[iScience]

I just got exposed to a little mass spectrometry today. Nothing deep, but the basic idea was to shoot a sample with and electron beam and pass that "cation" through an electric field. But, I was curious as to why shooting something with electrons would knock out electrons as opposed to shove more electrons in an orbital? is this more of a P. chem question?

[DrCMS]

Some questions for you that might help you figure this out:

How did you direct/detect the ions with a positive charge?

Where would any ions with a negative charge go under the same conditions?

So do you think you only ever made positively charged ions or that those were the only ones that got to the detector?

[magician4]

in addition:

under the usual conditions of MS, the velocity of the electrons you're bombarding the molecules with is too high to catch them: they can split a molecule, they can knock out electrons, but that's all.

if memory serves, the ratio of "catch" to "knock out" is calculated to be in the range of 1 : 100

regards

Ingo

[iScience]

it seems magician4's response pretty much answers my question. But then interpolating from DrCMS's reponse, it would seem that both anions and cations get made in respectable amounts but the detector is set up at an angle such that it only catches the path of the cations. Is this true?

[magician4]

as I said, the ratio that those ions occur at is like 1 : 100
(there are also other interesting things that take place, like double positively charged ions and thatlike for instance)

so, yes, you need kind of a "filter" to get rid of negatively charged ions  (and there is, yes)


regards

Ingo

[DrCMS]

My response was meant to make you think why you only saw the positive ions. 

magician4's answer explains why EI-MS is best set up for those positive ions. 

[orgopete]

I just got exposed to a little mass spectrometry today. Nothing deep, but the basic idea was to shoot a sample with and electron beam and pass that "cation" through an electric field. But, I was curious as to why shooting something with electrons would knock out electrons as opposed to shove more electrons in an orbital? is this more of a P. chem question?

This is a question that I have thought about quite a lot. As Ingo has suggested a 1:100 ratio, I would think the net result is very compound dependent. If you had a carboxylic acid, I think I would be surprised to see electron capture. Well, sort of, there will be electron capture and loss of a hydrogen radical to give a carboxylate.

I personally think they may all be electron capture events. The lower the electron volts, the smaller the number of possible interactions. With a high energy electron volt stream, the interactions may be less selective. I would draw an analogy with free radical reactions, fluorine radicals are the least stable and the least selective, bromine the most stable and the most selective.

If they are electron capture, then why would we get cations? There is a rational amount of chemistry we might consider. I did not find H3(-), but H3(+) is a common entity. It can capture an electron, however H3 has an odd number of electrons and it decomposes. H2(+) forms from cosmic radiation, but it too will decompose in collisions with H2. The result is H3(+) and H-radical. So seemingly there is a preference for even electrons. If electrons are captured, the result will have an odd number of electrons. Negative ions should only result if this were stable. If it is unstable, it could lose one electron and reform the original electron pair. As an alternate, it could lose an electron pair and form a cation with an odd number of electrons. This can decompose to a cation and radical. This now looks like the mass spectrum effect.

Now this may seem unusual, but we could consider a different mass spectrum effect, namely using protons rather than electrons. There is a solution equivalent of this, acidification. If protonation of the non-bonded electrons of water occurs, if electron-electron force were weak, then a hydrogen radical could result. Hydrogen radicals readily couple to form H2. This could tell us electron pair forces are stronger than Coulombic forces, that is the simple capture of an electron by a proton. Acidification does not form hydrogen gas. Heating water to 2500 degrees in a nuclear reactor will break the O-H bond and lead to H2 and O2 gases.

Just a different guess as to why this might occur.