoscale-mass-sensor.html Commented Jan 11, 2022 at 19:51
• @barlop - there are, of course, ways of getting at charge and mass separately, they are more complex than a simple TOF mass spec and generally are not needed if you understand that the TOF system is giving you what it is giving you. As a fun aside, the inventor of the TOF mass spec was a nuclear physicist who built and ran Van de Graaff-type accelerators for photofission experiments. Commented Jan 11, 2022 at 20:44

## 1 Answer

Is this the reason why time of flight mass spectrometers produce a mass spectagram with mass over charge on the x axis?

Because this is what the mass spectrometer is designed to measure - mass over charge of an ion. The term mass spectrometer is a misnomer. They do not measure mass. Initially, they were called parabola spectrographs. The reason arises from mathematics. If you look at the equation of the motion of ions in an electric field or magnetic field, you will always find mass to charge ratio somewhere. It can be difficult to calculate the motion of a charged particle in a field. Lots of differential equations.

Plotting of m/z on the x-axis has literally nothing to do how the molecules are ionized. Also, this has nothing to do with electron gun in a mass spectrometer's ionization source. When you remove sticky tape rapidly from it reel, this can also ionize molecules. Just for fun, some people have even shown emission of soft X-rays when you peel the tape off.